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already been added and the inconsistency made choosing names and changing code more annoying. Plus, wow are they better for this code! git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231347 91177308-0d34-0410-b5e6-96231b3b80d8
1204 lines
48 KiB
C++
1204 lines
48 KiB
C++
//===-- MachineBlockPlacement.cpp - Basic Block Code Layout optimization --===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements basic block placement transformations using the CFG
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// structure and branch probability estimates.
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//
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// The pass strives to preserve the structure of the CFG (that is, retain
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// a topological ordering of basic blocks) in the absence of a *strong* signal
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// to the contrary from probabilities. However, within the CFG structure, it
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// attempts to choose an ordering which favors placing more likely sequences of
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// blocks adjacent to each other.
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//
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// The algorithm works from the inner-most loop within a function outward, and
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// at each stage walks through the basic blocks, trying to coalesce them into
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// sequential chains where allowed by the CFG (or demanded by heavy
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// probabilities). Finally, it walks the blocks in topological order, and the
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// first time it reaches a chain of basic blocks, it schedules them in the
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// function in-order.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/CodeGen/MachineBasicBlock.h"
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#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
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#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
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#include "llvm/CodeGen/MachineDominators.h"
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#include "llvm/CodeGen/MachineFunction.h"
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#include "llvm/CodeGen/MachineFunctionPass.h"
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#include "llvm/CodeGen/MachineLoopInfo.h"
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#include "llvm/CodeGen/MachineModuleInfo.h"
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#include "llvm/Support/Allocator.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Target/TargetInstrInfo.h"
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#include "llvm/Target/TargetLowering.h"
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#include "llvm/Target/TargetSubtargetInfo.h"
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#include <algorithm>
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using namespace llvm;
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#define DEBUG_TYPE "block-placement"
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STATISTIC(NumCondBranches, "Number of conditional branches");
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STATISTIC(NumUncondBranches, "Number of uncondittional branches");
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STATISTIC(CondBranchTakenFreq,
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"Potential frequency of taking conditional branches");
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STATISTIC(UncondBranchTakenFreq,
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"Potential frequency of taking unconditional branches");
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static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
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cl::desc("Force the alignment of all "
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"blocks in the function."),
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cl::init(0), cl::Hidden);
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// FIXME: Find a good default for this flag and remove the flag.
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static cl::opt<unsigned> ExitBlockBias(
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"block-placement-exit-block-bias",
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cl::desc("Block frequency percentage a loop exit block needs "
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"over the original exit to be considered the new exit."),
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cl::init(0), cl::Hidden);
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static cl::opt<bool> OutlineOptionalBranches(
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"outline-optional-branches",
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cl::desc("Put completely optional branches, i.e. branches with a common "
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"post dominator, out of line."),
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cl::init(false), cl::Hidden);
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namespace {
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class BlockChain;
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/// \brief Type for our function-wide basic block -> block chain mapping.
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typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
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}
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namespace {
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/// \brief A chain of blocks which will be laid out contiguously.
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///
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/// This is the datastructure representing a chain of consecutive blocks that
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/// are profitable to layout together in order to maximize fallthrough
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/// probabilities and code locality. We also can use a block chain to represent
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/// a sequence of basic blocks which have some external (correctness)
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/// requirement for sequential layout.
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///
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/// Chains can be built around a single basic block and can be merged to grow
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/// them. They participate in a block-to-chain mapping, which is updated
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/// automatically as chains are merged together.
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class BlockChain {
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/// \brief The sequence of blocks belonging to this chain.
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///
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/// This is the sequence of blocks for a particular chain. These will be laid
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/// out in-order within the function.
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SmallVector<MachineBasicBlock *, 4> Blocks;
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/// \brief A handle to the function-wide basic block to block chain mapping.
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///
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/// This is retained in each block chain to simplify the computation of child
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/// block chains for SCC-formation and iteration. We store the edges to child
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/// basic blocks, and map them back to their associated chains using this
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/// structure.
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BlockToChainMapType &BlockToChain;
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public:
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/// \brief Construct a new BlockChain.
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///
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/// This builds a new block chain representing a single basic block in the
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/// function. It also registers itself as the chain that block participates
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/// in with the BlockToChain mapping.
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BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
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: Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) {
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assert(BB && "Cannot create a chain with a null basic block");
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BlockToChain[BB] = this;
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}
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/// \brief Iterator over blocks within the chain.
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typedef SmallVectorImpl<MachineBasicBlock *>::iterator iterator;
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/// \brief Beginning of blocks within the chain.
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iterator begin() { return Blocks.begin(); }
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/// \brief End of blocks within the chain.
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iterator end() { return Blocks.end(); }
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/// \brief Merge a block chain into this one.
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///
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/// This routine merges a block chain into this one. It takes care of forming
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/// a contiguous sequence of basic blocks, updating the edge list, and
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/// updating the block -> chain mapping. It does not free or tear down the
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/// old chain, but the old chain's block list is no longer valid.
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void merge(MachineBasicBlock *BB, BlockChain *Chain) {
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assert(BB);
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assert(!Blocks.empty());
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// Fast path in case we don't have a chain already.
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if (!Chain) {
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assert(!BlockToChain[BB]);
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Blocks.push_back(BB);
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BlockToChain[BB] = this;
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return;
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}
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assert(BB == *Chain->begin());
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assert(Chain->begin() != Chain->end());
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// Update the incoming blocks to point to this chain, and add them to the
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// chain structure.
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for (MachineBasicBlock *ChainBB : *Chain) {
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Blocks.push_back(ChainBB);
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assert(BlockToChain[ChainBB] == Chain && "Incoming blocks not in chain");
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BlockToChain[ChainBB] = this;
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}
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}
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#ifndef NDEBUG
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/// \brief Dump the blocks in this chain.
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LLVM_DUMP_METHOD void dump() {
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for (MachineBasicBlock *MBB : *this)
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MBB->dump();
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}
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#endif // NDEBUG
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/// \brief Count of predecessors within the loop currently being processed.
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///
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/// This count is updated at each loop we process to represent the number of
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/// in-loop predecessors of this chain.
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unsigned LoopPredecessors;
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};
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}
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namespace {
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class MachineBlockPlacement : public MachineFunctionPass {
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/// \brief A typedef for a block filter set.
