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993 lines
39 KiB
C++
993 lines
39 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 absense 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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#define DEBUG_TYPE "block-placement2"
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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/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/CodeGen/Passes.h"
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#include "llvm/Support/Allocator.h"
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#include "llvm/Support/Debug.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/Target/TargetInstrInfo.h"
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#include "llvm/Target/TargetLowering.h"
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#include <algorithm>
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using namespace llvm;
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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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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. We also can use a block chain to represent a sequence of
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/// basic blocks which have some external (correctness) requirement for
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/// sequential layout.
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///
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/// Eventually, the block chains will form a directed graph over the function.
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/// We provide an SCC-supporting-iterator in order to quicky build and walk the
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/// SCCs of block chains within a function.
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///
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/// The block chains also have support for calculating and caching probability
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/// information related to the chain itself versus other chains. This is used
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/// for ranking during the final layout of block chains.
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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 *>::const_iterator iterator;
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/// \brief Beginning of blocks within the chain.
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iterator begin() const { return Blocks.begin(); }
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/// \brief End of blocks within the chain.
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iterator end() const { 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 (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
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BI != BE; ++BI) {
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Blocks.push_back(*BI);
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assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
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BlockToChain[*BI] = this;
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}
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}
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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 TargetLowering *TLI;
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/// \brief Allocator and owner of BlockChain structures.
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///
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/// We build BlockChains lazily by merging together high probability BB
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/// sequences acording to the "Algo2" in the paper mentioned at the top of
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/// the file. To reduce malloc traffic, we allocate them using this slab-like
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/// allocator, and destroy them after the pass completes.
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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,
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MachineBasicBlock *LoopHeaderBB,
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SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
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const BlockFilterSet *BlockFilter = 0);
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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 *selectBestCandidateBlock(
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BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
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const BlockFilterSet *BlockFilter);
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MachineBasicBlock *getFirstUnplacedBlock(
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MachineFunction &F,
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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 = 0);
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MachineBasicBlock *findBestLoopTop(MachineFunction &F,
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MachineLoop &L,
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const BlockFilterSet &LoopBlockSet);
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void buildLoopChains(MachineFunction &F, MachineLoop &L);
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void buildCFGChains(MachineFunction &F);
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void AlignLoops(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);
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void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<MachineBranchProbabilityInfo>();
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AU.addRequired<MachineBlockFrequencyInfo>();
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AU.addRequired<MachineLoopInfo>();
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MachineFunctionPass::getAnalysisUsage(AU);
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}
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const char *getPassName() const { return "Block Placement"; }
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};
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}
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char MachineBlockPlacement::ID = 0;
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INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
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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(MachineLoopInfo)
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INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
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"Branch Probability Basic Block Placement", false, false)
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FunctionPass *llvm::createMachineBlockPlacementPass() {
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return new MachineBlockPlacement();
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}
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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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<< " (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,
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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 (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end();
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CBI != CBE; ++CBI) {
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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_iterator SI = (*CBI)->succ_begin(),
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SE = (*CBI)->succ_end();
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SI != SE; ++SI) {
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if (BlockFilter && !BlockFilter->count(*SI))
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continue;
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BlockChain &SuccChain = *BlockToChain[*SI];
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// Disregard edges within a fixed chain, or edges to the loop header.
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if (&Chain == &SuccChain || *SI == 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 *MachineBlockPlacement::selectBestSuccessor(
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MachineBasicBlock *BB, 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 = 0;
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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 effeciently 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_iterator SI = BB->succ_begin(),
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SE = BB->succ_end();
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SI != SE; ++SI) {
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if (BlockFilter && !BlockFilter->count(*SI))
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continue;
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BlockChain &SuccChain = *BlockToChain[*SI];
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if (&SuccChain == &Chain) {
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DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
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continue;
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}
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if (*SI != *SuccChain.begin()) {
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DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n");
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continue;
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}
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uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
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BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
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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(*SI) << " -> 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 important
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// 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_iterator PI = (*SI)->pred_begin(),
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PE = (*SI)->pred_end();
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PI != PE; ++PI) {
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if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
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BlockToChain[*PI] == &Chain)
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continue;
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BlockFrequency PredEdgeFreq
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= MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
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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(*SI)
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<< " -> 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(*SI) << " -> " << 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 = *SI;
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BestWeight = SuccWeight;
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}
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return BestSucc;
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}
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namespace {
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/// \brief Predicate struct to detect blocks already placed.
