| //===- 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. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/ADT/SmallSet.h" |
| #include "llvm/Analysis/TargetTransformInfo.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.")); |
| |
| namespace { |
| class SpeculativeExecution : public FunctionPass { |
| public: |
| static char ID; |
| SpeculativeExecution(): FunctionPass(ID) {} |
| |
| void getAnalysisUsage(AnalysisUsage &AU) const override; |
| bool runOnFunction(Function &F) override; |
| |
| private: |
| bool runOnBasicBlock(BasicBlock &B); |
| bool considerHoistingFromTo(BasicBlock &FromBlock, BasicBlock &ToBlock); |
| |
| const TargetTransformInfo *TTI = nullptr; |
| }; |
| } // namespace |
| |
| char SpeculativeExecution::ID = 0; |
| INITIALIZE_PASS_BEGIN(SpeculativeExecution, "speculative-execution", |
| "Speculatively execute instructions", false, false) |
| INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) |
| INITIALIZE_PASS_END(SpeculativeExecution, "speculative-execution", |
| "Speculatively execute instructions", false, false) |
| |
| void SpeculativeExecution::getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.addRequired<TargetTransformInfoWrapperPass>(); |
| } |
| |
| bool SpeculativeExecution::runOnFunction(Function &F) { |
| if (skipOptnoneFunction(F)) |
| return false; |
| |
| TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F); |
| |
| bool Changed = false; |
| for (auto& B : F) { |
| Changed |= runOnBasicBlock(B); |
| } |
| return Changed; |
| } |
| |
| bool SpeculativeExecution::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: |
| return TTI.getUserCost(I); |
| |
| default: |
| return UINT_MAX; // Disallow anything not whitelisted. |
| } |
| } |
| |
| bool SpeculativeExecution::considerHoistingFromTo(BasicBlock &FromBlock, |
| BasicBlock &ToBlock) { |
| SmallSet<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; |
| } |
| |
| namespace llvm { |
| |
| FunctionPass *createSpeculativeExecutionPass() { |
| return new SpeculativeExecution(); |
| } |
| |
| } // namespace llvm |