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typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;
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/// \brief A handle to the branch probability pass.
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const MachineBranchProbabilityInfo *MBPI;
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/// \brief A handle to the function-wide block frequency pass.
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const MachineBlockFrequencyInfo *MBFI;
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/// \brief A handle to the loop info.
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const MachineLoopInfo *MLI;
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/// \brief A handle to the target's instruction info.
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const TargetInstrInfo *TII;
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/// \brief A handle to the target's lowering info.
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const TargetLoweringBase *TLI;
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/// \brief A handle to the post dominator tree.
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MachineDominatorTree *MDT;
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/// \brief A set of blocks that are unavoidably execute, i.e. they dominate
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/// all terminators of the MachineFunction.
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SmallPtrSet<MachineBasicBlock *, 4> UnavoidableBlocks;
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/// \brief Allocator and owner of BlockChain structures.
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///
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/// We build BlockChains lazily while processing the loop structure of
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/// a function. To reduce malloc traffic, we allocate them using this
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/// slab-like allocator, and destroy them after the pass completes. An
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/// important guarantee is that this allocator produces stable pointers to
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/// the chains.
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SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
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/// \brief Function wide BasicBlock to BlockChain mapping.
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///
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/// This mapping allows efficiently moving from any given basic block to the
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/// BlockChain it participates in, if any. We use it to, among other things,
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/// allow implicitly defining edges between chains as the existing edges
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/// between basic blocks.
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DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
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void markChainSuccessors(BlockChain &Chain, MachineBasicBlock *LoopHeaderBB,
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SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
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const BlockFilterSet *BlockFilter = nullptr);
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MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
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BlockChain &Chain,
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const BlockFilterSet *BlockFilter);
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MachineBasicBlock *
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selectBestCandidateBlock(BlockChain &Chain,
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SmallVectorImpl<MachineBasicBlock *> &WorkList,
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const BlockFilterSet *BlockFilter);
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MachineBasicBlock *
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getFirstUnplacedBlock(MachineFunction &F, const BlockChain &PlacedChain,
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MachineFunction::iterator &PrevUnplacedBlockIt,
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const BlockFilterSet *BlockFilter);
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void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
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SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
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const BlockFilterSet *BlockFilter = nullptr);
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MachineBasicBlock *findBestLoopTop(MachineLoop &L,
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const BlockFilterSet &LoopBlockSet);
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MachineBasicBlock *findBestLoopExit(MachineFunction &F, MachineLoop &L,
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const BlockFilterSet &LoopBlockSet);
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void buildLoopChains(MachineFunction &F, MachineLoop &L);
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void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB,
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const BlockFilterSet &LoopBlockSet);
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void buildCFGChains(MachineFunction &F);
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public:
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static char ID; // Pass identification, replacement for typeid
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MachineBlockPlacement() : MachineFunctionPass(ID) {
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initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
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}
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bool runOnMachineFunction(MachineFunction &F) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<MachineBranchProbabilityInfo>();
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AU.addRequired<MachineBlockFrequencyInfo>();
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AU.addRequired<MachineDominatorTree>();
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AU.addRequired<MachineLoopInfo>();
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MachineFunctionPass::getAnalysisUsage(AU);
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}
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};
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}
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char MachineBlockPlacement::ID = 0;
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char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
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INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement",
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"Branch Probability Basic Block Placement", false, false)
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INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
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INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
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INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
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INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
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INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement",
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"Branch Probability Basic Block Placement", false, false)
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#ifndef NDEBUG
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/// \brief Helper to print the name of a MBB.
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///
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/// Only used by debug logging.
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static std::string getBlockName(MachineBasicBlock *BB) {
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std::string Result;
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raw_string_ostream OS(Result);
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OS << "BB#" << BB->getNumber();
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OS << " (derived from LLVM BB '" << BB->getName() << "')";
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OS.flush();
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return Result;
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}
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/// \brief Helper to print the number of a MBB.
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///
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/// Only used by debug logging.
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static std::string getBlockNum(MachineBasicBlock *BB) {
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std::string Result;
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raw_string_ostream OS(Result);
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OS << "BB#" << BB->getNumber();
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OS.flush();
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return Result;
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}
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#endif
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/// \brief Mark a chain's successors as having one fewer preds.
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///
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/// When a chain is being merged into the "placed" chain, this routine will
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/// quickly walk the successors of each block in the chain and mark them as
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/// having one fewer active predecessor. It also adds any successors of this
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/// chain which reach the zero-predecessor state to the worklist passed in.
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void MachineBlockPlacement::markChainSuccessors(
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BlockChain &Chain, MachineBasicBlock *LoopHeaderBB,
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SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
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const BlockFilterSet *BlockFilter) {
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// Walk all the blocks in this chain, marking their successors as having
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// a predecessor placed.
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for (MachineBasicBlock *MBB : Chain) {
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// Add any successors for which this is the only un-placed in-loop
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// predecessor to the worklist as a viable candidate for CFG-neutral
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// placement. No subsequent placement of this block will violate the CFG
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// shape, so we get to use heuristics to choose a favorable placement.
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for (MachineBasicBlock *Succ : MBB->successors()) {
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if (BlockFilter && !BlockFilter->count(Succ))
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continue;
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BlockChain &SuccChain = *BlockToChain[Succ];
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// Disregard edges within a fixed chain, or edges to the loop header.
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if (&Chain == &SuccChain || Succ == LoopHeaderBB)
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continue;
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// This is a cross-chain edge that is within the loop, so decrement the
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// loop predecessor count of the destination chain.
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if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
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BlockWorkList.push_back(*SuccChain.begin());
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}
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}
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}
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/// \brief Select the best successor for a block.
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///
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/// This looks across all successors of a particular block and attempts to
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/// select the "best" one to be the layout successor. It only considers direct
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/// successors which also pass the block filter. It will attempt to avoid
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/// breaking CFG structure, but cave and break such structures in the case of
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/// very hot successor edges.
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///
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/// \returns The best successor block found, or null if none are viable.