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class IsBlockPlaced {
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const BlockChain &PlacedChain;
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const BlockToChainMapType &BlockToChain;
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public:
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IsBlockPlaced(const BlockChain &PlacedChain,
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const BlockToChainMapType &BlockToChain)
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: PlacedChain(PlacedChain), BlockToChain(BlockToChain) {}
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bool operator()(MachineBasicBlock *BB) const {
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return BlockToChain.lookup(BB) == &PlacedChain;
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}
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};
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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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IsBlockPlaced(Chain, BlockToChain)),
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WorkList.end());
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MachineBasicBlock *BestBlock = 0;
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BlockFrequency BestFreq;
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for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
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WBE = WorkList.end();
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WBI != WBE; ++WBI) {
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assert(!BlockFilter || BlockFilter->count(*WBI));
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BlockChain &SuccChain = *BlockToChain[*WBI];
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if (&SuccChain == &Chain) {
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DEBUG(dbgs() << " " << getBlockName(*WBI)
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<< " -> 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(*WBI);
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DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> " << CandidateFreq
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<< " (freq)\n");
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if (BestBlock && BestFreq >= CandidateFreq)
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continue;
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BestBlock = *WBI;
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BestFreq = CandidateFreq;
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}
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return BestBlock;
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}
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/// \brief Retrieve the first unplaced basic block.
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///
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/// This routine is called when we are unable to use the CFG to walk through
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/// all of the basic blocks and form a chain due to unnatural loops in the CFG.
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/// We walk through the function's blocks in order, starting from the
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/// LastUnplacedBlockIt. We update this iterator on each call to avoid
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/// re-scanning the entire sequence on repeated calls to this routine.
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MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
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MachineFunction &F, const BlockChain &PlacedChain,
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MachineFunction::iterator &PrevUnplacedBlockIt,
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const BlockFilterSet *BlockFilter) {
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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 0;
|
|
}
|
|
|
|
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 = *llvm::prior(Chain.end());
|
|
for (;;) {
|
|
assert(BB);
|
|
assert(BlockToChain[BB] == &Chain);
|
|
assert(*llvm::prior(Chain.end()) == BB);
|
|
MachineBasicBlock *BestSucc = 0;
|
|
|
|
// Look for the best viable successor if there is one to place immediately
|
|
// after this block.
|
|
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 = *llvm::prior(Chain.end());
|
|
}
|
|
|
|
DEBUG(dbgs() << "Finished forming chain for header block "
|
|
<< getBlockNum(*Chain.begin()) << "\n");
|
|
}
|
|
|
|
/// \brief Find the best loop top 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::findBestLoopTop(MachineFunction &F,
|
|
MachineLoop &L,
|
|
const BlockFilterSet &LoopBlockSet) {
|
|
BlockFrequency BestExitEdgeFreq;
|
|
MachineBasicBlock *ExitingBB = 0;
|
|
MachineBasicBlock *LoopingBB = 0;
|
|
// 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 (MachineLoop::block_iterator I = L.block_begin(),
|
|
E = L.block_end();
|
|
I != E; ++I) {
|
|
BlockChain &Chain = *BlockToChain[*I];
|
|
// 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 (*I != *llvm::prior(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;
|
|
// We also compute and store the best looping successor for use in layout.
|
|
MachineBasicBlock *BestLoopSucc = 0;
|
|
// FIXME: Due to the performance of the probability and weight routines in
|
|
// the MBPI analysis, we use the internal weights. This is only valid
|
|
// because it is purely a ranking function, we don't care about anything
|
|
// but the relative values.
