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Revert "indvars: Improve LFTR by eliminating truncation when comparing

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against a constant."

This reverts commit r186107. It didn't handle wrapping arithmetic in the
loop correctly and thus caused the following C program to count from
0 to UINT64_MAX instead of from 0 to 255 as intended:

  #include <stdio.h>
  int main() {
    unsigned char first = 0, last = 255;
    do { printf("%d\n", first); } while (first++ != last);
  }

Full test case and instructions to reproduce with just the -indvars pass
sent to the original review thread rather than to r186107's commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186152 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chandler Carruth 2013-07-12 11:18:55 +00:00
parent dd51a0c1e0
commit 6f0ec20e8f
2 changed files with 4 additions and 48 deletions
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23
lib/Transforms/Scalar/IndVarSimplify.cpp
View File

@ -1612,30 +1612,11 @@ LinearFunctionTestReplace(Loop *L,
<< " IVCount:\t" << *IVCount << "\n");
IRBuilder<> Builder(BI);
unsigned CmpIndVarSize = SE->getTypeSizeInBits(CmpIndVar->getType());
unsigned ExitCntSize = SE->getTypeSizeInBits(ExitCnt->getType());
if (CmpIndVarSize > ExitCntSize) {
const SCEVAddRecExpr *AR = cast<SCEVAddRecExpr>(SE->getSCEV(IndVar));
const SCEV *ARStart = AR->getStart();
const SCEV *ARStep = AR->getStepRecurrence(*SE);
if (isa<SCEVConstant>(ARStart) && isa<SCEVConstant>(IVCount)) {
const APInt &Start = cast<SCEVConstant>(ARStart)->getValue()->getValue();
const APInt &Count = cast<SCEVConstant>(IVCount)->getValue()->getValue();
APInt NewLimit;
if (cast<SCEVConstant>(ARStep)->getValue()->isNegative())
NewLimit = Start - Count.zext(CmpIndVarSize);
else
NewLimit = Start + Count.zext(CmpIndVarSize);
ExitCnt = ConstantInt::get(CmpIndVar->getType(), NewLimit);
DEBUG(dbgs() << " Widen RHS:\t" << *ExitCnt << "\n");
} else {
if (SE->getTypeSizeInBits(CmpIndVar->getType())
> SE->getTypeSizeInBits(ExitCnt->getType())) {
CmpIndVar = Builder.CreateTrunc(CmpIndVar, ExitCnt->getType(),
"lftr.wideiv");
}
}
Value *Cond = Builder.CreateICmp(P, CmpIndVar, ExitCnt, "exitcond");
Value *OrigCond = BI->getCondition();

25
test/Transforms/IndVarSimplify/exitcnt-const-arstart-const-opt.ll
View File

@ -1,25 +0,0 @@
;RUN: opt -S %s -indvars | FileCheck %s
; Function Attrs: nounwind uwtable
define void @foo() #0 {
entry:
br label %for.body
for.body: ; preds = %entry, %for.body
%i.01 = phi i16 [ 0, %entry ], [ %inc, %for.body ]
%conv2 = sext i16 %i.01 to i32
call void @bar(i32 %conv2) #1
%inc = add i16 %i.01, 1
;CHECK-NOT: %lftr.wideiv = trunc i32 %indvars.iv.next to i16
;CHECK: %exitcond = icmp ne i32 %indvars.iv.next, 512
%cmp = icmp slt i16 %inc, 512
br i1 %cmp, label %for.body, label %for.end
for.end: ; preds = %for.body
ret void
}
declare void @bar(i32)
attributes #0 = { nounwind uwtable }
attributes #1 = { nounwind }