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[SCEV] ScalarEvolution::createSCEV(): clarify no-wrap flag propagation for shift by bitwidth-1 

Summary:
There was this comment here previously:
```
-        // It is currently not resolved how to interpret NSW for left
-        // shift by BitWidth - 1, so we avoid applying flags in that
-        // case. Remove this check (or this comment) once the situation
-        // is resolved. See
-        // http://lists.llvm.org/pipermail/llvm-dev/2015-April/084195.html
-        // and http://reviews.llvm.org/D8890 .
```
But langref was fixed in rL286785, and the behavior is pretty obvious:
http://volta.cs.utah.edu:8080/z/MM4WZP
^ nuw can always be propagated. nsw can be propagated if
either nuw is specified, or the shift is by *less* than bitwidth-1.

This mimics similar D81189 Reassociate change, alive2 is happy about that one.

I'm not sure `NUW` isn't being printed, but that seems unrelated.

Reviewers: mkazantsev, reames, sanjoy, nlopes, craig.topper, efriedma

Reviewed By: efriedma

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81243
This commit is contained in:
Roman Lebedev 2020-06-06 12:59:44 +03:00
parent ff1210edb6
commit c868335e24
No known key found for this signature in database
GPG Key ID: 083C3EBB4A1689E0
2 changed files with 14 additions and 10 deletions
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20
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -6453,15 +6453,19 @@ const SCEV *ScalarEvolution::createSCEV(Value *V) {
if (SA->getValue().uge(BitWidth))
break;
// It is currently not resolved how to interpret NSW for left
// shift by BitWidth - 1, so we avoid applying flags in that
// case. Remove this check (or this comment) once the situation
// is resolved. See
// http://lists.llvm.org/pipermail/llvm-dev/2015-April/084195.html
// and http://reviews.llvm.org/D8890 .
// We can safely preserve the nuw flag in all cases. It's also safe to
// turn a nuw nsw shl into a nuw nsw mul. However, nsw in isolation
// requires special handling. It can be preserved as long as we're not
// left shifting by bitwidth - 1.
auto Flags = SCEV::FlagAnyWrap;
if (BO->Op && SA->getValue().ult(BitWidth - 1))
Flags = getNoWrapFlagsFromUB(BO->Op);
if (BO->Op) {
auto MulFlags = getNoWrapFlagsFromUB(BO->Op);
if ((MulFlags & SCEV::FlagNSW) &&
((MulFlags & SCEV::FlagNUW) || SA->getValue().ult(BitWidth - 1)))
Flags = (SCEV::NoWrapFlags)(Flags | SCEV::FlagNSW);
if (MulFlags & SCEV::FlagNUW)
Flags = (SCEV::NoWrapFlags)(Flags | SCEV::FlagNUW);
}
Constant *X = ConstantInt::get(
getContext(), APInt::getOneBitSet(BitWidth, SA->getZExtValue()));

4
llvm/test/Analysis/ScalarEvolution/flags-from-poison.ll
View File

@ -559,11 +559,11 @@ loop:
%i = phi i32 [ %nexti, %loop ], [ %start, %entry ]
; CHECK: %index32 =
; CHECK: --> {(-2147483648 * %start),+,-2147483648}<%loop>
; CHECK: --> {(-2147483648 * %start),+,-2147483648}<nsw><%loop>
%index32 = shl nuw nsw i32 %i, 31
; CHECK: %index64 =
; CHECK: --> (sext i32 {(-2147483648 * %start),+,-2147483648}<%loop>
; CHECK: --> {(sext i32 (-2147483648 * %start) to i64),+,-2147483648}<nsw><%loop>
%index64 = sext i32 %index32 to i64
%ptr = getelementptr inbounds float, float* %input, i64 %index64