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