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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

llvm-svn: 230280
This commit is contained in:
Sanjoy Das 2015-02-23 23:22:58 +00:00
parent c9cf0151cf
commit 18c243b933
9 changed files with 69 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

33
llvm/lib/Analysis/ScalarEvolutionExpander.cpp
View File

@ -1063,6 +1063,34 @@ static bool canBeCheaplyTransformed(ScalarEvolution &SE,
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(const SCEVAddRecExpr *Normalized,
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);

30
llvm/test/Analysis/ScalarEvolution/scev-expander-incorrect-nowrap.ll Normal file
View File

@ -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
}

2
llvm/test/Analysis/ScalarEvolution/zext-signed-addrec.ll
View File

@ -43,7 +43,7 @@ if.end: ; preds = %if.end, %for.cond1.
%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

2
llvm/test/CodeGen/AArch64/arm64-scaled_iv.ll
View File

@ -20,7 +20,7 @@ for.body: ; preds = %for.body, %entry
%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

2
llvm/test/CodeGen/X86/avoid_complex_am.ll
View File

@ -22,7 +22,7 @@ for.body: ; preds = %for.body, %entry
%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

2
llvm/test/Transforms/IndVarSimplify/overflowcheck.ll
View File

@ -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

4
llvm/test/Transforms/IndVarSimplify/pr20680.ll
View File

@ -204,8 +204,8 @@ for.cond2.for.inc13_crit_edge: ; preds = %for.cond2.for.inc13
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

2
llvm/test/Transforms/LoopStrengthReduce/count-to-zero.ll
View File

@ -19,7 +19,7 @@ bb3: ; preds = %bb1
%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

2
llvm/test/Transforms/LoopStrengthReduce/uglygep.ll
View File

@ -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]