[llvm] [GlobalISel] Add computeNumSignBits for G_SHUFFLE_VECTOR (PR #139505)

[David Green via llvm-commits]

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@@ -400,9 +400,10 @@ define <8 x i16> @missing_insert(<8 x i8> %b) {
 ;
 ; CHECK-GI-LABEL: missing_insert:
 ; CHECK-GI:       // %bb.0: // %entry
-; CHECK-GI-NEXT:    sshll v0.8h, v0.8b, #0
-; CHECK-GI-NEXT:    ext v1.16b, v0.16b, v0.16b, #4
-; CHECK-GI-NEXT:    mul v0.8h, v1.8h, v0.8h
+; CHECK-GI-NEXT:    sshll v1.8h, v0.8b, #0
+; CHECK-GI-NEXT:    ext v1.16b, v1.16b, v1.16b, #4
+; CHECK-GI-NEXT:    xtn v1.8b, v1.8h
+; CHECK-GI-NEXT:    smull v0.8h, v1.8b, v0.8b
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davemgreen wrote:

Yeah - It looks like a missing trunc(shuffle(sext)) fold, you can see the SDAG code is still further ahead.

https://github.com/llvm/llvm-project/pull/139505