[PATCH] D48662: [InstCombine] reverse canonicalization of binops to allow more shuffle folding

[Sanjay Patel via Phabricator via llvm-commits]

spatel added inline comments.


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Comment at: test/Transforms/InstCombine/shuffle_select.ll:600
 ; CHECK-NEXT:    [[V0:%.*]] = lshr <4 x i8> [[V:%.*]], <i8 3, i8 3, i8 3, i8 3>
-; CHECK-NEXT:    [[T1:%.*]] = or <4 x i8> [[V0]], <i8 undef, i8 undef, i8 -64, i8 -64>
-; CHECK-NEXT:    [[T2:%.*]] = add nuw nsw <4 x i8> [[V0]], <i8 1, i8 2, i8 undef, i8 undef>
-; CHECK-NEXT:    [[T3:%.*]] = shufflevector <4 x i8> [[T1]], <4 x i8> [[T2]], <4 x i32> <i32 4, i32 5, i32 2, i32 3>
+; CHECK-NEXT:    [[T3:%.*]] = add nsw <4 x i8> [[V0]], <i8 1, i8 2, i8 -64, i8 -64>
 ; CHECK-NEXT:    ret <4 x i8> [[T3]]
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RKSimon wrote:
> spatel wrote:
> > lebedev.ri wrote:
> > > Too bad alive does not work with vectors :/
> > > I'm not sure i understand why we drop `nuw` but keep `nsw`.
> > > If anything, i'd guess the `or` is implicit `nuw`,
> > > so i would be less suprized if we dropped `nsw` instead.
> > This transform drops the flags since 'or' has no wrapping.
> > We add back the 'nsw' in visitAdd, but it doesn't find 'nuw' is possible (a shortcoming in computeOverflowForUnsignedAdd)...it should be both I think.
> Please can you raise this as a poor codegen bug?
https://bugs.llvm.org/show_bug.cgi?id=38021


https://reviews.llvm.org/D48662