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probability wouldn't be considered "hot" in some weird loop structures or other compounding probability patterns. This makes it much harder to confuse, but isn't really a principled fix. I'd actually like it if we could model a zero probability, as it would make this much easier to reason about. Suggestions for how to do this better are welcome. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147142 91177308-0d34-0410-b5e6-96231b3b80d8
511 lines
16 KiB
C++
511 lines
16 KiB
C++
//===-- BranchProbabilityInfo.cpp - Branch Probability Analysis -*- C++ -*-===//
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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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// Loops should be simplified before this analysis.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Constants.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/LLVMContext.h"
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#include "llvm/Metadata.h"
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#include "llvm/Analysis/BranchProbabilityInfo.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/ADT/PostOrderIterator.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/Debug.h"
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using namespace llvm;
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INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
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"Branch Probability Analysis", false, true)
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INITIALIZE_PASS_DEPENDENCY(LoopInfo)
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INITIALIZE_PASS_END(BranchProbabilityInfo, "branch-prob",
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"Branch Probability Analysis", false, true)
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char BranchProbabilityInfo::ID = 0;
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// Weights are for internal use only. They are used by heuristics to help to
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// estimate edges' probability. Example:
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//
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// Using "Loop Branch Heuristics" we predict weights of edges for the
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// block BB2.
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// ...
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// |
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// V
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// BB1<-+
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// | |
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// | | (Weight = 124)
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// V |
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// BB2--+
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// |
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// | (Weight = 4)
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// V
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// BB3
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//
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// Probability of the edge BB2->BB1 = 124 / (124 + 4) = 0.96875
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// Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125
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static const uint32_t LBH_TAKEN_WEIGHT = 124;
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static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
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/// \brief Unreachable-terminating branch taken weight.
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///
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/// This is the weight for a branch being taken to a block that terminates
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/// (eventually) in unreachable. These are predicted as unlikely as possible.
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static const uint32_t UR_TAKEN_WEIGHT = 1;
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/// \brief Unreachable-terminating branch not-taken weight.
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///
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/// This is the weight for a branch not being taken toward a block that
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/// terminates (eventually) in unreachable. Such a branch is essentially never
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/// taken. Set the weight to an absurdly high value so that nested loops don't
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/// easily subsume it.
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static const uint32_t UR_NONTAKEN_WEIGHT = 1024*1024 - 1;
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static const uint32_t PH_TAKEN_WEIGHT = 20;
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static const uint32_t PH_NONTAKEN_WEIGHT = 12;
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static const uint32_t ZH_TAKEN_WEIGHT = 20;
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static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
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static const uint32_t FPH_TAKEN_WEIGHT = 20;
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static const uint32_t FPH_NONTAKEN_WEIGHT = 12;
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// Standard weight value. Used when none of the heuristics set weight for
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// the edge.
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static const uint32_t NORMAL_WEIGHT = 16;
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// Minimum weight of an edge. Please note, that weight is NEVER 0.
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static const uint32_t MIN_WEIGHT = 1;
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static uint32_t getMaxWeightFor(BasicBlock *BB) {
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return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
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}
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/// \brief Calculate edge weights for successors lead to unreachable.
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///
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/// Predict that a successor which leads necessarily to an
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/// unreachable-terminated block as extremely unlikely.
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bool BranchProbabilityInfo::calcUnreachableHeuristics(BasicBlock *BB) {
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TerminatorInst *TI = BB->getTerminator();
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if (TI->getNumSuccessors() == 0) {
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if (isa<UnreachableInst>(TI))
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PostDominatedByUnreachable.insert(BB);
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return false;
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}
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SmallPtrSet<BasicBlock *, 4> UnreachableEdges;
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SmallPtrSet<BasicBlock *, 4> ReachableEdges;
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for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
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if (PostDominatedByUnreachable.count(*I))
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UnreachableEdges.insert(*I);
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else
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ReachableEdges.insert(*I);
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}
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// If all successors are in the set of blocks post-dominated by unreachable,
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// this block is too.