|
|
uint32_t BestLoopSuccWeight = 0;
|
|
// FIXME: We also manually compute the probabilities to avoid quadratic
|
|
// behavior.
|
|
uint32_t WeightScale = 0;
|
|
uint32_t SumWeight = MBPI->getSumForBlock(*I, WeightScale);
|
|
for (MachineBasicBlock::succ_iterator SI = (*I)->succ_begin(),
|
|
SE = (*I)->succ_end();
|
|
SI != SE; ++SI) {
|
|
if ((*SI)->isLandingPad())
|
|
continue;
|
|
if (*SI == *I)
|
|
continue;
|
|
BlockChain &SuccChain = *BlockToChain[*SI];
|
|
// Don't split chains, either this chain or the successor's chain.
|
|
if (&Chain == &SuccChain || *SI != *SuccChain.begin()) {
|
|
DEBUG(dbgs() << " " << (LoopBlockSet.count(*SI) ? "looping: "
|
|
: "exiting: ")
|
|
<< getBlockName(*I) << " -> "
|
|
<< getBlockName(*SI) << " (chain conflict)\n");
|
|
continue;
|
|
}
|
|
|
|
uint32_t SuccWeight = MBPI->getEdgeWeight(*I, *SI);
|
|
if (LoopBlockSet.count(*SI)) {
|
|
DEBUG(dbgs() << " looping: " << getBlockName(*I) << " -> "
|
|
<< getBlockName(*SI) << " (" << SuccWeight << ")\n");
|
|
if (BestLoopSucc && BestLoopSuccWeight >= SuccWeight)
|
|
continue;
|
|
|
|
BestLoopSucc = *SI;
|
|
BestLoopSuccWeight = SuccWeight;
|
|
continue;
|
|
}
|
|
|
|
BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
|
|
BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(*I) * SuccProb;
|
|
DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
|
|
<< getBlockName(*SI) << " (" << ExitEdgeFreq << ")\n");
|
|
// Note that we slightly bias this toward an existing layout successor to
|
|
// retain incoming order in the absence of better information.
|
|
// FIXME: Should we bias this more strongly? It's pretty weak.
|
|
if (!ExitingBB || ExitEdgeFreq > BestExitEdgeFreq ||
|
|
((*I)->isLayoutSuccessor(*SI) &&
|
|
!(ExitEdgeFreq < BestExitEdgeFreq))) {
|
|
BestExitEdgeFreq = ExitEdgeFreq;
|
|
ExitingBB = *I;
|
|
}
|
|
|
|
if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI))
|
|
if (ExitLoop->contains(&L))
|
|
BlocksExitingToOuterLoop.insert(*I);
|
|
}
|
|
|
|
// Restore the old exiting state, no viable looping successor was found.
|
|
if (!BestLoopSucc) {
|
|
ExitingBB = OldExitingBB;
|
|
BestExitEdgeFreq = OldBestExitEdgeFreq;
|
|
continue;
|
|
}
|
|
|
|
// If this was best exiting block thus far, also record the looping block.