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MachineBasicBlock *
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MachineBlockPlacement::selectBestSuccessor(MachineBasicBlock *BB,
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BlockChain &Chain,
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const BlockFilterSet *BlockFilter) {
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const BranchProbability HotProb(4, 5); // 80%
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MachineBasicBlock *BestSucc = nullptr;
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// FIXME: Due to the performance of the probability and weight routines in
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// the MBPI analysis, we manually compute probabilities using the edge
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// weights. This is suboptimal as it means that the somewhat subtle
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// definition of edge weight semantics is encoded here as well. We should
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// improve the MBPI interface to efficiently support query patterns such as
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// this.
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uint32_t BestWeight = 0;
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uint32_t WeightScale = 0;
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uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
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DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
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for (MachineBasicBlock *Succ : BB->successors()) {
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if (BlockFilter && !BlockFilter->count(Succ))
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continue;
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BlockChain &SuccChain = *BlockToChain[Succ];
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if (&SuccChain == &Chain) {
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DEBUG(dbgs() << " " << getBlockName(Succ) << " -> Already merged!\n");
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continue;
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}
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if (Succ != *SuccChain.begin()) {
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DEBUG(dbgs() << " " << getBlockName(Succ) << " -> Mid chain!\n");
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continue;
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}
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uint32_t SuccWeight = MBPI->getEdgeWeight(BB, Succ);
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BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
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// If we outline optional branches, look whether Succ is unavoidable, i.e.
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// dominates all terminators of the MachineFunction. If it does, other
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// successors must be optional. Don't do this for cold branches.
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if (OutlineOptionalBranches && SuccProb > HotProb.getCompl() &&
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UnavoidableBlocks.count(Succ) > 0)
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return Succ;
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// Only consider successors which are either "hot", or wouldn't violate
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// any CFG constraints.
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if (SuccChain.LoopPredecessors != 0) {
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if (SuccProb < HotProb) {
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DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
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<< " (prob) (CFG conflict)\n");
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continue;
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}
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// Make sure that a hot successor doesn't have a globally more
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// important predecessor.
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BlockFrequency CandidateEdgeFreq =
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MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
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bool BadCFGConflict = false;
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for (MachineBasicBlock *Pred : Succ->predecessors()) {
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if (Pred == Succ || (BlockFilter && !BlockFilter->count(Pred)) ||
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BlockToChain[Pred] == &Chain)
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continue;
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BlockFrequency PredEdgeFreq =
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MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ);
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if (PredEdgeFreq >= CandidateEdgeFreq) {
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BadCFGConflict = true;
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break;
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}
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}
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if (BadCFGConflict) {
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DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
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<< " (prob) (non-cold CFG conflict)\n");
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continue;
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}
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}
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DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
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<< " (prob)"
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<< (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
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<< "\n");
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if (BestSucc && BestWeight >= SuccWeight)
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continue;
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BestSucc = Succ;
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BestWeight = SuccWeight;
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}
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return BestSucc;
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}
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/// \brief Select the best block from a worklist.
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///
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/// This looks through the provided worklist as a list of candidate basic
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/// blocks and select the most profitable one to place. The definition of
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/// profitable only really makes sense in the context of a loop. This returns
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/// the most frequently visited block in the worklist, which in the case of
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/// a loop, is the one most desirable to be physically close to the rest of the
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/// loop body in order to improve icache behavior.
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///
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/// \returns The best block found, or null if none are viable.
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MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
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BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
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const BlockFilterSet *BlockFilter) {
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// Once we need to walk the worklist looking for a candidate, cleanup the
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// worklist of already placed entries.
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// FIXME: If this shows up on profiles, it could be folded (at the cost of
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// some code complexity) into the loop below.
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WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
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[&](MachineBasicBlock *BB) {
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return BlockToChain.lookup(BB) == &Chain;
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}),
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WorkList.end());
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MachineBasicBlock *BestBlock = nullptr;
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BlockFrequency BestFreq;
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for (MachineBasicBlock *MBB : WorkList) {
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BlockChain &SuccChain = *BlockToChain[MBB];
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if (&SuccChain == &Chain) {
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DEBUG(dbgs() << " " << getBlockName(MBB) << " -> Already merged!\n");
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continue;
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}
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assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
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BlockFrequency CandidateFreq = MBFI->getBlockFreq(MBB);
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DEBUG(dbgs() << " " << getBlockName(MBB) << " -> ";
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MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n");
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if (BestBlock && BestFreq >= CandidateFreq)
|
|
continue;
|
|
BestBlock = MBB;
|
|
BestFreq = CandidateFreq;
|
|
}
|
|
return BestBlock;
|
|
}
|
|
|
|
/// \brief Retrieve the first unplaced basic block.
|
|
///
|
|
/// This routine is called when we are unable to use the CFG to walk through
|
|
/// all of the basic blocks and form a chain due to unnatural loops in the CFG.
|
|
/// We walk through the function's blocks in order, starting from the
|
|
/// LastUnplacedBlockIt. We update this iterator on each call to avoid
|
|
/// re-scanning the entire sequence on repeated calls to this routine.
|
|
MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
|
|
MachineFunction &F, const BlockChain &PlacedChain,
|
|
MachineFunction::iterator &PrevUnplacedBlockIt,
|
|
const BlockFilterSet *BlockFilter) {
|
|
for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E;
|
|
++I) {
|
|
if (BlockFilter && !BlockFilter->count(I))
|
|
continue;
|
|
if (BlockToChain[I] != &PlacedChain) {
|
|
PrevUnplacedBlockIt = I;
|
|
// Now select the head of the chain to which the unplaced block belongs
|
|
// as the block to place. This will force the entire chain to be placed,
|
|
// and satisfies the requirements of merging chains.
|
|
return *BlockToChain[I]->begin();
|
|
}
|
|
}
|
|
return nullptr;
|
|
}
|
|
|
|
void MachineBlockPlacement::buildChain(
|
|
MachineBasicBlock *BB, BlockChain &Chain,
|
|
SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
|
|
const BlockFilterSet *BlockFilter) {
|
|
assert(BB);
|
|
assert(BlockToChain[BB] == &Chain);
|
|
MachineFunction &F = *BB->getParent();
|
|
MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
|
|
|
|
MachineBasicBlock *LoopHeaderBB = BB;
|
|
markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
|
|
BB = *std::prev(Chain.end());
|
|
for (;;) {
|
|
assert(BB);
|
|
assert(BlockToChain[BB] == &Chain);
|
|
assert(*std::prev(Chain.end()) == BB);
|
|
|
|
// Look for the best viable successor if there is one to place immediately
|
|
// after this block.