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if (UnreachableEdges.size() == TI->getNumSuccessors())
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PostDominatedByUnreachable.insert(BB);
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// Skip probabilities if this block has a single successor or if all were
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// reachable.
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if (TI->getNumSuccessors() == 1 || UnreachableEdges.empty())
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return false;
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uint32_t UnreachableWeight =
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std::max(UR_TAKEN_WEIGHT / UnreachableEdges.size(), MIN_WEIGHT);
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for (SmallPtrSet<BasicBlock *, 4>::iterator I = UnreachableEdges.begin(),
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E = UnreachableEdges.end();
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I != E; ++I)
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setEdgeWeight(BB, *I, UnreachableWeight);
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if (ReachableEdges.empty())
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return true;
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uint32_t ReachableWeight =
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std::max(UR_NONTAKEN_WEIGHT / ReachableEdges.size(), NORMAL_WEIGHT);
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for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReachableEdges.begin(),
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E = ReachableEdges.end();
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I != E; ++I)
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setEdgeWeight(BB, *I, ReachableWeight);
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return true;
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}
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// Propagate existing explicit probabilities from either profile data or
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// 'expect' intrinsic processing.
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bool BranchProbabilityInfo::calcMetadataWeights(BasicBlock *BB) {
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TerminatorInst *TI = BB->getTerminator();
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if (TI->getNumSuccessors() == 1)
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return false;
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if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
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return false;
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MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
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if (!WeightsNode)
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return false;
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// Ensure there are weights for all of the successors. Note that the first
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// operand to the metadata node is a name, not a weight.
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if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
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return false;
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// Build up the final weights that will be used in a temporary buffer, but
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// don't add them until all weihts are present. Each weight value is clamped
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// to [1, getMaxWeightFor(BB)].
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uint32_t WeightLimit = getMaxWeightFor(BB);
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SmallVector<uint32_t, 2> Weights;
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Weights.reserve(TI->getNumSuccessors());
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for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
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ConstantInt *Weight = dyn_cast<ConstantInt>(WeightsNode->getOperand(i));
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if (!Weight)
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return false;
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Weights.push_back(
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std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
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}
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assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
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for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
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setEdgeWeight(BB, TI->getSuccessor(i), Weights[i]);
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return true;
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}
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// Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
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// between two pointer or pointer and NULL will fail.
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bool BranchProbabilityInfo::calcPointerHeuristics(BasicBlock *BB) {
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BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
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if (!BI || !BI->isConditional())
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return false;
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Value *Cond = BI->getCondition();
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ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
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if (!CI || !CI->isEquality())
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return false;
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Value *LHS = CI->getOperand(0);
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if (!LHS->getType()->isPointerTy())
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return false;
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assert(CI->getOperand(1)->getType()->isPointerTy());
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BasicBlock *Taken = BI->getSuccessor(0);
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BasicBlock *NonTaken = BI->getSuccessor(1);
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// p != 0 -> isProb = true
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// p == 0 -> isProb = false
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// p != q -> isProb = true
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// p == q -> isProb = false;
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bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
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if (!isProb)
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std::swap(Taken, NonTaken);
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setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
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setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
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return true;
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}
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// Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
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// as taken, exiting edges as not-taken.