|
|
if (ExitingBB == *I)
|
|
LoopingBB = BestLoopSucc;
|
|
}
|
|
// Without a candidate exitting 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 L.getHeader();
|
|
|
|
// 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 L.getHeader();
|
|
|
|
assert(LoopingBB && "All successors of a loop block are exit blocks!");
|
|
DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
|
|
DEBUG(dbgs() << " Best top block: " << getBlockName(LoopingBB) << "\n");
|
|
return LoopingBB;
|
|
}
|
|
|
|
/// \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::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
|
|
buildLoopChains(F, **LI);
|
|
|
|
SmallVector<MachineBasicBlock *, 16> BlockWorkList;
|
|
BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
|
|
|
|
MachineBasicBlock *LayoutTop = findBestLoopTop(F, L, LoopBlockSet);
|
|
BlockChain &LoopChain = *BlockToChain[LayoutTop];
|
|
|
|
// 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 (MachineLoop::block_iterator BI = L.block_begin(),
|
|
BE = L.block_end();
|
|
BI != BE; ++BI) {
|
|
BlockChain &Chain = *BlockToChain[*BI];
|
|
if (!UpdatedPreds.insert(&Chain))
|
|
continue;
|
|
|
|
assert(Chain.LoopPredecessors == 0);
|
|
for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
|
|
BCI != BCE; ++BCI) {
|
|
assert(BlockToChain[*BCI] == &Chain);
|
|
for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
|
|
PE = (*BCI)->pred_end();
|
|
PI != PE; ++PI) {
|
|
if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
|
|
continue;
|
|
++Chain.LoopPredecessors;
|
|
}
|
|
}
|
|
|
|
if (Chain.LoopPredecessors == 0)
|
|
BlockWorkList.push_back(*Chain.begin());
|
|
}
|
|
|
|
buildChain(LayoutTop, LoopChain, BlockWorkList, &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 (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
|
|
BCI != BCE; ++BCI)
|
|
if (!LoopBlockSet.erase(*BCI)) {
|
|
// 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(*BCI) << "\n";
|
|
}
|
|
|
|
if (!LoopBlockSet.empty()) {
|
|
BadLoop = true;
|
|
for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
|
|
LBE = LoopBlockSet.end();
|
|
LBI != LBE; ++LBI)
|
|
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(*LBI) << "\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 = 0, *FBB = 0; // For AnalyzeBranch.
|
|
if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
|
|
break;
|
|
|
|
MachineFunction::iterator NextFI(llvm::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, 0);
|
|
FI = NextFI;
|
|
BB = NextBB;
|
|
}
|
|
}
|
|
|
|
// Build any loop-based chains.
|
|
for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
|
|
++LI)
|
|
buildLoopChains(F, **LI);
|
|
|
|
SmallVector<MachineBasicBlock *, 16> BlockWorkList;
|
|
|
|
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
|
|
for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
|
|
MachineBasicBlock *BB = &*FI;
|
|
BlockChain &Chain = *BlockToChain[BB];
|
|
if (!UpdatedPreds.insert(&Chain))
|
|
continue;
|
|
|
|
assert(Chain.LoopPredecessors == 0);
|
|
for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
|
|
BCI != BCE; ++BCI) {
|
|
assert(BlockToChain[*BCI] == &Chain);
|
|
for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
|
|
PE = (*BCI)->pred_end();
|
|
PI != PE; ++PI) {
|
|
if (BlockToChain[*PI] == &Chain)
|
|
continue;
|
|
++Chain.LoopPredecessors;
|
|
}
|
|
}
|
|
|
|
if (Chain.LoopPredecessors == 0)
|
|
BlockWorkList.push_back(*Chain.begin());
|
|
}
|
|
|
|
BlockChain &FunctionChain = *BlockToChain[&F.front()];
|
|
buildChain(&F.front(), FunctionChain, BlockWorkList);
|
|
|
|
typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
|
|
DEBUG({
|
|
// Crash at the end so we get all of the debugging output first.
|
|
bool BadFunc = false;
|
|
FunctionBlockSetType FunctionBlockSet;
|
|
for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
|
|
FunctionBlockSet.insert(FI);
|
|
|
|
for (BlockChain::iterator BCI = FunctionChain.begin(),
|
|
BCE = FunctionChain.end();
|
|
BCI != BCE; ++BCI)
|
|
if (!FunctionBlockSet.erase(*BCI)) {
|
|
BadFunc = true;
|
|
dbgs() << "Function chain contains a block not in the function!\n"
|
|
<< " Bad block: " << getBlockName(*BCI) << "\n";
|
|
}
|
|
|
|
if (!FunctionBlockSet.empty()) {
|
|
BadFunc = true;
|
|
for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
|
|
FBE = FunctionBlockSet.end();
|
|
FBI != FBE; ++FBI)
|
|
dbgs() << "Function contains blocks never placed into a chain!\n"
|
|
<< " Bad block: " << getBlockName(*FBI) << "\n";
|
|
}
|
|
assert(!BadFunc && "Detected problems with the block placement.");
|
|
});
|
|
|
|
// Splice the blocks into place.
