[llvm] r230280 - Bugfix: SCEVExpander incorrectly marks increment operations as no-wrap

Hans Wennborg hans at chromium.org
Tue Feb 24 08:22:14 PST 2015


I've reverted this in r230341 to unbreak our builds until this can be
fixed properly.

 - Hans

On Tue, Feb 24, 2015 at 5:52 AM, H.J. Lu <hjl.tools at gmail.com> wrote:
> It caused:
>
> http://llvm.org/bugs/show_bug.cgi?id=22674
>
> On Mon, Feb 23, 2015 at 3:23 PM, Sanjoy Das
> <sanjoy at playingwithpointers.com> wrote:
>> Author: sanjoy
>> Date: Mon Feb 23 17:22:58 2015
>> New Revision: 230280
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=230280&view=rev
>> Log:
>> Bugfix: SCEVExpander incorrectly marks increment operations as no-wrap
>>
>> When emitting the increment operation, SCEVExpander marks the
>> operation as nuw or nsw based on the flags on the preincrement SCEV.
>> This is incorrect because, for instance, it is possible that {-6,+,1}
>> is <nuw> while {-6,+,1}+1 = {-5,+,1} is not.
>>
>> This change teaches SCEV to mark the increment as nuw/nsw only if it
>> can explicitly prove that the increment operation won't overflow.
>>
>> Apart from the attached test case, another (more realistic) manifestation
>> of the bug can be seen in Transforms/IndVarSimplify/pr20680.ll.
>>
>> NOTE: this change was landed with an incorrect commit message in
>> rL230275 and was reverted for that reason in rL230279.  This commit
>> message is the correct one.
>>
>> Differential Revision: http://reviews.llvm.org/D7778
>>
>>
>> Added:
>>     llvm/trunk/test/Analysis/ScalarEvolution/scev-expander-incorrect-nowrap.ll
>> Modified:
>>     llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp
>>     llvm/trunk/test/Analysis/ScalarEvolution/zext-signed-addrec.ll
>>     llvm/trunk/test/CodeGen/AArch64/arm64-scaled_iv.ll
>>     llvm/trunk/test/CodeGen/X86/avoid_complex_am.ll
>>     llvm/trunk/test/Transforms/IndVarSimplify/overflowcheck.ll
>>     llvm/trunk/test/Transforms/IndVarSimplify/pr20680.ll
>>     llvm/trunk/test/Transforms/LoopStrengthReduce/count-to-zero.ll
>>     llvm/trunk/test/Transforms/LoopStrengthReduce/uglygep.ll
>>
>> Modified: llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp?rev=230280&r1=230279&r2=230280&view=diff
>> ==============================================================================
>> --- llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp (original)
>> +++ llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp Mon Feb 23 17:22:58 2015
>> @@ -1063,6 +1063,34 @@ static bool canBeCheaplyTransformed(Scal
>>    return false;
>>  }
>>
>> +static bool IsIncrementNSW(ScalarEvolution &SE, const SCEVAddRecExpr *AR) {
>> +  if (!isa<IntegerType>(AR->getType()))
>> +    return false;
>> +
>> +  unsigned BitWidth = cast<IntegerType>(AR->getType())->getBitWidth();
>> +  Type *WideTy = IntegerType::get(AR->getType()->getContext(), BitWidth * 2);
>> +  const SCEV *Step = AR->getStepRecurrence(SE);
>> +  const SCEV *OpAfterExtend = SE.getAddExpr(SE.getSignExtendExpr(Step, WideTy),
>> +                                            SE.getSignExtendExpr(AR, WideTy));
>> +  const SCEV *ExtendAfterOp =
>> +    SE.getSignExtendExpr(SE.getAddExpr(AR, Step), WideTy);
>> +  return ExtendAfterOp == OpAfterExtend;
>> +}
>> +
>> +static bool IsIncrementNUW(ScalarEvolution &SE, const SCEVAddRecExpr *AR) {
>> +  if (!isa<IntegerType>(AR->getType()))
>> +    return false;
>> +
>> +  unsigned BitWidth = cast<IntegerType>(AR->getType())->getBitWidth();
>> +  Type *WideTy = IntegerType::get(AR->getType()->getContext(), BitWidth * 2);
>> +  const SCEV *Step = AR->getStepRecurrence(SE);
>> +  const SCEV *OpAfterExtend = SE.getAddExpr(SE.getZeroExtendExpr(Step, WideTy),
>> +                                            SE.getZeroExtendExpr(AR, WideTy));
>> +  const SCEV *ExtendAfterOp =
>> +    SE.getZeroExtendExpr(SE.getAddExpr(AR, Step), WideTy);
>> +  return ExtendAfterOp == OpAfterExtend;
>> +}
>> +
>>  /// getAddRecExprPHILiterally - Helper for expandAddRecExprLiterally. Expand
>>  /// the base addrec, which is the addrec without any non-loop-dominating
>>  /// values, and return the PHI.