|
|
MachineBasicBlock *BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
|
|
|
|
// If an immediate successor isn't available, look for the best viable
|
|
// block among those we've identified as not violating the loop's CFG at
|
|
// this point. This won't be a fallthrough, but it will increase locality.
|
|
if (!BestSucc)
|
|
BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
|
|
|
|
if (!BestSucc) {
|
|
BestSucc =
|
|
getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt, BlockFilter);
|
|
if (!BestSucc)
|
|
break;
|
|
|
|
DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
|
|
"layout successor until the CFG reduces\n");
|
|
}
|
|
|
|
// Place this block, updating the datastructures to reflect its placement.
|
|
BlockChain &SuccChain = *BlockToChain[BestSucc];
|
|
// Zero out LoopPredecessors for the successor we're about to merge in case
|
|
// we selected a successor that didn't fit naturally into the CFG.
|
|
SuccChain.LoopPredecessors = 0;
|
|
DEBUG(dbgs() << "Merging from " << getBlockNum(BB) << " to "
|
|
<< getBlockNum(BestSucc) << "\n");
|
|
markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
|
|
Chain.merge(BestSucc, &SuccChain);
|
|
BB = *std::prev(Chain.end());
|
|
}
|
|
|
|
DEBUG(dbgs() << "Finished forming chain for header block "
|
|
<< getBlockNum(*Chain.begin()) << "\n");
|
|
}
|
|
|
|
/// \brief Find the best loop top block for layout.
|
|
///
|
|
/// Look for a block which is strictly better than the loop header for laying
|
|
/// out at the top of the loop. This looks for one and only one pattern:
|
|
/// a latch block with no conditional exit. This block will cause a conditional
|
|
/// jump around it or will be the bottom of the loop if we lay it out in place,
|
|
/// but if it it doesn't end up at the bottom of the loop for any reason,
|
|
/// rotation alone won't fix it. Because such a block will always result in an
|
|
/// unconditional jump (for the backedge) rotating it in front of the loop
|
|
/// header is always profitable.
|
|
MachineBasicBlock *
|
|
MachineBlockPlacement::findBestLoopTop(MachineLoop &L,
|
|
const BlockFilterSet &LoopBlockSet) {
|
|
// Check that the header hasn't been fused with a preheader block due to
|
|
// crazy branches. If it has, we need to start with the header at the top to
|
|
// prevent pulling the preheader into the loop body.
|
|
BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
|
|
if (!LoopBlockSet.count(*HeaderChain.begin()))
|
|
return L.getHeader();
|
|
|
|
DEBUG(dbgs() << "Finding best loop top for: " << getBlockName(L.getHeader())
|
|
<< "\n");
|
|
|
|
BlockFrequency BestPredFreq;
|
|
MachineBasicBlock *BestPred = nullptr;
|
|
for (MachineBasicBlock *Pred : L.getHeader()->predecessors()) {
|
|
if (!LoopBlockSet.count(Pred))
|
|
continue;
|
|
DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", "
|
|
<< Pred->succ_size() << " successors, ";
|
|
MBFI->printBlockFreq(dbgs(), Pred) << " freq\n");
|
|
if (Pred->succ_size() > 1)
|
|
continue;
|
|
|
|
BlockFrequency PredFreq = MBFI->getBlockFreq(Pred);
|
|
if (!BestPred || PredFreq > BestPredFreq ||
|
|
(!(PredFreq < BestPredFreq) &&
|
|
Pred->isLayoutSuccessor(L.getHeader()))) {
|
|
BestPred = Pred;
|
|
BestPredFreq = PredFreq;
|
|
}
|
|
}
|
|
|
|
// If no direct predecessor is fine, just use the loop header.
|
|
if (!BestPred)
|
|
return L.getHeader();
|
|
|
|
// Walk backwards through any straight line of predecessors.
|
|
while (BestPred->pred_size() == 1 &&
|
|
(*BestPred->pred_begin())->succ_size() == 1 &&
|
|
*BestPred->pred_begin() != L.getHeader())
|
|
BestPred = *BestPred->pred_begin();
|
|
|
|
DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n");
|
|
return BestPred;
|
|
}
|
|
|
|
/// \brief Find the best loop exiting block for layout.
|
|
///
|
|
/// This routine implements the logic to analyze the loop looking for the best
|
|
/// block to layout at the top of the loop. Typically this is done to maximize
|
|
/// fallthrough opportunities.
|
|
MachineBasicBlock *
|
|
MachineBlockPlacement::findBestLoopExit(MachineFunction &F, MachineLoop &L,
|
|
const BlockFilterSet &LoopBlockSet) {
|
|
// We don't want to layout the loop linearly in all cases. If the loop header
|
|
// is just a normal basic block in the loop, we want to look for what block
|
|
// within the loop is the best one to layout at the top. However, if the loop
|
|
// header has be pre-merged into a chain due to predecessors not having
|
|
// analyzable branches, *and* the predecessor it is merged with is *not* part
|
|
// of the loop, rotating the header into the middle of the loop will create
|
|
// a non-contiguous range of blocks which is Very Bad. So start with the
|
|
// header and only rotate if safe.
|
|
BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
|
|
if (!LoopBlockSet.count(*HeaderChain.begin()))
|
|
return nullptr;
|
|
|
|
BlockFrequency BestExitEdgeFreq;
|
|
unsigned BestExitLoopDepth = 0;
|
|
MachineBasicBlock *ExitingBB = nullptr;
|
|
// If there are exits to outer loops, loop rotation can severely limit
|
|
// fallthrough opportunites unless it selects such an exit. Keep a set of
|
|
// blocks where rotating to exit with that block will reach an outer loop.
|
|
SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
|
|
|
|
DEBUG(dbgs() << "Finding best loop exit for: " << getBlockName(L.getHeader())
|
|
<< "\n");
|
|
for (MachineBasicBlock *MBB : L.getBlocks()) {
|
|
BlockChain &Chain = *BlockToChain[MBB];
|
|
// Ensure that this block is at the end of a chain; otherwise it could be
|
|
// mid-way through an inner loop or a successor of an analyzable branch.
