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

Sanjoy Das sanjoy at playingwithpointers.com
Mon Feb 23 15:23:01 PST 2015


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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