[PATCH] D109151: [InstCombine] Convert xor (ashr X, BW-1), C -> select(X >=s 0, C, ~C)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri accepted this revision.
lebedev.ri added a comment.
This revision is now accepted and ready to land.

Seems fine to me.



================
Comment at: llvm/test/Transforms/InstCombine/xor-ashr.ll:66-76
 define <4 x i8> @testv4i16i8_undef(<4 x i16> %add) {
 ; CHECK-LABEL: @testv4i16i8_undef(
-; CHECK-NEXT:    [[SH:%.*]] = ashr <4 x i16> [[ADD:%.*]], <i16 15, i16 undef, i16 15, i16 15>
-; CHECK-NEXT:    [[T:%.*]] = trunc <4 x i16> [[SH]] to <4 x i8>
-; CHECK-NEXT:    [[X:%.*]] = xor <4 x i8> [[T]], <i8 27, i8 27, i8 undef, i8 27>
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp sgt <4 x i16> [[ADD:%.*]], <i16 -1, i16 -1, i16 -1, i16 -1>
+; CHECK-NEXT:    [[X:%.*]] = select <4 x i1> [[TMP1]], <4 x i8> <i8 27, i8 27, i8 undef, i8 27>, <4 x i8> <i8 -28, i8 -28, i8 undef, i8 -28>
 ; CHECK-NEXT:    ret <4 x i8> [[X]]
 ;
   %sh = ashr <4 x i16> %add, <i16 15, i16 undef, i16 15, i16 15>
----------------
We fail to fully propagate undefs. This should be
https://alive2.llvm.org/ce/z/ZGMsRq
(see `Constant::mergeUndefsWith()`)


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https://reviews.llvm.org/D109151