|
|
if (MBB != *std::prev(Chain.end()))
|
|
continue;
|
|
|
|
// Now walk the successors. We need to establish whether this has a viable
|
|
// exiting successor and whether it has a viable non-exiting successor.
|
|
// We store the old exiting state and restore it if a viable looping
|
|
// successor isn't found.
|
|
MachineBasicBlock *OldExitingBB = ExitingBB;
|
|
BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq;
|
|
bool HasLoopingSucc = false;
|
|
// FIXME: Due to the performance of the probability and weight routines in
|
|
// the MBPI analysis, we use the internal weights and manually compute the
|
|
// probabilities to avoid quadratic behavior.
|
|
uint32_t WeightScale = 0;
|
|
uint32_t SumWeight = MBPI->getSumForBlock(MBB, WeightScale);
|
|
for (MachineBasicBlock *Succ : MBB->successors()) {
|
|
if (Succ->isLandingPad())
|
|
continue;
|
|
if (Succ == MBB)
|
|
continue;
|
|
BlockChain &SuccChain = *BlockToChain[Succ];
|
|
// Don't split chains, either this chain or the successor's chain.
|
|
if (&Chain == &SuccChain) {
|
|
DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> "
|
|
<< getBlockName(Succ) << " (chain conflict)\n");
|
|
continue;
|
|
}
|
|
|
|
uint32_t SuccWeight = MBPI->getEdgeWeight(MBB, Succ);
|
|
if (LoopBlockSet.count(Succ)) {
|
|
DEBUG(dbgs() << " looping: " << getBlockName(MBB) << " -> "
|
|
<< getBlockName(Succ) << " (" << SuccWeight << ")\n");
|
|
HasLoopingSucc = true;
|
|
continue;
|
|
}
|
|
|
|
unsigned SuccLoopDepth = 0;
|
|
if (MachineLoop *ExitLoop = MLI->getLoopFor(Succ)) {
|
|
SuccLoopDepth = ExitLoop->getLoopDepth();
|
|
if (ExitLoop->contains(&L))
|
|
BlocksExitingToOuterLoop.insert(MBB);
|
|
}
|
|
|
|
BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
|
|
BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(MBB) * SuccProb;
|
|
DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> "
|
|
<< getBlockName(Succ) << " [L:" << SuccLoopDepth << "] (";
|
|
MBFI->printBlockFreq(dbgs(), ExitEdgeFreq) << ")\n");
|
|
// Note that we bias this toward an existing layout successor to retain
|
|
// incoming order in the absence of better information. The exit must have
|
|
// a frequency higher than the current exit before we consider breaking
|
|
// the layout.
|
|
BranchProbability Bias(100 - ExitBlockBias, 100);
|
|
if (!ExitingBB || BestExitLoopDepth < SuccLoopDepth ||
|
|
ExitEdgeFreq > BestExitEdgeFreq ||
|
|
(MBB->isLayoutSuccessor(Succ) &&
|
|
!(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
|
|
BestExitEdgeFreq = ExitEdgeFreq;
|
|
ExitingBB = MBB;
|
|
}
|
|
}
|
|
|
|
// Restore the old exiting state, no viable looping successor was found.
|
|
if (!HasLoopingSucc) {
|
|
ExitingBB = OldExitingBB;
|
|
BestExitEdgeFreq = OldBestExitEdgeFreq;
|
|
continue;
|
|
}
|
|
}
|
|
// Without a candidate exiting block or with only a single block in the
|
|
// loop, just use the loop header to layout the loop.
|
|
if (!ExitingBB || L.getNumBlocks() == 1)
|
|
return nullptr;
|
|
|
|
// Also, if we have exit blocks which lead to outer loops but didn't select
|
|
// one of them as the exiting block we are rotating toward, disable loop
|
|
// rotation altogether.
|
|
if (!BlocksExitingToOuterLoop.empty() &&
|
|
!BlocksExitingToOuterLoop.count(ExitingBB))
|
|
return nullptr;
|
|
|
|
DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
|
|
return ExitingBB;
|
|
}
|
|
|
|
/// \brief Attempt to rotate an exiting block to the bottom of the loop.
|
|
///
|
|
/// Once we have built a chain, try to rotate it to line up the hot exit block
|
|
/// with fallthrough out of the loop if doing so doesn't introduce unnecessary
|
|
/// branches. For example, if the loop has fallthrough into its header and out
|
|
/// of its bottom already, don't rotate it.
|
|
void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
|
|
MachineBasicBlock *ExitingBB,
|
|
const BlockFilterSet &LoopBlockSet) {
|
|
if (!ExitingBB)
|
|
return;
|
|
|
|
MachineBasicBlock *Top = *LoopChain.begin();
|
|
bool ViableTopFallthrough = false;
|
|
for (MachineBasicBlock *Pred : Top->predecessors()) {
|
|
BlockChain *PredChain = BlockToChain[Pred];
|
|
if (!LoopBlockSet.count(Pred) &&
|
|
(!PredChain || Pred == *std::prev(PredChain->end()))) {
|
|
ViableTopFallthrough = true;
|
|
break;
|
|
}
|
|
}
|
|
|
|
// If the header has viable fallthrough, check whether the current loop
|
|
// bottom is a viable exiting block. If so, bail out as rotating will
|
|
// introduce an unnecessary branch.
|
|
if (ViableTopFallthrough) {
|
|
MachineBasicBlock *Bottom = *std::prev(LoopChain.end());
|
|
for (MachineBasicBlock *Succ : Bottom->successors()) {
|
|
BlockChain *SuccChain = BlockToChain[Succ];
|
|
if (!LoopBlockSet.count(Succ) &&
|
|
(!SuccChain || Succ == *SuccChain->begin()))
|
|
return;
|
|
}
|
|
}
|
|
|
|
BlockChain::iterator ExitIt =
|
|
std::find(LoopChain.begin(), LoopChain.end(), ExitingBB);
|
|
if (ExitIt == LoopChain.end())
|
|
return;
|
|
|
|
std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end());
|
|
}
|
|
|
|
/// \brief Forms basic block chains from the natural loop structures.