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bool BranchProbabilityInfo::calcLoopBranchHeuristics(BasicBlock *BB) {
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Loop *L = LI->getLoopFor(BB);
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if (!L)
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return false;
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SmallPtrSet<BasicBlock *, 8> BackEdges;
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SmallPtrSet<BasicBlock *, 8> ExitingEdges;
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SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
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for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
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if (!L->contains(*I))
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ExitingEdges.insert(*I);
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else if (L->getHeader() == *I)
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BackEdges.insert(*I);
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else
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InEdges.insert(*I);
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}
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if (uint32_t numBackEdges = BackEdges.size()) {
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uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
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if (backWeight < NORMAL_WEIGHT)
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backWeight = NORMAL_WEIGHT;
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for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
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EE = BackEdges.end(); EI != EE; ++EI) {
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BasicBlock *Back = *EI;
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setEdgeWeight(BB, Back, backWeight);
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}
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}
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if (uint32_t numInEdges = InEdges.size()) {
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uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
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if (inWeight < NORMAL_WEIGHT)
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inWeight = NORMAL_WEIGHT;
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for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
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EE = InEdges.end(); EI != EE; ++EI) {
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BasicBlock *Back = *EI;
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setEdgeWeight(BB, Back, inWeight);
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}
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}
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if (uint32_t numExitingEdges = ExitingEdges.size()) {
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uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numExitingEdges;
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if (exitWeight < MIN_WEIGHT)
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exitWeight = MIN_WEIGHT;
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for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
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EE = ExitingEdges.end(); EI != EE; ++EI) {
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BasicBlock *Exiting = *EI;
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setEdgeWeight(BB, Exiting, exitWeight);
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}
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}
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return true;
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}
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bool BranchProbabilityInfo::calcZeroHeuristics(BasicBlock *BB) {
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BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
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if (!BI || !BI->isConditional())
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return false;
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Value *Cond = BI->getCondition();
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ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
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if (!CI)
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return false;
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Value *RHS = CI->getOperand(1);
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ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
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if (!CV)
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return false;
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bool isProb;
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if (CV->isZero()) {
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switch (CI->getPredicate()) {
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case CmpInst::ICMP_EQ:
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// X == 0 -> Unlikely
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isProb = false;
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break;
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case CmpInst::ICMP_NE:
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// X != 0 -> Likely
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isProb = true;
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break;
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case CmpInst::ICMP_SLT:
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// X < 0 -> Unlikely
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isProb = false;
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break;
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case CmpInst::ICMP_SGT:
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// X > 0 -> Likely
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isProb = true;
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break;
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default:
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return false;
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}
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} else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
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// InstCombine canonicalizes X <= 0 into X < 1.
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// X <= 0 -> Unlikely
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isProb = false;
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} else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
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// InstCombine canonicalizes X >= 0 into X > -1.
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// X >= 0 -> Likely
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isProb = true;
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} else {
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return false;
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}
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BasicBlock *Taken = BI->getSuccessor(0);
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BasicBlock *NonTaken = BI->getSuccessor(1);
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if (!isProb)
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std::swap(Taken, NonTaken);
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setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
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setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
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return true;
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}
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bool BranchProbabilityInfo::calcFloatingPointHeuristics(BasicBlock *BB) {
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BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
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if (!BI || !BI->isConditional())
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return false;
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Value *Cond = BI->getCondition();
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FCmpInst *FCmp = dyn_cast<FCmpInst>(Cond);
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if (!FCmp)
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return false;
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bool isProb;
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if (FCmp->isEquality()) {
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// f1 == f2 -> Unlikely
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// f1 != f2 -> Likely
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isProb = !FCmp->isTrueWhenEqual();
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} else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
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// !isnan -> Likely
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isProb = true;
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} else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
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// isnan -> Unlikely
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isProb = false;
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} else {
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return false;
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}
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BasicBlock *Taken = BI->getSuccessor(0);
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BasicBlock *NonTaken = BI->getSuccessor(1);
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if (!isProb)
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std::swap(Taken, NonTaken);
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setEdgeWeight(BB, Taken, FPH_TAKEN_WEIGHT);
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setEdgeWeight(BB, NonTaken, FPH_NONTAKEN_WEIGHT);
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return true;
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}
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void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<LoopInfo>();
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AU.setPreservesAll();
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}
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bool BranchProbabilityInfo::runOnFunction(Function &F) {
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LastF = &F; // Store the last function we ran on for printing.
