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the mailing list. Suggestions for other statistics to collect would be awesome. =] Currently these are implemented as a separate pass guarded by a separate flag. I'm not thrilled by that, but I wanted to be able to collect the statistics for the old code placement as well as the new in order to have a point of comparison. I'm planning on folding them into the single pass if / when there is only one pass of interest. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143537 91177308-0d34-0410-b5e6-96231b3b80d8
567 lines
22 KiB
C++
567 lines
22 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/Support/ErrorHandling.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/PostOrderIterator.h"
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#include "llvm/ADT/SCCIterator.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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/// \brief A structure for storing a weighted edge.
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///
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/// This stores an edge and its weight, computed as the product of the
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/// frequency that the starting block is entered with the probability of
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/// a particular exit block.
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struct WeightedEdge {
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BlockFrequency EdgeFrequency;
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MachineBasicBlock *From, *To;
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bool operator<(const WeightedEdge &RHS) const {
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return EdgeFrequency < RHS.EdgeFrequency;
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}
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};
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}
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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) {
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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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assert(Blocks.back()->isSuccessor(BB));
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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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};
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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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BlockChain *CreateChain(MachineBasicBlock *BB);
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void mergeSuccessor(MachineBasicBlock *BB, BlockChain *Chain,
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BlockFilterSet *Filter = 0);
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void buildLoopChains(MachineFunction &F, MachineLoop &L);
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void buildCFGChains(MachineFunction &F);
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void placeChainsTopologically(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 Helper to create a new chain for a single BB.
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///
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/// Takes care of growing the Chains, setting up the BlockChain object, and any
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/// debug checking logic.
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/// \returns A pointer to the new BlockChain.
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BlockChain *MachineBlockPlacement::CreateChain(MachineBasicBlock *BB) {
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BlockChain *Chain =
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new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
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return Chain;
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}
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/// \brief Merge a chain with any viable successor.
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///
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/// This routine walks the predecessors of the current block, looking for
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/// viable merge candidates. It has strict rules it uses to determine when
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/// a predecessor can be merged with the current block, which center around
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/// preserving the CFG structure. It performs the merge if any viable candidate
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/// is found.
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void MachineBlockPlacement::mergeSuccessor(MachineBasicBlock *BB,
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BlockChain *Chain,
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BlockFilterSet *Filter) {
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assert(BB);
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assert(Chain);
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// If this block is not at the end of its chain, it cannot merge with any
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// other chain.
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if (Chain && *llvm::prior(Chain->end()) != BB)
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return;
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// Walk through the successors looking for the highest probability edge.
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MachineBasicBlock *Successor = 0;
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BranchProbability BestProb = BranchProbability::getZero();
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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 (BB == *SI || (Filter && !Filter->count(*SI)))
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continue;
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BranchProbability SuccProb = MBPI->getEdgeProbability(BB, *SI);
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DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb << "\n");
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if (!Successor || SuccProb > BestProb || (!(SuccProb < BestProb) &&
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BB->isLayoutSuccessor(*SI))) {
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Successor = *SI;
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BestProb = SuccProb;
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}
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}
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if (!Successor)
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return;
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// Grab a chain if it exists already for this successor and make sure the
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// successor is at the start of the chain as we can't merge mid-chain. Also,
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// if the successor chain is the same as our chain, we're already merged.
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BlockChain *SuccChain = BlockToChain[Successor];
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if (SuccChain && (SuccChain == Chain || Successor != *SuccChain->begin()))
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return;
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// We only merge chains across a CFG merge when the desired merge path is
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// significantly hotter than the incoming edge. We define a hot edge more
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// strictly than the BranchProbabilityInfo does, as the two predecessor
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// blocks may have dramatically different incoming probabilities we need to
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// account for. Therefor we use the "global" edge weight which is the
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// branch's probability times the block frequency of the predecessor.
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BlockFrequency MergeWeight = MBFI->getBlockFreq(BB);
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MergeWeight *= MBPI->getEdgeProbability(BB, Successor);
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// We only want to consider breaking the CFG when the merge weight is much
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// higher (80% vs. 20%), so multiply it by 1/4. This will require the merged
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// edge to be 4x more likely before we disrupt the CFG. This number matches
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// the definition of "hot" in BranchProbabilityAnalysis (80% vs. 20%).
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MergeWeight *= BranchProbability(1, 4);
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for (MachineBasicBlock::pred_iterator PI = Successor->pred_begin(),
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PE = Successor->pred_end();
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PI != PE; ++PI) {
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if (BB == *PI || Successor == *PI) continue;
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BlockFrequency PredWeight = MBFI->getBlockFreq(*PI);
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PredWeight *= MBPI->getEdgeProbability(*PI, Successor);
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// Return on the first predecessor we find which outstrips our merge weight.