|
|
///
|
|
/// These chains are designed to preserve the existing *structure* of the code
|
|
/// as much as possible. We can then stitch the chains together in a way which
|
|
/// both preserves the topological structure and minimizes taken conditional
|
|
/// branches.
|
|
void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
|
|
MachineLoop &L) {
|
|
// First recurse through any nested loops, building chains for those inner
|
|
// loops.
|
|
for (MachineLoop *InnerLoop : L)
|
|
buildLoopChains(F, *InnerLoop);
|
|
|
|
SmallVector<MachineBasicBlock *, 16> BlockWorkList;
|
|
BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
|
|
|
|
// First check to see if there is an obviously preferable top block for the
|
|
// loop. This will default to the header, but may end up as one of the
|
|
// predecessors to the header if there is one which will result in strictly
|
|
// fewer branches in the loop body.
|
|
MachineBasicBlock *LoopTop = findBestLoopTop(L, LoopBlockSet);
|
|
|
|
// If we selected just the header for the loop top, look for a potentially
|
|
// profitable exit block in the event that rotating the loop can eliminate
|
|
// branches by placing an exit edge at the bottom.
|
|
MachineBasicBlock *ExitingBB = nullptr;
|
|
if (LoopTop == L.getHeader())
|
|
ExitingBB = findBestLoopExit(F, L, LoopBlockSet);
|
|
|
|
BlockChain &LoopChain = *BlockToChain[LoopTop];
|
|
|
|
// FIXME: This is a really lame way of walking the chains in the loop: we
|
|
// walk the blocks, and use a set to prevent visiting a particular chain
|
|
// twice.
|
|
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
|
|
assert(LoopChain.LoopPredecessors == 0);
|
|
UpdatedPreds.insert(&LoopChain);
|
|
for (MachineBasicBlock *LoopBB : L.getBlocks()) {
|
|
BlockChain &Chain = *BlockToChain[LoopBB];
|
|
if (!UpdatedPreds.insert(&Chain).second)
|
|
continue;
|
|
|
|
assert(Chain.LoopPredecessors == 0);
|
|
for (MachineBasicBlock *ChainBB : Chain) {
|
|
assert(BlockToChain[ChainBB] == &Chain);
|
|
for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
|
|
if (BlockToChain[Pred] == &Chain || !LoopBlockSet.count(Pred))
|
|
continue;
|
|
++Chain.LoopPredecessors;
|
|
}
|
|
}
|
|
|
|
if (Chain.LoopPredecessors == 0)
|
|
BlockWorkList.push_back(*Chain.begin());
|
|
}
|
|
|
|
buildChain(LoopTop, LoopChain, BlockWorkList, &LoopBlockSet);
|
|
rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
|
|
|
|
DEBUG({
|
|
// Crash at the end so we get all of the debugging output first.
|
|
bool BadLoop = false;
|
|
if (LoopChain.LoopPredecessors) {
|
|
BadLoop = true;
|
|
dbgs() << "Loop chain contains a block without its preds placed!\n"
|
|
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
|
|
<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
|
|
}
|
|
for (MachineBasicBlock *ChainBB : LoopChain) {
|
|
dbgs() << " ... " << getBlockName(ChainBB) << "\n";
|
|
if (!LoopBlockSet.erase(ChainBB)) {
|
|
// We don't mark the loop as bad here because there are real situations
|
|
// where this can occur. For example, with an unanalyzable fallthrough
|
|
// from a loop block to a non-loop block or vice versa.
|
|
dbgs() << "Loop chain contains a block not contained by the loop!\n"
|
|
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
|
|
<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
|
|
<< " Bad block: " << getBlockName(ChainBB) << "\n";
|
|
}
|
|
}
|
|
|
|
if (!LoopBlockSet.empty()) {
|
|
BadLoop = true;
|
|
for (MachineBasicBlock *LoopBB : LoopBlockSet)
|
|
dbgs() << "Loop contains blocks never placed into a chain!\n"
|
|
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
|
|
<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
|
|
<< " Bad block: " << getBlockName(LoopBB) << "\n";
|
|
}
|
|
assert(!BadLoop && "Detected problems with the placement of this loop.");
|
|
});
|
|
}
|
|
|
|
void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
|
|
// Ensure that every BB in the function has an associated chain to simplify
|
|
// the assumptions of the remaining algorithm.
|
|
SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
|
|
for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
|
|
MachineBasicBlock *BB = FI;
|
|
BlockChain *Chain =
|
|
new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
|
|
// Also, merge any blocks which we cannot reason about and must preserve
|
|
// the exact fallthrough behavior for.
|
|
for (;;) {
|
|
Cond.clear();
|
|
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
|
|
if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
|
|
break;
|
|
|
|
MachineFunction::iterator NextFI(std::next(FI));
|
|
MachineBasicBlock *NextBB = NextFI;
|
|
// Ensure that the layout successor is a viable block, as we know that
|
|
// fallthrough is a possibility.
|
|
assert(NextFI != FE && "Can't fallthrough past the last block.");
|
|
DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
|
|
<< getBlockName(BB) << " -> " << getBlockName(NextBB)
|
|
<< "\n");
|
|
Chain->merge(NextBB, nullptr);
|
|
FI = NextFI;
|
|
BB = NextBB;
|
|
}
|
|
}
|
|
|
|
if (OutlineOptionalBranches) {
|
|
// Find the nearest common dominator of all of F's terminators.
|
|
MachineBasicBlock *Terminator = nullptr;
|
|
for (MachineBasicBlock &MBB : F) {
|
|
if (MBB.succ_size() == 0) {
|
|
if (Terminator == nullptr)
|
|
Terminator = &MBB;
|
|
else
|
|
Terminator = MDT->findNearestCommonDominator(Terminator, &MBB);
|
|
}
|
|
}
|
|
|
|
// MBBs dominating this common dominator are unavoidable.
|
|
UnavoidableBlocks.clear();
|
|
for (MachineBasicBlock &MBB : F) {
|
|
if (MDT->dominates(&MBB, Terminator)) {
|
|
UnavoidableBlocks.insert(&MBB);
|
|
}
|
|
}
|
|
}
|
|
|
|
// Build any loop-based chains.
