//===- 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/SmallSet.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>();
}
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()) {
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) {
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;
}
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>();
return PA;
}
} // namespace llvm