>> @@ -1213,10 +1241,11 @@ SCEVExpander::getAddRecExprPHILiterally(
>>        IVIncInsertPos : Pred->getTerminator();
>>      Builder.SetInsertPoint(InsertPos);
>>      Value *IncV = expandIVInc(PN, StepV, L, ExpandTy, IntTy, useSubtract);
>> +
>>      if (isa<OverflowingBinaryOperator>(IncV)) {
>> -      if (Normalized->getNoWrapFlags(SCEV::FlagNUW))
>> +      if (IsIncrementNUW(SE, Normalized))
>>          cast<BinaryOperator>(IncV)->setHasNoUnsignedWrap();
>> -      if (Normalized->getNoWrapFlags(SCEV::FlagNSW))
>> +      if (IsIncrementNSW(SE, Normalized))
>>          cast<BinaryOperator>(IncV)->setHasNoSignedWrap();
>>      }
>>      PN->addIncoming(IncV, Pred);
>>
>> Added: llvm/trunk/test/Analysis/ScalarEvolution/scev-expander-incorrect-nowrap.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/ScalarEvolution/scev-expander-incorrect-nowrap.ll?rev=230280&view=auto
>> ==============================================================================
>> --- llvm/trunk/test/Analysis/ScalarEvolution/scev-expander-incorrect-nowrap.ll (added)
>> +++ llvm/trunk/test/Analysis/ScalarEvolution/scev-expander-incorrect-nowrap.ll Mon Feb 23 17:22:58 2015
>> @@ -0,0 +1,30 @@
>> +; RUN: opt -indvars -S < %s | FileCheck %s
>> +
>> +declare void @use(i32)
>> +declare void @use.i8(i8)
>> +
>> +define void @f() {
>> +; CHECK-LABEL: @f
>> + entry:
>> +  br label %loop
>> +
>> + loop:
>> +; The only use for idx.mirror is to induce an nuw for %idx.  It does
>> +; not induce an nuw for %idx.inc
>> +  %idx.mirror = phi i8 [ -6, %entry ], [ %idx.mirror.inc, %loop ]
>> +  %idx = phi i8 [ -5, %entry ], [ %idx.inc, %loop ]
>> +
>> +  %idx.sext = sext i8 %idx to i32
>> +  call void @use(i32 %idx.sext)
>> +
>> +  %idx.mirror.inc = add nuw i8 %idx.mirror, 1
>> +  call void @use.i8(i8 %idx.mirror.inc)
>> +
>> +  %idx.inc = add i8 %idx, 1
>> +; CHECK-NOT: %indvars.iv.next = add nuw nsw i32 %indvars.iv, 1
>> +  %cmp = icmp ugt i8 %idx.inc, 0
>> +  br i1 %cmp, label %loop, label %exit
>> +
>> + exit:
>> +  ret void
>> +}
>>
>> Modified: llvm/trunk/test/Analysis/ScalarEvolution/zext-signed-addrec.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/ScalarEvolution/zext-signed-addrec.ll?rev=230280&r1=230279&r2=230280&view=diff
>> ==============================================================================
>> --- llvm/trunk/test/Analysis/ScalarEvolution/zext-signed-addrec.ll (original)
>> +++ llvm/trunk/test/Analysis/ScalarEvolution/zext-signed-addrec.ll Mon Feb 23 17:22:58 2015
>> @@ -43,7 +43,7 @@ if.end:
>>    %shl = and i32 %conv7, 510
>>    store i32 %shl, i32* @c, align 4
>>
>> -; CHECK: %lsr.iv.next = add i32 %lsr.iv, -258
>> +; CHECK: %lsr.iv.next = add nsw i32 %lsr.iv, -258
>>    %dec = add i8 %2, -1
>>
>>    %cmp2 = icmp sgt i8 %dec, -1
