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Diffstat (limited to 'gnu/llvm/lib/Transforms/Scalar/SpeculativeExecution.cpp')
| -rw-r--r-- | gnu/llvm/lib/Transforms/Scalar/SpeculativeExecution.cpp | 321 |
1 files changed, 0 insertions, 321 deletions
diff --git a/gnu/llvm/lib/Transforms/Scalar/SpeculativeExecution.cpp b/gnu/llvm/lib/Transforms/Scalar/SpeculativeExecution.cpp deleted file mode 100644 index f5e1dd6ed85..00000000000 --- a/gnu/llvm/lib/Transforms/Scalar/SpeculativeExecution.cpp +++ /dev/null @@ -1,321 +0,0 @@ -//===- SpeculativeExecution.cpp ---------------------------------*- C++ -*-===// -// -// The LLVM Compiler Infrastructure -// -// This file is distributed under the University of Illinois Open Source -// License. See LICENSE.TXT for details. -// -//===----------------------------------------------------------------------===// -// -// This pass hoists instructions to enable speculative execution on -// targets where branches are expensive. This is aimed at GPUs. It -// currently works on simple if-then and if-then-else -// patterns. -// -// Removing branches is not the only motivation for this -// pass. E.g. consider this code and assume that there is no -// addressing mode for multiplying by sizeof(*a): -// -// if (b > 0) -// c = a[i + 1] -// if (d > 0) -// e = a[i + 2] -// -// turns into -// -// p = &a[i + 1]; -// if (b > 0) -// c = *p; -// q = &a[i + 2]; -// if (d > 0) -// e = *q; -// -// which could later be optimized to -// -// r = &a[i]; -// if (b > 0) -// c = r[1]; -// if (d > 0) -// e = r[2]; -// -// Later passes sink back much of the speculated code that did not enable -// further optimization. -// -// This pass is more aggressive than the function SpeculativeyExecuteBB in -// SimplifyCFG. SimplifyCFG will not speculate if no selects are introduced and -// it will speculate at most one instruction. It also will not speculate if -// there is a value defined in the if-block that is only used in the then-block. -// These restrictions make sense since the speculation in SimplifyCFG seems -// aimed at introducing cheap selects, while this pass is intended to do more -// aggressive speculation while counting on later passes to either capitalize on -// that or clean it up. -// -// If the pass was created by calling -// createSpeculativeExecutionIfHasBranchDivergencePass or the -// -spec-exec-only-if-divergent-target option is present, this pass only has an -// effect on targets where TargetTransformInfo::hasBranchDivergence() is true; -// on other targets, it is a nop. -// -// This lets you include this pass unconditionally in the IR pass pipeline, but -// only enable it for relevant targets. -// -//===----------------------------------------------------------------------===// - -#include "llvm/Transforms/Scalar/SpeculativeExecution.h" -#include "llvm/ADT/SmallPtrSet.h" -#include "llvm/Analysis/GlobalsModRef.h" -#include "llvm/Analysis/ValueTracking.h" -#include "llvm/IR/Instructions.h" -#include "llvm/IR/Module.h" -#include "llvm/IR/Operator.h" -#include "llvm/Support/CommandLine.h" -#include "llvm/Support/Debug.h" - -using namespace llvm; - -#define DEBUG_TYPE "speculative-execution" - -// The risk that speculation will not pay off increases with the -// number of instructions speculated, so we put a limit on that. -static cl::opt<unsigned> SpecExecMaxSpeculationCost( - "spec-exec-max-speculation-cost", cl::init(7), cl::Hidden, - cl::desc("Speculative execution is not applied to basic blocks where " - "the cost of the instructions to speculatively execute " - "exceeds this limit.")); - -// Speculating just a few instructions from a larger block tends not -// to be