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Allow to generate a loop without the GuardBB
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  This allows us to omit the GuardBB in front of created loops
  if we can show the loop trip count is at least one. It also
  simplifies the dominance relation inside the new created region.
  A GuardBB (even with a constant branch condition) might trigger
  false dominance errors during function verification.

Differential Revision: http://reviews.llvm.org/D5297

llvm-svn: 217525
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Johannes Doerfert committed Sep 10, 2014
1 parent 1c9547e commit dd5c144
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Showing 4 changed files with 91 additions and 43 deletions.
8 changes: 6 additions & 2 deletions polly/include/polly/CodeGen/LoopGenerators.h
View file
Expand Up @@ -27,7 +27,7 @@ class BasicBlock;
namespace polly {
using namespace llvm;

/// @brief Create a scalar loop.
/// @brief Create a scalar do/for-style loop.
///
/// @param LowerBound The starting value of the induction variable.
/// @param UpperBound The upper bound of the induction variable.
Expand All @@ -43,12 +43,16 @@ using namespace llvm;
/// @param Annotator This function can (optionally) take a LoopAnnotator which
/// tracks the loop structure.
/// @param Parallel If this loop should be marked parallel in the Annotator.
/// @param UseGuard Create a guard in front of the header to check if the
/// loop is executed at least once, otherwise just assume it.
///
/// @return Value* The newly created induction variable for this loop.
Value *createLoop(Value *LowerBound, Value *UpperBound, Value *Stride,
PollyIRBuilder &Builder, Pass *P, LoopInfo &LI,
DominatorTree &DT, BasicBlock *&ExitBlock,
ICmpInst::Predicate Predicate,
LoopAnnotator *Annotator = NULL, bool Parallel = false);
LoopAnnotator *Annotator = NULL, bool Parallel = false,
bool UseGuard = true);

class OMPGenerator {
public:
Expand Down
6 changes: 5 additions & 1 deletion polly/lib/CodeGen/IslCodeGeneration.cpp
View file
Expand Up @@ -352,8 +352,12 @@ void IslNodeBuilder::createForSequential(__isl_take isl_ast_node *For) {
if (MaxType != ValueInc->getType())
ValueInc = Builder.CreateSExt(ValueInc, MaxType);

// If we can show that LB <Predicate> UB holds at least once, we can
// omit the GuardBB in front of the loop.
bool UseGuardBB =
!SE.isKnownPredicate(Predicate, SE.getSCEV(ValueLB), SE.getSCEV(ValueUB));
IV = createLoop(ValueLB, ValueUB, ValueInc, Builder, P, LI, DT, ExitBlock,
Predicate, &Annotator, Parallel);
Predicate, &Annotator, Parallel, UseGuardBB);
IDToValue[IteratorID] = IV;

create(Body);
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87 changes: 47 additions & 40 deletions polly/lib/CodeGen/LoopGenerators.cpp
View file
Expand Up @@ -23,34 +23,33 @@
using namespace llvm;
using namespace polly;

// We generate a loop of the following structure
// We generate a loop of either of the following structures:
//
// BeforeBB
// |
// v
// GuardBB
// / |
// __ PreHeaderBB |
// / \ / |
// latch HeaderBB |
// \ / \ /
// < \ /
// \ /
// ExitBB
// BeforeBB BeforeBB
// | |
// v v
// GuardBB PreHeaderBB
// / | | _____
// __ PreHeaderBB | v \/ |
// / \ / | HeaderBB latch
// latch HeaderBB | |\ |
// \ / \ / | \------/
// < \ / |
// \ / v
// ExitBB ExitBB
//
// GuardBB checks if the loop is executed at least once. If this is the case
// we branch to PreHeaderBB and subsequently to the HeaderBB, which contains the
// loop iv 'polly.indvar', the incremented loop iv 'polly.indvar_next' as well
// as the condition to check if we execute another iteration of the loop. After
// the loop has finished, we branch to ExitBB.
//
// TODO: We currently always create the GuardBB. If we can prove the loop is
// always executed at least once, we can get rid of this branch.
// depending on whether or not we know that it is executed at least once. If
// not, GuardBB checks if the loop is executed at least once. If this is the
// case we branch to PreHeaderBB and subsequently to the HeaderBB, which
// contains the loop iv 'polly.indvar', the incremented loop iv
// 'polly.indvar_next' as well as the condition to check if we execute another
// iteration of the loop. After the loop has finished, we branch to ExitBB.
Value *polly::createLoop(Value *LB, Value *UB, Value *Stride,
PollyIRBuilder &Builder, Pass *P, LoopInfo &LI,
DominatorTree &DT, BasicBlock *&ExitBB,
ICmpInst::Predicate Predicate,
LoopAnnotator *Annotator, bool Parallel) {
LoopAnnotator *Annotator, bool Parallel,
bool UseGuard) {
Function *F = Builder.GetInsertBlock()->getParent();
LLVMContext &Context = F->getContext();

Expand All @@ -59,7 +58,8 @@ Value *polly::createLoop(Value *LB, Value *UB, Value *Stride,
assert(LoopIVType && "UB is not integer?");

