[PATCH] Little improvement in IndVarSimplify

[Chandler Carruth]

I've reverted in r186152. It caused a really serious regression. Test case
attached here (it's not quite in the right form for committing yet -- you
can probably write it more directly against indvars once debugged.

% cat bad2.ll
; ModuleID = '<stdin>'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@.str = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1

; Function Attrs: nounwind uwtable
define i32 @main() #0 {
entry:
  br label %do.body

do.body:                                          ; preds = %do.body, %entry
  %first.0 = phi i8 [ 0, %entry ], [ %inc, %do.body ]
  %conv = zext i8 %first.0 to i32
  %call = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([4 x
i8]* @.str, i64 0, i64 0), i32 %conv) #1
  %inc = add i8 %first.0, 1
  %cmp = icmp eq i8 %first.0, -1
  br i1 %cmp, label %do.end, label %do.body

do.end:                                           ; preds = %do.body
  ret i32 0
}

declare i32 @printf(i8*, ...)

attributes #0 = { nounwind uwtable }
attributes #1 = { nounwind }

% ./bin/llc -O0 <bad2.ll | ./bin/clang -x assembler -
% ./a.out
0
1
2
<snip>
255

% ./bin/opt -S -indvars <bad2.ll | ./bin/llc -O0 | ./bin/clang -x assembler
-
0
1
2
<snip>
255
256
257
...

This goes on until the 64-bit register wraps back to zero. ;] It looks like
the problem is extending the integer induction variable and exit value
without accounting for wrapping.

The bad2.ll test case is attached for easier access.



On Thu, Jul 11, 2013 at 10:38 AM, Andrew Trick <atrick at apple.com> wrote:

> On Jul 11, 2013, at 12:29 AM, Michele Scandale <michele.scandale at gmail.com>
> wrote:
>
>  Here an updated version of the patch fixing code style as suggested by
> Andrew.
>
>
> Committed r186107. Thanks.
> -Andy
>
>
>  Thanks again.
>
>  Michele Scandale
>
> Hi atrick,
>
> http://llvm-reviews.chandlerc.com/D1112
>
> CHANGE SINCE LAST DIFF
>  http://llvm-reviews.chandlerc.com/D1112?vs=2713&id=2765#toc
>
> Files:
>  lib/Transforms/Scalar/IndVarSimplify.cpp
>  test/Transforms/IndVarSimplify/exitcnt-const-arstart-const-opt.ll
>
> Index: lib/Transforms/Scalar/IndVarSimplify.cpp
> ===================================================================
> --- lib/Transforms/Scalar/IndVarSimplify.cpp
> +++ lib/Transforms/Scalar/IndVarSimplify.cpp
> @@ -1612,10 +1612,29 @@
>                << "  IVCount:\t" << *IVCount << "\n");
>
>   IRBuilder<> Builder(BI);
> -  if (SE->getTypeSizeInBits(CmpIndVar->getType())
> -      > SE->getTypeSizeInBits(ExitCnt->getType())) {
> -    CmpIndVar = Builder.CreateTrunc(CmpIndVar, ExitCnt->getType(),
> -                                    "lftr.wideiv");
> +
> +  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 {
> +      CmpIndVar = Builder.CreateTrunc(CmpIndVar, ExitCnt->getType(),
> +                                      "lftr.wideiv");
> +    }
>   }
>
>   Value *Cond = Builder.CreateICmp(P, CmpIndVar, ExitCnt, "exitcond");
> Index: test/Transforms/IndVarSimplify/exitcnt-const-arstart-const-opt.ll
> ===================================================================
> --- test/Transforms/IndVarSimplify/exitcnt-const-arstart-const-opt.ll
> +++ test/Transforms/IndVarSimplify/exitcnt-const-arstart-const-opt.ll
> @@ -0,0 +1,25 @@
> +;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 }
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