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Make ScalarEvolution less aggressive with respect to no-wrap flags.

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ScalarEvolution currently lowers a subtraction recurrence to an add
recurrence with the same no-wrap flags as the subtraction.  This is
incorrect because `sub nsw X, Y` is not the same as `add nsw X, -Y`
and `sub nuw X, Y` is not the same as `add nuw X, -Y`.  This patch
fixes the issue, and adds two test cases demonstrating the bug.

Differential Revision: http://reviews.llvm.org/D7081



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226755 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjoy Das 2015-01-22 00:48:47 +00:00
parent 5b17b4ea2b
commit 48fcc07856
2 changed files with 48 additions and 8 deletions
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15
lib/Analysis/ScalarEvolution.cpp
View File

@ -3154,8 +3154,9 @@ const SCEV *ScalarEvolution::getMinusSCEV(const SCEV *LHS, const SCEV *RHS,
if (LHS == RHS)
return getConstant(LHS->getType(), 0);
// X - Y --> X + -Y
return getAddExpr(LHS, getNegativeSCEV(RHS), Flags);
// X - Y --> X + -Y.
// X -(nsw || nuw) Y --> X + -Y.
return getAddExpr(LHS, getNegativeSCEV(RHS));
}
/// getTruncateOrZeroExtend - Return a SCEV corresponding to a conversion of the
@ -3461,12 +3462,10 @@ const SCEV *ScalarEvolution::createNodeForPHI(PHINode *PN) {
if (isKnownPositive(getMinusSCEV(getSCEV(GEP), Ptr)))
Flags = setFlags(Flags, SCEV::FlagNUW);
}
} else if (const SubOperator *OBO =
dyn_cast<SubOperator>(BEValueV)) {
if (OBO->hasNoUnsignedWrap())
Flags = setFlags(Flags, SCEV::FlagNUW);
if (OBO->hasNoSignedWrap())
Flags = setFlags(Flags, SCEV::FlagNSW);
// We cannot transfer nuw and nsw flags from subtraction
// operations -- sub nuw X, Y is not the same as add nuw X, -Y
// for instance.
}
const SCEV *StartVal = getSCEV(StartValueV);

41
test/Analysis/ScalarEvolution/nw-sub-is-not-nw-add.ll Normal file
View File

@ -0,0 +1,41 @@
; RUN: opt -S -indvars < %s | FileCheck %s
; Check that SCEV does not assume sub nuw X Y == add nuw X, -Y
define void @f(i32* %loc) {
; CHECK-LABEL: @f
entry:
br label %loop
loop:
%idx = phi i32 [ 6, %entry ], [ %idx.dec, %loop ]
store i32 %idx, i32* %loc
%idx.dec = sub nuw i32 %idx, 1
%cond = icmp uge i32 %idx.dec, 5
br i1 %cond, label %loop, label %exit
; CHECK-NOT: br i1 true, label %loop, label %exit
exit:
ret void
}
declare void @use_i1(i1)
; Check that SCEV does not assume sub nsw X Y == add nsw X, -Y
define void @g(i32 %lim) {
; CHECK-LABEL: @g
entry:
br label %loop
loop:
%idx = phi i32 [ -1, %entry ], [ %idx.dec, %loop ]
%t = icmp sgt i32 %idx, 0
; CHECK-NOT: call void @use_i1(i1 false)
; CHECK: call void @use_i1(i1 %t)
call void @use_i1(i1 %t)
%idx.dec = sub nsw i32 %idx, -2147483648
%cond = icmp eq i32 %idx.dec, %lim
br i1 %cond, label %loop, label %exit
exit:
ret void
}