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Allow to generate a loop without the GuardBB 

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
This commit is contained in:
Johannes Doerfert 2014-09-10 17:33:32 +00:00
parent 1c9547ed51
commit dd5c144246
4 changed files with 90 additions and 42 deletions
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8
polly/include/polly/CodeGen/LoopGenerators.h
View File

@ -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.
@ -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:

6
polly/lib/CodeGen/IslCodeGeneration.cpp
View File

@ -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);

85
polly/lib/CodeGen/LoopGenerators.cpp
View File

@ -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();
@ -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);
@ -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());
@ -92,18 +91,23 @@ Value *polly::createLoop(Value *LB, Value *UB, Value *Stride,
ExitBB->setName("polly.loop_exit");
// BeforeBB
BeforeBB->getTerminator()->setSuccessor(0, GuardBB);
if (GuardBB) {
BeforeBB->getTerminator()->setSuccessor(0, GuardBB);
DT.addNewBlock(GuardBB, BeforeBB);
// 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);
// 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);
@ -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());
@ -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());

33
polly/test/Isl/CodeGen/no_guard_bb.ll Normal file
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
}