[llvm] r237459 - Add a speculative execution pass
Philip Reames
listmail at philipreames.com
Tue May 19 10:12:46 PDT 2015
On 05/19/2015 10:06 AM, Jingyue Wu wrote:
> Sure, I'll update that. Bjarke said something about it in the review
> thread
> (http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150427/274219.html).
>
That was exactly what I was looking for, thanks!
>
> On Tue, May 19, 2015 at 9:58 AM, Philip Reames
> <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>
> Jingyue,
>
> Can you spell out how this is different from the existing
> speculation done in SimplifyCFG? I'd really prefer to see this
> addressed in the comments at the top of the pass. It's something
> I imagine many readers are going to be wondering about.
>
> Philip
>
>
> On 05/15/2015 10:54 AM, Jingyue Wu wrote:
>
> Author: jingyue
> Date: Fri May 15 12:54:48 2015
> New Revision: 237459
>
> URL: http://llvm.org/viewvc/llvm-project?rev=237459&view=rev
> Log:
> 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
>
> Added:
> llvm/trunk/lib/Transforms/Scalar/SpeculativeExecution.cpp
> llvm/trunk/test/Transforms/SpeculativeExecution/
> llvm/trunk/test/Transforms/SpeculativeExecution/spec.ll
> Modified:
> llvm/trunk/include/llvm/IR/BasicBlock.h
> llvm/trunk/include/llvm/InitializePasses.h
> llvm/trunk/include/llvm/LinkAllPasses.h
> llvm/trunk/include/llvm/Transforms/Scalar.h
> llvm/trunk/lib/IR/BasicBlock.cpp
> llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp
> llvm/trunk/lib/Transforms/Scalar/CMakeLists.txt
> llvm/trunk/lib/Transforms/Scalar/Scalar.cpp
>
> Modified: llvm/trunk/include/llvm/IR/BasicBlock.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/BasicBlock.h?rev=237459&r1=237458&r2=237459&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/IR/BasicBlock.h (original)
> +++ llvm/trunk/include/llvm/IR/BasicBlock.h Fri May 15
> 12:54:48 2015
> @@ -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();
>
> Modified: llvm/trunk/include/llvm/InitializePasses.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/InitializePasses.h?rev=237459&r1=237458&r2=237459&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/InitializePasses.h (original)
> +++ llvm/trunk/include/llvm/InitializePasses.h Fri May 15
> 12:54:48 2015
> @@ -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&);
>
> Modified: llvm/trunk/include/llvm/LinkAllPasses.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/LinkAllPasses.h?rev=237459&r1=237458&r2=237459&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/LinkAllPasses.h (original)
> +++ llvm/trunk/include/llvm/LinkAllPasses.h Fri May 15
> 12:54:48 2015
> @@ -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();
>
> Modified: llvm/trunk/include/llvm/Transforms/Scalar.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Transforms/Scalar.h?rev=237459&r1=237458&r2=237459&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/Transforms/Scalar.h (original)
> +++ llvm/trunk/include/llvm/Transforms/Scalar.h Fri May 15
> 12:54:48 2015
> @@ -423,6 +423,13 @@ createSeparateConstOffsetFromGEPPass(con
> //===----------------------------------------------------------------------===//
> //
> +// SpeculativeExecution - Aggressively hoist instructions to
> enable
> +// speculative execution on targets where branches are expensive.
> +//
> +FunctionPass *createSpeculativeExecutionPass();
> +
> +//===----------------------------------------------------------------------===//
> +//
> // LoadCombine - Combine loads into bigger loads.
> //
> BasicBlockPass *createLoadCombinePass();
>
> Modified: llvm/trunk/lib/IR/BasicBlock.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/BasicBlock.cpp?rev=237459&r1=237458&r2=237459&view=diff
> ==============================================================================
> --- llvm/trunk/lib/IR/BasicBlock.cpp (original)
> +++ llvm/trunk/lib/IR/BasicBlock.cpp Fri May 15 12:54:48 2015
> @@ -238,6 +238,14 @@ BasicBlock *BasicBlock::getUniquePredece
> 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
>
> Modified: llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp?rev=237459&r1=237458&r2=237459&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp
> (original)
> +++ llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp Fri
> May 15 12:54:48 2015
> @@ -231,6 +231,7 @@ void PassManagerBuilder::populateModuleP
> 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
>
> Modified: llvm/trunk/lib/Transforms/Scalar/CMakeLists.txt
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/CMakeLists.txt?rev=237459&r1=237458&r2=237459&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/CMakeLists.txt (original)
> +++ llvm/trunk/lib/Transforms/Scalar/CMakeLists.txt Fri May 15
> 12:54:48 2015
> @@ -45,6 +45,7 @@ add_llvm_library(LLVMScalarOpts
> SeparateConstOffsetFromGEP.cpp
> SimplifyCFGPass.cpp
> Sink.cpp
> + SpeculativeExecution.cpp
> StraightLineStrengthReduce.cpp
> StructurizeCFG.cpp
> TailRecursionElimination.cpp
>
> Modified: llvm/trunk/lib/Transforms/Scalar/Scalar.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/Scalar.cpp?rev=237459&r1=237458&r2=237459&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/Scalar.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/Scalar.cpp Fri May 15
> 12:54:48 2015
> @@ -74,6 +74,7 @@ void llvm::initializeScalarOpts(PassRegi
> initializeSinkingPass(Registry);
> initializeTailCallElimPass(Registry);
> initializeSeparateConstOffsetFromGEPPass(Registry);
> + initializeSpeculativeExecutionPass(Registry);
> initializeStraightLineStrengthReducePass(Registry);
> initializeLoadCombinePass(Registry);
> initializePlaceBackedgeSafepointsImplPass(Registry);
>
> Added: llvm/trunk/lib/Transforms/Scalar/SpeculativeExecution.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/SpeculativeExecution.cpp?rev=237459&view=auto
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/SpeculativeExecution.cpp
> (added)
> +++ llvm/trunk/lib/Transforms/Scalar/SpeculativeExecution.cpp
> Fri May 15 12:54:48 2015
> @@ -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
>
> Added: llvm/trunk/test/Transforms/SpeculativeExecution/spec.ll
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SpeculativeExecution/spec.ll?rev=237459&view=auto
> ==============================================================================
> --- llvm/trunk/test/Transforms/SpeculativeExecution/spec.ll
> (added)
> +++ llvm/trunk/test/Transforms/SpeculativeExecution/spec.ll
> Fri May 15 12:54:48 2015
> @@ -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
> +}
>
>
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