BasicBlock *BeforeBB = Builder.GetInsertBlock();
BasicBlock *GuardBB = BasicBlock::Create(Context, "polly.loop_if", F);
BasicBlock *GuardBB =
UseGuard ? BasicBlock::Create(Context, "polly.loop_if", F) : nullptr;
BasicBlock *HeaderBB = BasicBlock::Create(Context, "polly.loop_header", F);
BasicBlock *PreHeaderBB =
BasicBlock::Create(Context, "polly.loop_preheader", F);
Expand All @@ -74,16 +74,15 @@ Value *polly::createLoop(Value *LB, Value *UB, Value *Stride,
Loop *OuterLoop = LI.getLoopFor(BeforeBB);
Loop *NewLoop = new Loop();

if (OuterLoop) {
if (OuterLoop)
OuterLoop->addChildLoop(NewLoop);
} else {
else
LI.addTopLevelLoop(NewLoop);
}

if (OuterLoop) {
if (OuterLoop && GuardBB)
OuterLoop->addBasicBlockToLoop(GuardBB, LI.getBase());
else if (OuterLoop)
OuterLoop->addBasicBlockToLoop(PreHeaderBB, LI.getBase());
}

NewLoop->addBasicBlockToLoop(HeaderBB, LI.getBase());

Expand All @@ -92,18 +91,23 @@ Value *polly::createLoop(Value *LB, Value *UB, Value *Stride,
ExitBB->setName("polly.loop_exit");

// BeforeBB
BeforeBB->getTerminator()->setSuccessor(0, GuardBB);

// GuardBB
DT.addNewBlock(GuardBB, BeforeBB);
Builder.SetInsertPoint(GuardBB);
Value *LoopGuard;
LoopGuard = Builder.CreateICmp(Predicate, LB, UB);
LoopGuard->setName("polly.loop_guard");
Builder.CreateCondBr(LoopGuard, PreHeaderBB, ExitBB);
if (GuardBB) {
BeforeBB->getTerminator()->setSuccessor(0, GuardBB);
DT.addNewBlock(GuardBB, BeforeBB);

// GuardBB
Builder.SetInsertPoint(GuardBB);
Value *LoopGuard;
LoopGuard = Builder.CreateICmp(Predicate, LB, UB);
LoopGuard->setName("polly.loop_guard");
Builder.CreateCondBr(LoopGuard, PreHeaderBB, ExitBB);
DT.addNewBlock(PreHeaderBB, GuardBB);
} else {
BeforeBB->getTerminator()->setSuccessor(0, PreHeaderBB);
DT.addNewBlock(PreHeaderBB, BeforeBB);
}

// PreHeaderBB
DT.addNewBlock(PreHeaderBB, GuardBB);
Builder.SetInsertPoint(PreHeaderBB);
Builder.CreateBr(HeaderBB);

Expand All @@ -120,7 +124,10 @@ Value *polly::createLoop(Value *LB, Value *UB, Value *Stride,
LoopCondition->setName("polly.loop_cond");
Builder.CreateCondBr(LoopCondition, HeaderBB, ExitBB);
IV->addIncoming(IncrementedIV, HeaderBB);
DT.changeImmediateDominator(ExitBB, GuardBB);
if (GuardBB)
DT.changeImmediateDominator(ExitBB, GuardBB);
else
DT.changeImmediateDominator(ExitBB, BeforeBB);

// The loop body should be added here.
Builder.SetInsertPoint(HeaderBB->getFirstNonPHI());
Expand Down Expand Up @@ -322,7 +329,7 @@ Value *OMPGenerator::createSubfunction(Value *Stride, Value *StructData,
Builder.SetInsertPoint(--Builder.GetInsertPoint());
LoopInfo &LI = P->getAnalysis<LoopInfo>();
IV = createLoop(LowerBound, UpperBound, Stride, Builder, P, LI, DT, AfterBB,
ICmpInst::ICMP_SLE);
ICmpInst::ICMP_SLE, nullptr, true, /* UseGuard */ false);

BasicBlock::iterator LoopBody = Builder.GetInsertPoint();
Builder.SetInsertPoint(AfterBB->begin());
Expand Down
33 changes: 33 additions & 0 deletions polly/test/Isl/CodeGen/no_guard_bb.ll
View file
@@ -0,0 +1,33 @@
; RUN: opt %loadPolly -polly-codegen-isl -S < %s | FileCheck %s
;
; CHECK-NOT: br i1 true, label %polly.{{.*}}, label %polly.{{.*}}
;
; void jd(int *A) {
; for (int i = 0; i < 1024; i++)
; A[i] = i;
; }
;
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"

define void @jd(i32* %A) {
entry:
br label %for.cond

for.cond: ; preds = %for.inc, %entry
%indvars.iv = phi i64 [ %indvars.iv.next, %for.inc ], [ 0, %entry ]
%exitcond = icmp ne i64 %indvars.iv, 1024
br i1 %exitcond, label %for.body, label %for.end

for.body: ; preds = %for.cond
%arrayidx = getelementptr inbounds i32* %A, i64 %indvars.iv
%tmp = trunc i64 %indvars.iv to i32
store i32 %tmp, i32* %arrayidx, align 4
br label %for.inc

for.inc: ; preds = %for.body
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
br label %for.cond

for.end: ; preds = %for.cond
ret void
}

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