Add a speculative execution pass

Summary:
This is a pass for speculative execution of instructions for simple if-then (triangle) control flow. It's aimed at GPUs, but could perhaps be used in other contexts. Enabling this pass gives us a 1.0% geomean improvement on Google benchmark suites, with one benchmark improving 33%.

Credit goes to Jingyue Wu for writing an earlier version of this pass.

Patched by Bjarke Roune. 

Test Plan:
This patch adds a set of tests in test/Transforms/SpeculativeExecution/spec.ll
The pass is controlled by a flag which defaults to having the pass not run.

Reviewers: eliben, dberlin, meheff, jingyue, hfinkel

Reviewed By: jingyue, hfinkel

Subscribers: majnemer, jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D9360

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237459 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jingyue Wu 2015-05-15 17:54:48 +00:00
parent c683eb2d37
commit 85e632de29
10 changed files with 460 additions and 3 deletions

View File

@ -206,9 +206,19 @@ public:
return const_cast<BasicBlock*>(this)->getUniquePredecessor();
}
/// Return the successor of this block if it has a unique successor.
/// Otherwise return a null pointer. This method is analogous to
/// getUniquePredeccessor above.
/// \brief Return the successor of this block if it has a single successor.
/// Otherwise return a null pointer.
///
/// This method is analogous to getSinglePredecessor above.
BasicBlock *getSingleSuccessor();
const BasicBlock *getSingleSuccessor() const {
return const_cast<BasicBlock*>(this)->getSingleSuccessor();
}
/// \brief Return the successor of this block if it has a unique successor.
/// Otherwise return a null pointer.
///
/// This method is analogous to getUniquePredecessor above.
BasicBlock *getUniqueSuccessor();
const BasicBlock *getUniqueSuccessor() const {
return const_cast<BasicBlock*>(this)->getUniqueSuccessor();

View File

@ -256,6 +256,7 @@ void initializeSinkingPass(PassRegistry&);
void initializeSeparateConstOffsetFromGEPPass(PassRegistry &);
void initializeSlotIndexesPass(PassRegistry&);
void initializeSpillPlacementPass(PassRegistry&);
void initializeSpeculativeExecutionPass(PassRegistry&);
void initializeStackProtectorPass(PassRegistry&);
void initializeStackColoringPass(PassRegistry&);
void initializeStackSlotColoringPass(PassRegistry&);

View File

@ -170,6 +170,7 @@ namespace {
(void) llvm::createPartiallyInlineLibCallsPass();
(void) llvm::createScalarizerPass();
(void) llvm::createSeparateConstOffsetFromGEPPass();
(void) llvm::createSpeculativeExecutionPass();
(void) llvm::createRewriteSymbolsPass();
(void) llvm::createStraightLineStrengthReducePass();
(void) llvm::createMemDerefPrinter();

View File

@ -421,6 +421,13 @@ FunctionPass *
createSeparateConstOffsetFromGEPPass(const TargetMachine *TM = nullptr,
bool LowerGEP = false);
//===----------------------------------------------------------------------===//
//
// SpeculativeExecution - Aggressively hoist instructions to enable
// speculative execution on targets where branches are expensive.
//
FunctionPass *createSpeculativeExecutionPass();
//===----------------------------------------------------------------------===//
//
// LoadCombine - Combine loads into bigger loads.

View File

@ -238,6 +238,14 @@ BasicBlock *BasicBlock::getUniquePredecessor() {
return PredBB;
}
BasicBlock *BasicBlock::getSingleSuccessor() {
succ_iterator SI = succ_begin(this), E = succ_end(this);
if (SI == E) return nullptr; // no successors
BasicBlock *TheSucc = *SI;
++SI;
return (SI == E) ? TheSucc : nullptr /* multiple successors */;
}
BasicBlock *BasicBlock::getUniqueSuccessor() {
succ_iterator SI = succ_begin(this), E = succ_end(this);
if (SI == E) return NULL; // No successors

View File

@ -231,6 +231,7 @@ void PassManagerBuilder::populateModulePassManager(
MPM.add(createSROAPass(/*RequiresDomTree*/ false));
else
MPM.add(createScalarReplAggregatesPass(-1, false));
MPM.add(createEarlyCSEPass()); // Catch trivial redundancies
MPM.add(createJumpThreadingPass()); // Thread jumps.
MPM.add(createCorrelatedValuePropagationPass()); // Propagate conditionals

View File

@ -45,6 +45,7 @@ add_llvm_library(LLVMScalarOpts
SeparateConstOffsetFromGEP.cpp
SimplifyCFGPass.cpp
Sink.cpp
SpeculativeExecution.cpp
StraightLineStrengthReduce.cpp
StructurizeCFG.cpp
TailRecursionElimination.cpp

View File

@ -74,6 +74,7 @@ void llvm::initializeScalarOpts(PassRegistry &Registry) {
initializeSinkingPass(Registry);
initializeTailCallElimPass(Registry);
initializeSeparateConstOffsetFromGEPPass(Registry);
initializeSpeculativeExecutionPass(Registry);
initializeStraightLineStrengthReducePass(Registry);
initializeLoadCombinePass(Registry);
initializePlaceBackedgeSafepointsImplPass(Registry);

