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Bugfix: SCEV incorrectly marks certain add recurrences as nsw

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When creating a scev for sext({X,+,Y}), scev checks if the expression
is equivalent to {sext X,+,zext Y}.  If it can prove that, it also
tags the original {X,+,Y} as <nsw>, which is not correct.

In the test case I run `-scalar-evolution` twice because the bug
manifests only once SCEV has run through and seen the `sext`
expressions (and then does a in-place mutation on {X,+,Y}).

Differential Revision: http://reviews.llvm.org/D7495



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228586 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjoy Das 2015-02-09 18:34:55 +00:00
parent 5a81520668
commit 2ceba77dfa
2 changed files with 36 additions and 2 deletions
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12
lib/Analysis/ScalarEvolution.cpp
View File

@ -1550,8 +1550,16 @@ const SCEV *ScalarEvolution::getSignExtendExpr(const SCEV *Op,
getMulExpr(WideMaxBECount,
getZeroExtendExpr(Step, WideTy)));
if (SAdd == OperandExtendedAdd) {
// Cache knowledge of AR NSW, which is propagated to this AddRec.
const_cast<SCEVAddRecExpr *>(AR)->setNoWrapFlags(SCEV::FlagNSW);
// If AR wraps around then
//
// abs(Step) * MaxBECount > unsigned-max(AR->getType())
// => SAdd != OperandExtendedAdd
//
// Thus (AR is not NW => SAdd != OperandExtendedAdd) <=>
// (SAdd == OperandExtendedAdd => AR is NW)
const_cast<SCEVAddRecExpr *>(AR)->setNoWrapFlags(SCEV::FlagNW);
// Return the expression with the addrec on the outside.
return getAddRecExpr(getSignExtendAddRecStart(AR, Ty, this),
getZeroExtendExpr(Step, Ty),

26
test/Analysis/ScalarEvolution/incorrect-nsw.ll Normal file
View File

@ -0,0 +1,26 @@
; RUN: opt -analyze -scalar-evolution -scalar-evolution < %s | FileCheck %s
define void @bad.nsw() {
; CHECK-LABEL: Classifying expressions for: @bad.nsw
; CHECK-LABEL: Classifying expressions for: @bad.nsw
entry:
br label %loop
loop:
%i = phi i8 [ -1, %entry ], [ %i.inc, %loop ]
; CHECK: %i = phi i8 [ -1, %entry ], [ %i.inc, %loop ]
; CHECK-NEXT: --> {-1,+,-128}<nw><%loop>
; CHECK-NOT: --> {-1,+,-128}<nsw><%loop>
%counter = phi i8 [ 0, %entry ], [ %counter.inc, %loop ]
%i.inc = add i8 %i, -128
%i.sext = sext i8 %i to i16
%counter.inc = add i8 %counter, 1
%continue = icmp eq i8 %counter, 1
br i1 %continue, label %exit, label %loop
exit:
ret void
}