>>
>> Modified: llvm/trunk/test/CodeGen/AArch64/arm64-scaled_iv.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/arm64-scaled_iv.ll?rev=230280&r1=230279&r2=230280&view=diff
>> ==============================================================================
>> --- llvm/trunk/test/CodeGen/AArch64/arm64-scaled_iv.ll (original)
>> +++ llvm/trunk/test/CodeGen/AArch64/arm64-scaled_iv.ll Mon Feb 23 17:22:58 2015
>> @@ -20,7 +20,7 @@ for.body:
>>    %arrayidx = getelementptr inbounds double* %b, i64 %tmp
>>    %tmp1 = load double* %arrayidx, align 8
>>  ; The induction variable should carry the scaling factor: 1 * 8 = 8.
>> -; CHECK: [[IVNEXT]] = add nuw i64 [[IV]], 8
>> +; CHECK: [[IVNEXT]] = add nuw nsw i64 [[IV]], 8
>>    %indvars.iv.next = add i64 %indvars.iv, 1
>>    %arrayidx2 = getelementptr inbounds double* %c, i64 %indvars.iv.next
>>    %tmp2 = load double* %arrayidx2, align 8
>>
>> Modified: llvm/trunk/test/CodeGen/X86/avoid_complex_am.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/avoid_complex_am.ll?rev=230280&r1=230279&r2=230280&view=diff
>> ==============================================================================
>> --- llvm/trunk/test/CodeGen/X86/avoid_complex_am.ll (original)
>> +++ llvm/trunk/test/CodeGen/X86/avoid_complex_am.ll Mon Feb 23 17:22:58 2015
>> @@ -22,7 +22,7 @@ for.body:
>>    %arrayidx = getelementptr inbounds double* %b, i64 %tmp
>>    %tmp1 = load double* %arrayidx, align 8
>>  ; The induction variable should carry the scaling factor: 1.
>> -; CHECK: [[IVNEXT]] = add nuw i64 [[IV]], 1
>> +; CHECK: [[IVNEXT]] = add nuw nsw i64 [[IV]], 1
>>    %indvars.iv.next = add i64 %indvars.iv, 1
>>    %arrayidx2 = getelementptr inbounds double* %c, i64 %indvars.iv.next
>>    %tmp2 = load double* %arrayidx2, align 8
>>
>> Modified: llvm/trunk/test/Transforms/IndVarSimplify/overflowcheck.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IndVarSimplify/overflowcheck.ll?rev=230280&r1=230279&r2=230280&view=diff
>> ==============================================================================
>> --- llvm/trunk/test/Transforms/IndVarSimplify/overflowcheck.ll (original)
>> +++ llvm/trunk/test/Transforms/IndVarSimplify/overflowcheck.ll Mon Feb 23 17:22:58 2015
>> @@ -9,7 +9,7 @@ target triple = "x86_64-apple-macosx"
>>  ; CHECK: @llvm.sadd.with.overflow
>>  ; CHECK-LABEL: loop2:
>>  ; CHECK-NOT: extractvalue
>> -; CHECK: add nuw nsw
>> +; CHECK: add nuw
>>  ; CHECK: @llvm.sadd.with.overflow
>>  ; CHECK-LABEL: loop3:
>>  ; CHECK-NOT: extractvalue
>>
>> Modified: llvm/trunk/test/Transforms/IndVarSimplify/pr20680.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IndVarSimplify/pr20680.ll?rev=230280&r1=230279&r2=230280&view=diff
>> ==============================================================================