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LI = &getAnalysis<LoopInfo>();
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assert(PostDominatedByUnreachable.empty());
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// Walk the basic blocks in post-order so that we can build up state about
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// the successors of a block iteratively.
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for (po_iterator<BasicBlock *> I = po_begin(&F.getEntryBlock()),
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E = po_end(&F.getEntryBlock());
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I != E; ++I) {
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DEBUG(dbgs() << "Computing probabilities for " << I->getName() << "\n");
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if (calcUnreachableHeuristics(*I))
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continue;
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if (calcMetadataWeights(*I))
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continue;
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if (calcLoopBranchHeuristics(*I))
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continue;
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if (calcPointerHeuristics(*I))
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continue;
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if (calcZeroHeuristics(*I))
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continue;
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calcFloatingPointHeuristics(*I);
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}
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PostDominatedByUnreachable.clear();
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return false;
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}
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void BranchProbabilityInfo::print(raw_ostream &OS, const Module *) const {
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OS << "---- Branch Probabilities ----\n";
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// We print the probabilities from the last function the analysis ran over,
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// or the function it is currently running over.
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assert(LastF && "Cannot print prior to running over a function");
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for (Function::const_iterator BI = LastF->begin(), BE = LastF->end();
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BI != BE; ++BI) {
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for (succ_const_iterator SI = succ_begin(BI), SE = succ_end(BI);
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SI != SE; ++SI) {
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printEdgeProbability(OS << " ", BI, *SI);
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}
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}
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}
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uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
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uint32_t Sum = 0;
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for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
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const BasicBlock *Succ = *I;
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uint32_t Weight = getEdgeWeight(BB, Succ);
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uint32_t PrevSum = Sum;
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Sum += Weight;
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assert(Sum > PrevSum); (void) PrevSum;
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}
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return Sum;
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}
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bool BranchProbabilityInfo::
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isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
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// Hot probability is at least 4/5 = 80%
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// FIXME: Compare against a static "hot" BranchProbability.
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return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
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}
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BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
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uint32_t Sum = 0;
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uint32_t MaxWeight = 0;
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BasicBlock *MaxSucc = 0;
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for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
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BasicBlock *Succ = *I;
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uint32_t Weight = getEdgeWeight(BB, Succ);
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uint32_t PrevSum = Sum;
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Sum += Weight;
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assert(Sum > PrevSum); (void) PrevSum;
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if (Weight > MaxWeight) {
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MaxWeight = Weight;
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MaxSucc = Succ;
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}
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}
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// Hot probability is at least 4/5 = 80%
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if (BranchProbability(MaxWeight, Sum) > BranchProbability(4, 5))
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return MaxSucc;
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return 0;
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}
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// Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
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uint32_t BranchProbabilityInfo::
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getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
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Edge E(Src, Dst);
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DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
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if (I != Weights.end())
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return I->second;
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return DEFAULT_WEIGHT;
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}
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void BranchProbabilityInfo::
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setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
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Weights[std::make_pair(Src, Dst)] = Weight;
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DEBUG(dbgs() << "set edge " << Src->getName() << " -> "
|
|
<< Dst->getName() << " weight to " << Weight
|
|
<< (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
|
|
}
|
|
|
|
|
|
BranchProbability BranchProbabilityInfo::
|
|
getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
|
|
|
|
uint32_t N = getEdgeWeight(Src, Dst);
|
|
uint32_t D = getSumForBlock(Src);
|
|
|
|
return BranchProbability(N, D);
|
|
}
|
|
|
|
raw_ostream &
|
|
BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS,
|
|
const BasicBlock *Src,
|
|
const BasicBlock *Dst) const {
|
|
|
|
const BranchProbability Prob = getEdgeProbability(Src, Dst);
|
|
OS << "edge " << Src->getName() << " -> " << Dst->getName()
|
|
<< " probability is " << Prob
|
|
<< (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
|
|
|
|
return OS;
|
|
}
|