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if (MergeWeight < PredWeight)
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return;
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DEBUG(dbgs() << "Breaking CFG edge!\n"
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<< " Edge from " << getBlockNum(BB) << " to "
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<< getBlockNum(Successor) << ": " << MergeWeight << "\n"
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<< " vs. " << getBlockNum(BB) << " to "
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<< getBlockNum(*PI) << ": " << PredWeight << "\n");
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}
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DEBUG(dbgs() << "Merging from " << getBlockNum(BB) << " to "
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<< getBlockNum(Successor) << "\n");
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Chain->merge(Successor, SuccChain);
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}
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/// \brief Forms basic block chains from the natural loop structures.
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///
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/// These chains are designed to preserve the existing *structure* of the code
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/// as much as possible. We can then stitch the chains together in a way which
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/// both preserves the topological structure and minimizes taken conditional
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/// branches.
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void MachineBlockPlacement::buildLoopChains(MachineFunction &F, MachineLoop &L) {
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// First recurse through any nested loops, building chains for those inner
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// loops.
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for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
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buildLoopChains(F, **LI);
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SmallPtrSet<MachineBasicBlock *, 16> LoopBlockSet(L.block_begin(),
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L.block_end());
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// Begin building up a set of chains of blocks within this loop which should
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// remain contiguous. Some of the blocks already belong to a chain which
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// represents an inner loop.
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for (MachineLoop::block_iterator BI = L.block_begin(), BE = L.block_end();
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BI != BE; ++BI) {
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MachineBasicBlock *BB = *BI;
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BlockChain *Chain = BlockToChain[BB];
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if (!Chain) Chain = CreateChain(BB);
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mergeSuccessor(BB, Chain, &LoopBlockSet);
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}
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}
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void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
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// First build any loop-based chains.
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for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
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++LI)
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buildLoopChains(F, **LI);
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// Now walk the blocks of the function forming chains where they don't
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// violate any CFG structure.
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for (MachineFunction::iterator BI = F.begin(), BE = F.end();
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BI != BE; ++BI) {
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MachineBasicBlock *BB = BI;
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BlockChain *Chain = BlockToChain[BB];
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if (!Chain) Chain = CreateChain(BB);
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mergeSuccessor(BB, Chain);
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}
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}
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void MachineBlockPlacement::placeChainsTopologically(MachineFunction &F) {
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MachineBasicBlock *EntryB = &F.front();
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assert(BlockToChain[EntryB] && "Missing chain for entry block");
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assert(*BlockToChain[EntryB]->begin() == EntryB &&
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"Entry block is not the head of the entry block chain");
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// Walk the blocks in RPO, and insert each block for a chain in order the
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// first time we see that chain.
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MachineFunction::iterator InsertPos = F.begin();
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SmallPtrSet<BlockChain *, 16> VisitedChains;
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ReversePostOrderTraversal<MachineBasicBlock *> RPOT(EntryB);
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typedef ReversePostOrderTraversal<MachineBasicBlock *>::rpo_iterator
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rpo_iterator;
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for (rpo_iterator I = RPOT.begin(), E = RPOT.end(); I != E; ++I) {
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BlockChain *Chain = BlockToChain[*I];
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assert(Chain);
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if(!VisitedChains.insert(Chain))
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continue;
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for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end(); BI != BE;
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++BI) {
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DEBUG(dbgs() << (BI == Chain->begin() ? "Placing chain "
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: " ... ")
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<< getBlockName(*BI) << "\n");
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if (InsertPos != MachineFunction::iterator(*BI))
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F.splice(InsertPos, *BI);
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else
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++InsertPos;
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}
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}
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// Now that every block is in its final position, update all of the
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// terminators.
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SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
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for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
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// FIXME: It would be awesome of updateTerminator would just return rather
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// than assert when the branch cannot be analyzed in order to remove this
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// boiler plate.
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Cond.clear();
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MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
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if (!TII->AnalyzeBranch(*FI, TBB, FBB, Cond))
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FI->updateTerminator();
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}
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}
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/// \brief Recursive helper to align a loop and any nested loops.
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static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
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// Recurse through nested loops.
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for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
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AlignLoop(F, *I, Align);
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L->getTopBlock()->setAlignment(Align);
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}
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/// \brief Align loop headers to target preferred alignments.
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void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
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if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
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return;
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unsigned Align = TLI->getPrefLoopAlignment();
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if (!Align)
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return; // Don't care about loop alignment.
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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);
|
|
placeChainsTopologically(F);
|
|
AlignLoops(F);
|
|
|
|
BlockToChain.clear();
|
|
|
|
// 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;
|
|
}
|
|
|