|
|
for (MachineLoop *L : *MLI)
|
|
buildLoopChains(F, *L);
|
|
|
|
SmallVector<MachineBasicBlock *, 16> BlockWorkList;
|
|
|
|
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
|
|
for (MachineBasicBlock &MBB : F) {
|
|
BlockChain &Chain = *BlockToChain[&MBB];
|
|
if (!UpdatedPreds.insert(&Chain).second)
|
|
continue;
|
|
|
|
assert(Chain.LoopPredecessors == 0);
|
|
for (MachineBasicBlock *ChainBB : Chain) {
|
|
assert(BlockToChain[ChainBB] == &Chain);
|
|
for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
|
|
if (BlockToChain[Pred] == &Chain)
|
|
continue;
|
|
++Chain.LoopPredecessors;
|
|
}
|
|
}
|
|
|
|
if (Chain.LoopPredecessors == 0)
|
|
BlockWorkList.push_back(*Chain.begin());
|
|
}
|
|
|
|
BlockChain &FunctionChain = *BlockToChain[&F.front()];
|
|
buildChain(&F.front(), FunctionChain, BlockWorkList);
|
|
|
|
#ifndef NDEBUG
|
|
typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
|
|
#endif
|
|
DEBUG({
|
|
// Crash at the end so we get all of the debugging output first.
|
|
bool BadFunc = false;
|
|
FunctionBlockSetType FunctionBlockSet;
|
|
for (MachineBasicBlock &MBB : F)
|
|
FunctionBlockSet.insert(&MBB);
|
|
|
|
for (MachineBasicBlock *ChainBB : FunctionChain)
|
|
if (!FunctionBlockSet.erase(ChainBB)) {
|
|
BadFunc = true;
|
|
dbgs() << "Function chain contains a block not in the function!\n"
|
|
<< " Bad block: " << getBlockName(ChainBB) << "\n";
|
|
}
|
|
|
|
if (!FunctionBlockSet.empty()) {
|
|
BadFunc = true;
|
|
for (MachineBasicBlock *RemainingBB : FunctionBlockSet)
|
|
dbgs() << "Function contains blocks never placed into a chain!\n"
|
|
<< " Bad block: " << getBlockName(RemainingBB) << "\n";
|
|
}
|
|
assert(!BadFunc && "Detected problems with the block placement.");
|
|
});
|
|
|
|
// Splice the blocks into place.
|
|
MachineFunction::iterator InsertPos = F.begin();
|
|
for (MachineBasicBlock *ChainBB : FunctionChain) {
|
|
DEBUG(dbgs() << (ChainBB == *FunctionChain.begin() ? "Placing chain "
|
|
: " ... ")
|
|
<< getBlockName(ChainBB) << "\n");
|
|
if (InsertPos != MachineFunction::iterator(ChainBB))
|
|
F.splice(InsertPos, ChainBB);
|
|
else
|
|
++InsertPos;
|
|
|
|
// Update the terminator of the previous block.
|
|
if (ChainBB == *FunctionChain.begin())
|
|
continue;
|
|
MachineBasicBlock *PrevBB = std::prev(MachineFunction::iterator(ChainBB));
|
|
|
|
// FIXME: It would be awesome of updateTerminator would just return rather
|
|
// than assert when the branch cannot be analyzed in order to remove this
|
|
// boiler plate.
|
|
Cond.clear();
|
|
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
|
|
if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
|
|
// The "PrevBB" is not yet updated to reflect current code layout, so,
|
|
// o. it may fall-through to a block without explict "goto" instruction
|
|
// before layout, and no longer fall-through it after layout; or
|
|
// o. just opposite.
|
|
//
|
|
// AnalyzeBranch() may return erroneous value for FBB when these two
|
|
// situations take place. For the first scenario FBB is mistakenly set
|
|
// NULL; for the 2nd scenario, the FBB, which is expected to be NULL,
|
|
// is mistakenly pointing to "*BI".
|
|
//
|
|
bool needUpdateBr = true;
|
|
if (!Cond.empty() && (!FBB || FBB == ChainBB)) {
|
|
PrevBB->updateTerminator();
|
|
needUpdateBr = false;
|
|
Cond.clear();
|
|
TBB = FBB = nullptr;
|
|
if (TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
|
|
// FIXME: This should never take place.
|
|
TBB = FBB = nullptr;
|
|
}
|
|
}
|
|
|
|
// If PrevBB has a two-way branch, try to re-order the branches
|
|
// such that we branch to the successor with higher weight first.
|
|
if (TBB && !Cond.empty() && FBB &&
|
|
MBPI->getEdgeWeight(PrevBB, FBB) > MBPI->getEdgeWeight(PrevBB, TBB) &&
|
|
!TII->ReverseBranchCondition(Cond)) {
|
|
DEBUG(dbgs() << "Reverse order of the two branches: "
|
|
<< getBlockName(PrevBB) << "\n");
|
|
DEBUG(dbgs() << " Edge weight: " << MBPI->getEdgeWeight(PrevBB, FBB)
|
|
<< " vs " << MBPI->getEdgeWeight(PrevBB, TBB) << "\n");
|
|
DebugLoc dl; // FIXME: this is nowhere
|
|
TII->RemoveBranch(*PrevBB);
|
|
TII->InsertBranch(*PrevBB, FBB, TBB, Cond, dl);
|
|
needUpdateBr = true;
|
|
}
|
|
if (needUpdateBr)
|
|
PrevBB->updateTerminator();
|
|
}
|
|
}
|
|
|
|
// Fixup the last block.
|
|
Cond.clear();
|
|
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
|
|
if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
|
|
F.back().updateTerminator();
|
|
|
|
// Walk through the backedges of the function now that we have fully laid out
|
|
// the basic blocks and align the destination of each backedge. We don't rely
|
|
// exclusively on the loop info here so that we can align backedges in
|
|
// unnatural CFGs and backedges that were introduced purely because of the
|
|
// loop rotations done during this layout pass.
|
|
if (F.getFunction()->hasFnAttribute(Attribute::OptimizeForSize))
|
|
return;
|
|
if (FunctionChain.begin() == FunctionChain.end())
|
|
return; // Empty chain.