profitable and this limit prevents that. A reason for that is -// that small basic blocks are more likely to be candidates for -// further optimization. -static cl::opt<unsigned> SpecExecMaxNotHoisted( - "spec-exec-max-not-hoisted", cl::init(5), cl::Hidden, - cl::desc("Speculative execution is not applied to basic blocks where the " - "number of instructions that would not be speculatively executed " - "exceeds this limit.")); - -static cl::opt<bool> SpecExecOnlyIfDivergentTarget( - "spec-exec-only-if-divergent-target", cl::init(false), cl::Hidden, - cl::desc("Speculative execution is applied only to targets with divergent " - "branches, even if the pass was configured to apply only to all " - "targets.")); - -namespace { - -class SpeculativeExecutionLegacyPass : public FunctionPass { -public: - static char ID; - explicit SpeculativeExecutionLegacyPass(bool OnlyIfDivergentTarget = false) - : FunctionPass(ID), OnlyIfDivergentTarget(OnlyIfDivergentTarget || - SpecExecOnlyIfDivergentTarget), - Impl(OnlyIfDivergentTarget) {} - - void getAnalysisUsage(AnalysisUsage &AU) const override; - bool runOnFunction(Function &F) override; - - StringRef getPassName() const override { - if (OnlyIfDivergentTarget) - return "Speculatively execute instructions if target has divergent " - "branches"; - return "Speculatively execute instructions"; - } - -private: - // Variable preserved purely for correct name printing. - const bool OnlyIfDivergentTarget; - - SpeculativeExecutionPass Impl; -}; -} // namespace - -char SpeculativeExecutionLegacyPass::ID = 0; -INITIALIZE_PASS_BEGIN(SpeculativeExecutionLegacyPass, "speculative-execution", - "Speculatively execute instructions", false, false) -INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) -INITIALIZE_PASS_END(SpeculativeExecutionLegacyPass, "speculative-execution", - "Speculatively execute instructions", false, false) - -void SpeculativeExecutionLegacyPass::getAnalysisUsage(AnalysisUsage &AU) const { - AU.addRequired<TargetTransformInfoWrapperPass>(); - AU.addPreserved<GlobalsAAWrapperPass>(); - AU.setPreservesCFG(); -} - -bool SpeculativeExecutionLegacyPass::runOnFunction(Function &F) { - if (skipFunction(F)) - return false; - - auto *TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F); - return Impl.runImpl(F, TTI); -} - -namespace llvm { - -bool SpeculativeExecutionPass::runImpl(Function &F, TargetTransformInfo *TTI) { - if (OnlyIfDivergentTarget && !TTI->hasBranchDivergence()) { - LLVM_DEBUG(dbgs() << "Not running SpeculativeExecution because " - "TTI->hasBranchDivergence() is false.\n"); - return false; - } - - this->TTI = TTI; - bool Changed = false; - for (auto& B : F) { - Changed |= runOnBasicBlock(B); - } - return Changed; -} - -bool SpeculativeExecutionPass::runOnBasicBlock(BasicBlock &B) { - BranchInst *BI = dyn_cast<BranchInst>(B.getTerminator()); - if (BI == nullptr) - return false; - - if (BI->getNumSuccessors() != 2) - return false; - BasicBlock &Succ0 = *BI->getSuccessor(0); - BasicBlock &Succ1 = *BI->getSuccessor(1); - - if (&B == &Succ0 || &B == &Succ1 || &Succ0 == &Succ1) { - return false; - } - - // Hoist from if-then (triangle). - if (Succ0.getSinglePredecessor() != nullptr && - Succ0.getSingleSuccessor() == &Succ1) { - return considerHoistingFromTo(Succ0, B); - } - - // Hoist from if-else (triangle). - if (Succ1.getSinglePredecessor() != nullptr && - Succ1.getSingleSuccessor() == &Succ0) { - return considerHoistingFromTo(Succ1, B); - } - - // Hoist from if-then-else (diamond), but only if it is equivalent to - // an if-else or if-then due to one of the branches doing nothing. - if (Succ0.getSinglePredecessor() != nullptr && - Succ1.getSinglePredecessor() != nullptr && - Succ1.getSingleSuccessor() != nullptr && - Succ1.getSingleSuccessor() != &B && - Succ1.getSingleSuccessor() == Succ0.getSingleSuccessor()) { - // If a block has only one instruction, then that is a terminator - // instruction so that the block does nothing. This does happen. - if (Succ1.size() == 1) // equivalent to if-then - return considerHoistingFromTo(Succ0, B); - if (Succ0.size() == 1) // equivalent to if-else - return considerHoistingFromTo(Succ1, B); - } - - return false; -} - -static unsigned ComputeSpeculationCost(const Instruction *I, - const TargetTransformInfo &TTI) { - switch (Operator::getOpcode(I)) { - case Instruction::GetElementPtr: - case Instruction::Add: - case Instruction::Mul: - case Instruction::And: - case Instruction::Or: - case Instruction::Select: - case Instruction::Shl: - case Instruction::Sub: - case Instruction::LShr: - case Instruction::AShr: - case Instruction::Xor: - case Instruction::ZExt: - case Instruction::SExt: - case Instruction::Call: - case Instruction::BitCast: - case Instruction::PtrToInt: - case Instruction::IntToPtr: - case Instruction::AddrSpaceCast: - case Instruction::FPToUI: - case Instruction::FPToSI: - case Instruction::UIToFP: - case Instruction::SIToFP: - case Instruction::FPExt: - case Instruction::FPTrunc: - case Instruction::FAdd: - case Instruction::FSub: - case Instruction::FMul: - case Instruction::FDiv: - case Instruction::FRem: - case Instruction::ICmp: - case Instruction::FCmp: - return TTI.getUserCost(I); - - default: - return UINT_MAX; // Disallow anything not whitelisted. - } -} - -bool SpeculativeExecutionPass::considerHoistingFromTo( - BasicBlock &FromBlock, BasicBlock &ToBlock) { - SmallPtrSet<const Instruction *, 8> NotHoisted; - const auto AllPrecedingUsesFromBlockHoisted = [&NotHoisted](User *U) { - for (Value* V : U->operand_values()) { - if (Instruction *I = dyn_cast<Instruction>(V)) { - if (NotHoisted.count(I) > 0) - return false; - } - } - return true; - }; - - unsigned TotalSpeculationCost = 0; - for (auto& I : FromBlock) { - const unsigned Cost = ComputeSpeculationCost(&I, *TTI); - if (Cost != UINT_MAX && isSafeToSpeculativelyExecute(&I) && - AllPrecedingUsesFromBlockHoisted(&I)) { - TotalSpeculationCost += Cost; - if (TotalSpeculationCost > SpecExecMaxSpeculationCost) - return false; // too much to hoist - } else { - NotHoisted.insert(&I); - if (NotHoisted.size() > SpecExecMaxNotHoisted) - return false; // too much left behind - } - } - - if (TotalSpeculationCost == 0) - return false; // nothing to hoist - - for (auto I = FromBlock.begin(); I != FromBlock.end();) { - // We have to increment I before moving Current as moving Current - // changes the list that I is iterating through. - auto Current = I; - ++I; - if (!NotHoisted.count(&*Current)) { - Current->moveBefore(ToBlock.getTerminator()); - } - } - return true; -} - -FunctionPass *createSpeculativeExecutionPass() { - return new SpeculativeExecutionLegacyPass(); -} - -FunctionPass *createSpeculativeExecutionIfHasBranchDivergencePass() { - return new SpeculativeExecutionLegacyPass(/* OnlyIfDivergentTarget = */ true); -} - -SpeculativeExecutionPass::SpeculativeExecutionPass(bool OnlyIfDivergentTarget) - : OnlyIfDivergentTarget(OnlyIfDivergentTarget || - SpecExecOnlyIfDivergentTarget) {} - -PreservedAnalyses SpeculativeExecutionPass::run(Function &F, - FunctionAnalysisManager &AM) { - auto *TTI = &AM.getResult<TargetIRAnalysis>(F); - - bool Changed = runImpl(F, TTI); - - if (!Changed) - return PreservedAnalyses::all(); - PreservedAnalyses PA; - PA.preserve<GlobalsAA>(); - PA.preserveSet<CFGAnalyses>(); - return PA; -} -} // namespace llvm |