View File

@ -0,0 +1,232 @@
//===- 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.
//
//===----------------------------------------------------------------------===//
#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."));
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;
};
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

View File

@ -0,0 +1,195 @@
; RUN: opt < %s -S -speculative-execution \
; RUN: -spec-exec-max-speculation-cost 4 -spec-exec-max-not-hoisted 3 \
; RUN: | FileCheck %s
target datalayout = "e-i64:64-v16:16-v32:32-n16:32:64"
; Hoist in if-then pattern.
define void @ifThen() {
; CHECK-LABEL: @ifThen(
; CHECK: %x = add i32 2, 3
; CHECK: br i1 true
br i1 true, label %a, label %b
; CHECK: a:
a:
%x = add i32 2, 3
; CHECK: br label
br label %b
; CHECK: b:
b:
; CHECK: ret void
ret void
}
; Hoist in if-else pattern.
define void @ifElse() {
; CHECK-LABEL: @ifElse(
; CHECK: %x = add i32 2, 3
; CHECK: br i1 true
br i1 true, label %b, label %a
; CHECK: a:
a:
%x = add i32 2, 3
; CHECK: br label
br label %b
; CHECK: b:
b:
; CHECK: ret void
ret void
}
; Hoist in if-then-else pattern if it is equivalent to if-then.
define void @ifElseThenAsIfThen() {
; CHECK-LABEL: @ifElseThenAsIfThen(
; CHECK: %x = add i32 2, 3
; CHECK: br
br i1 true, label %a, label %b
; CHECK: a:
a:
%x = add i32 2, 3
; CHECK: br label
br label %c
; CHECK: b:
b:
br label %c
; CHECK: c
c:
ret void
}
; Hoist in if-then-else pattern if it is equivalent to if-else.
define void @ifElseThenAsIfElse() {
; CHECK-LABEL: @ifElseThenAsIfElse(
; CHECK: %x = add i32 2, 3
; CHECK: br
br i1 true, label %b, label %a
; CHECK: a:
a:
%x = add i32 2, 3
; CHECK: br label
br label %c
; CHECK: b:
b:
br label %c
; CHECK: c
c:
ret void
}
; Do not hoist if-then-else pattern if it is not equivalent to if-then
; or if-else.
define void @ifElseThen() {
; CHECK-LABEL: @ifElseThen(
; CHECK: br
br i1 true, label %a, label %b
; CHECK: a:
a:
; CHECK: %x = add
%x = add i32 2, 3
; CHECK: br label
br label %c
; CHECK: b:
b:
; CHECK: %y = add
%y = add i32 2, 3
br label %c
; CHECK: c
c:
ret void
}
; Do not hoist loads and do not hoist an instruction past a definition of
; an operand.
define void @doNotHoistPastDef() {
; CHECK-LABEL: @doNotHoistPastDef(
br i1 true, label %b, label %a
; CHECK-NOT: load
; CHECK-NOT: add
; CHECK: a:
a:
; CHECK: %def = load
%def = load i32, i32* null
; CHECK: %use = add
%use = add i32 %def, 0
br label %b
; CHECK: b:
b:
ret void
}
; Case with nothing to speculate.
define void @nothingToSpeculate() {
; CHECK-LABEL: @nothingToSpeculate(
br i1 true, label %b, label %a
; CHECK: a:
a:
; CHECK: %def = load
%def = load i32, i32* null
br label %b
; CHECK: b:
b:
ret void
}
; Still hoist if an operand is defined before the block or is itself hoisted.
define void @hoistIfNotPastDef() {
; CHECK-LABEL: @hoistIfNotPastDef(
; CHECK: %x = load
%x = load i32, i32* null
; CHECK: %y = add i32 %x, 1
; CHECK: %z = add i32 %y, 1
; CHECK: br
br i1 true, label %b, label %a
; CHECK: a:
a:
%y = add i32 %x, 1
%z = add i32 %y, 1
br label %b
; CHECK: b:
b:
ret void
}
; Do not hoist if the speculation cost is too high.
define void @costTooHigh() {
; CHECK-LABEL: @costTooHigh(
; CHECK: br
br i1 true, label %b, label %a
; CHECK: a:
a:
; CHECK: %r1 = add
%r1 = add i32 1, 1
; CHECK: %r2 = add
%r2 = add i32 1, 1
; CHECK: %r3 = add
%r3 = add i32 1, 1
; CHECK: %r4 = add
%r4 = add i32 1, 1
; CHECK: %r5 = add
%r5 = add i32 1, 1
br label %b
; CHECK: b:
b:
ret void
}
; Do not hoist if too many instructions are left behind.
define void @tooMuchLeftBehind() {
; CHECK-LABEL: @tooMuchLeftBehind(
; CHECK: br
br i1 true, label %b, label %a
; CHECK: a:
a:
; CHECK: %x = load
%x = load i32, i32* null
; CHECK: %r1 = add
%r1 = add i32 %x, 1
; CHECK: %r2 = add
%r2 = add i32 %x, 1
; CHECK: %r3 = add
%r3 = add i32 %x, 1
br label %b
; CHECK: b:
b:
ret void
}