>> --- llvm/trunk/test/Transforms/IndVarSimplify/pr20680.ll (original)
>> +++ llvm/trunk/test/Transforms/IndVarSimplify/pr20680.ll Mon Feb 23 17:22:58 2015
>> @@ -204,8 +204,8 @@ for.cond2.for.inc13_crit_edge:
>>    br label %for.inc13
>>
>>  ; CHECK: [[for_inc13]]:
>> -; CHECK-NEXT: %[[indvars_iv_next]] = add nuw nsw i32 %[[indvars_iv]], 1
>> -; CHECK-NEXT: %[[exitcond4:.*]] = icmp ne i32 %[[indvars_iv]], -1
>> +; CHECK-NEXT: %[[indvars_iv_next]] = add nsw i32 %[[indvars_iv]], 1
>> +; CHECK-NEXT: %[[exitcond4:.*]] = icmp ne i32 %[[indvars_iv_next]], 0
>>  ; CHECK-NEXT: br i1 %[[exitcond4]], label %[[for_cond2_preheader]], label %[[for_end15:.*]]
>>  for.inc13:                                        ; preds = %for.cond2.for.inc13_crit_edge, %for.cond2.preheader
>>    %inc14 = add i8 %storemerge15, 1
>>
>> Modified: llvm/trunk/test/Transforms/LoopStrengthReduce/count-to-zero.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopStrengthReduce/count-to-zero.ll?rev=230280&r1=230279&r2=230280&view=diff
>> ==============================================================================
>> --- llvm/trunk/test/Transforms/LoopStrengthReduce/count-to-zero.ll (original)
>> +++ llvm/trunk/test/Transforms/LoopStrengthReduce/count-to-zero.ll Mon Feb 23 17:22:58 2015
>> @@ -19,7 +19,7 @@ bb3:
>>    %tmp4 = add i32 %c_addr.1, -1                   ; <i32> [#uses=1]
>>    %c_addr.1.be = select i1 %tmp2, i32 %tmp3, i32 %tmp4 ; <i32> [#uses=1]
>>    %indvar.next = add i32 %indvar, 1               ; <i32> [#uses=1]
>> -; CHECK: add i32 %lsr.iv, -1
>> +; CHECK: add nsw i32 %lsr.iv, -1
>>    br label %bb6
>>
>>  bb6:                                              ; preds = %bb3, %entry
>>
>> Modified: llvm/trunk/test/Transforms/LoopStrengthReduce/uglygep.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopStrengthReduce/uglygep.ll?rev=230280&r1=230279&r2=230280&view=diff
>> ==============================================================================
>> --- llvm/trunk/test/Transforms/LoopStrengthReduce/uglygep.ll (original)
>> +++ llvm/trunk/test/Transforms/LoopStrengthReduce/uglygep.ll Mon Feb 23 17:22:58 2015
>> @@ -59,7 +59,7 @@ bb:
>>  ; CHECK: loop0:
>>  ; Induction variable is initialized to -2.
>>  ; CHECK-NEXT: [[PHIIV:%[^ ]+]] = phi i32 [ [[IVNEXT:%[^ ]+]], %loop0 ], [ -2, %bb ]
>> -; CHECK-NEXT: [[IVNEXT]] = add i32 [[PHIIV]], 1
>> +; CHECK-NEXT: [[IVNEXT]] = add nuw nsw i32 [[PHIIV]], 1
>>  ; CHECK-NEXT: br i1 false, label %loop0, label %bb0
>>  loop0:                                            ; preds = %loop0, %bb
>>    %i0 = phi i32 [ %i0.next, %loop0 ], [ 0, %bb ]  ; <i32> [#uses=2]
>>
>>
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>
>
>
> --
> H.J.
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