|
|
MachineFunction::iterator InsertPos = F.begin();
|
|
for (BlockChain::iterator BI = FunctionChain.begin(),
|
|
BE = FunctionChain.end();
|
|
BI != BE; ++BI) {
|
|
DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
|
|
: " ... ")
|
|
<< getBlockName(*BI) << "\n");
|
|
if (InsertPos != MachineFunction::iterator(*BI))
|
|
F.splice(InsertPos, *BI);
|
|
else
|
|
++InsertPos;
|
|
|
|
// Update the terminator of the previous block.
|
|
if (BI == FunctionChain.begin())
|
|
continue;
|
|
MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI));
|
|
|
|
// 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 = 0, *FBB = 0; // For AnalyzeBranch.
|
|
if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond))
|
|
PrevBB->updateTerminator();
|
|
}
|
|
|
|
// Fixup the last block.
|
|
Cond.clear();
|
|
MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
|
|
if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
|
|
F.back().updateTerminator();
|
|
}
|
|
|
|
/// \brief Recursive helper to align a loop and any nested loops.
|
|
static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
|
|
// Recurse through nested loops.
|
|
for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
|
|
AlignLoop(F, *I, Align);
|
|
|
|
L->getTopBlock()->setAlignment(Align);
|
|
}
|
|
|
|
/// \brief Align loop headers to target preferred alignments.
|
|
void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
|
|
if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
|
|
return;
|
|
|
|
unsigned Align = TLI->getPrefLoopAlignment();
|
|
if (!Align)
|
|
return; // Don't care about loop alignment.
|
|
|
|
for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
|
|
AlignLoop(F, *I, Align);
|
|
}
|
|
|
|
bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
|
|
// Check for single-block functions and skip them.
|
|
if (llvm::next(F.begin()) == F.end())
|
|
return false;
|
|
|
|
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
|
|
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
|
|
MLI = &getAnalysis<MachineLoopInfo>();
|
|
TII = F.getTarget().getInstrInfo();
|
|
TLI = F.getTarget().getTargetLowering();
|
|
assert(BlockToChain.empty());
|
|
|
|
buildCFGChains(F);
|
|
AlignLoops(F);
|
|
|
|
BlockToChain.clear();
|
|
ChainAllocator.DestroyAll();
|
|
|
|
// 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 absense 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);
|
|
|
|
void getAnalysisUsage(AnalysisUsage &AU) const {
|
|
AU.addRequired<MachineBranchProbabilityInfo>();
|
|
AU.addRequired<MachineBlockFrequencyInfo>();
|
|
AU.setPreservesAll();
|
|
MachineFunctionPass::getAnalysisUsage(AU);
|
|
}
|
|
|
|
const char *getPassName() const { return "Block Placement Stats"; }
|
|
};
|
|
}
|
|
|
|
char MachineBlockPlacementStats::ID = 0;
|
|
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)
|
|
|
|
FunctionPass *llvm::createMachineBlockPlacementStatsPass() {
|
|
return new MachineBlockPlacementStats();
|
|
}
|
|
|
|
bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
|
|
// Check for single-block functions and skip them.
|
|
if (llvm::next(F.begin()) == F.end())
|
|
return false;
|
|
|
|
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
|
|
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
|
|
|
|
for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
|
|
BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
|
|
Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
|
|
: NumUncondBranches;
|
|
Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
|
|
: UncondBranchTakenFreq;
|
|
for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
|
|
SE = I->succ_end();
|
|
SI != SE; ++SI) {
|
|
// Skip if this successor is a fallthrough.
|
|
if (I->isLayoutSuccessor(*SI))
|
|
continue;
|
|
|
|
BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
|
|
++NumBranches;
|
|
BranchTakenFreq += EdgeFreq.getFrequency();
|
|
}
|
|
}
|
|
|
|
return false;
|
|
}
|
|
|