|
|
|
|
const BranchProbability ColdProb(1, 5); // 20%
|
|
BlockFrequency EntryFreq = MBFI->getBlockFreq(F.begin());
|
|
BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
|
|
for (MachineBasicBlock *ChainBB : FunctionChain) {
|
|
if (ChainBB == *FunctionChain.begin())
|
|
continue;
|
|
|
|
// Don't align non-looping basic blocks. These are unlikely to execute
|
|
// enough times to matter in practice. Note that we'll still handle
|
|
// unnatural CFGs inside of a natural outer loop (the common case) and
|
|
// rotated loops.
|
|
MachineLoop *L = MLI->getLoopFor(ChainBB);
|
|
if (!L)
|
|
continue;
|
|
|
|
unsigned Align = TLI->getPrefLoopAlignment(L);
|
|
if (!Align)
|
|
continue; // Don't care about loop alignment.
|
|
|
|
// If the block is cold relative to the function entry don't waste space
|
|
// aligning it.
|
|
BlockFrequency Freq = MBFI->getBlockFreq(ChainBB);
|
|
if (Freq < WeightedEntryFreq)
|
|
continue;
|
|
|
|
// If the block is cold relative to its loop header, don't align it
|
|
// regardless of what edges into the block exist.
|
|
MachineBasicBlock *LoopHeader = L->getHeader();
|
|
BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(LoopHeader);
|
|
if (Freq < (LoopHeaderFreq * ColdProb))
|
|
continue;
|
|
|
|
// Check for the existence of a non-layout predecessor which would benefit
|
|
// from aligning this block.
|
|
MachineBasicBlock *LayoutPred =
|
|
&*std::prev(MachineFunction::iterator(ChainBB));
|
|
|
|
// Force alignment if all the predecessors are jumps. We already checked
|
|
// that the block isn't cold above.
|
|
if (!LayoutPred->isSuccessor(ChainBB)) {
|
|
ChainBB->setAlignment(Align);
|
|
continue;
|
|
}
|
|
|
|
// Align this block if the layout predecessor's edge into this block is
|
|
// cold relative to the block. When this is true, other predecessors make up
|
|
// all of the hot entries into the block and thus alignment is likely to be
|
|
// important.
|
|
BranchProbability LayoutProb =
|
|
MBPI->getEdgeProbability(LayoutPred, ChainBB);
|
|
BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
|
|
if (LayoutEdgeFreq <= (Freq * ColdProb))
|
|
ChainBB->setAlignment(Align);
|
|
}
|
|
}
|
|
|
|
bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
|
|
// Check for single-block functions and skip them.
|
|
if (std::next(F.begin()) == F.end())
|
|
return false;
|
|
|
|
if (skipOptnoneFunction(*F.getFunction()))
|
|
return false;
|
|
|
|
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
|
|
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
|
|
MLI = &getAnalysis<MachineLoopInfo>();
|
|
TII = F.getSubtarget().getInstrInfo();
|
|
TLI = F.getSubtarget().getTargetLowering();
|
|
MDT = &getAnalysis<MachineDominatorTree>();
|
|
assert(BlockToChain.empty());
|
|
|
|
buildCFGChains(F);
|
|
|
|
BlockToChain.clear();
|
|
ChainAllocator.DestroyAll();
|
|
|
|
if (AlignAllBlock)
|
|
// Align all of the blocks in the function to a specific alignment.
|
|
for (MachineBasicBlock &MBB : F)
|
|
MBB.setAlignment(AlignAllBlock);
|
|
|
|
// We always return true as we have no way to track whether the final order
|
|
// differs from the original order.
|
|
return true;
|
|
}
|
|
|
|
namespace {
|
|
/// \brief A pass to compute block placement statistics.
|
|
///
|
|
/// A separate pass to compute interesting statistics for evaluating block
|
|
/// placement. This is separate from the actual placement pass so that they can
|
|
/// be computed in the absence of any placement transformations or when using
|
|
/// alternative placement strategies.
|
|
class MachineBlockPlacementStats : public MachineFunctionPass {
|
|
/// \brief A handle to the branch probability pass.
|
|
const MachineBranchProbabilityInfo *MBPI;
|
|
|
|
/// \brief A handle to the function-wide block frequency pass.
|
|
const MachineBlockFrequencyInfo *MBFI;
|
|
|
|
public:
|
|
static char ID; // Pass identification, replacement for typeid
|
|
MachineBlockPlacementStats() : MachineFunctionPass(ID) {
|
|
initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
|
|
}
|
|
|
|
bool runOnMachineFunction(MachineFunction &F) override;
|
|
|
|
void getAnalysisUsage(AnalysisUsage &AU) const override {
|
|
AU.addRequired<MachineBranchProbabilityInfo>();
|
|
AU.addRequired<MachineBlockFrequencyInfo>();
|
|
AU.setPreservesAll();
|
|
MachineFunctionPass::getAnalysisUsage(AU);
|
|
}
|
|
};
|
|
}
|
|
|
|
char MachineBlockPlacementStats::ID = 0;
|
|
char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID;
|
|
INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
|
|
"Basic Block Placement Stats", false, false)
|
|
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
|
|
INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
|
|
INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
|
|
"Basic Block Placement Stats", false, false)
|
|
|
|
bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
|
|
// Check for single-block functions and skip them.
|
|
if (std::next(F.begin()) == F.end())
|
|
return false;
|
|
|
|
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
|
|
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
|
|
|
|
for (MachineBasicBlock &MBB : F) {
|
|
BlockFrequency BlockFreq = MBFI->getBlockFreq(&MBB);
|
|
Statistic &NumBranches =
|
|
(MBB.succ_size() > 1) ? NumCondBranches : NumUncondBranches;
|
|
Statistic &BranchTakenFreq =
|
|
(MBB.succ_size() > 1) ? CondBranchTakenFreq : UncondBranchTakenFreq;
|
|
for (MachineBasicBlock *Succ : MBB.successors()) {
|
|
// Skip if this successor is a fallthrough.
|
|
if (MBB.isLayoutSuccessor(Succ))
|
|
continue;
|
|
|
|
BlockFrequency EdgeFreq =
|
|
BlockFreq * MBPI->getEdgeProbability(&MBB, Succ);
|
|
++NumBranches;
|
|
BranchTakenFreq += EdgeFreq.getFrequency();
|
|
}
|
|
}
|
|
|
|
return false;
|
|
}
|
|
|