[llvm] [InstCombine] Pull extract through broadcast (PR #143380)

[Nikita Popov via llvm-commits]

================
@@ -0,0 +1,54 @@
+; RUN: opt -passes=instcombine -S < %s | FileCheck %s
+
+define float @extract_from_zero_init_shuffle(<2 x float> %1, i64 %idx) {
+; CHECK-LABEL: @extract_from_zero_init_shuffle(
+; CHECK-NEXT:    [[TMP1:%.*]] = extractelement <2 x float> [[W:%.*]], i64 0
+; CHECK-NEXT:    ret float [[TMP1]]
+;
+  %3 = shufflevector <2 x float> %1, <2 x float> poison, <4 x i32> zeroinitializer
+  %4 = extractelement <4 x float> %3, i64 %idx
+  ret float %4
+}
+
+
+define float @extract_from_general_splat(<2 x float> %1, i64 %idx) {
+; CHECK-LABEL: @extract_from_general_splat(
+; CHECK-NEXT:    [[TMP1:%.*]] = extractelement <2 x float> [[W:%.*]], i64 1
+; CHECK-NEXT:    ret float [[TMP1]]
+;
+  %3 = shufflevector <2 x float> %1, <2 x float> poison, <4 x i32> <i32 1, i32 1, i32 1, i32 1>
+  %4 = extractelement <4 x float> %3, i64 %idx
+  ret float %4
+}
+
+define float @extract_from_general_scalable_splat(<vscale x 2 x float> %1, i64 %idx) {
+; CHECK-LABEL: @extract_from_general_scalable_splat(
+; CHECK-NEXT:    [[TMP1:%.*]] = extractelement <vscale x 2 x float> [[W:%.*]], i64 0
+; CHECK-NEXT:    ret float [[TMP1]]
+;
+  %3 = shufflevector <vscale x 2 x float> %1, <vscale x 2 x float> poison, <vscale x 4 x i32> zeroinitializer
+  %4 = extractelement <vscale x 4 x float> %3, i64 %idx
+  ret float %4
+}
+
+define float @no_extract_from_general_no_splat_0(<2 x float> %1, i64 %idx) {
+; CHECK-LABEL: @no_extract_from_general_no_splat_0(
+; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <2 x float> [[W:%.*]], <2 x float> poison, <4 x i32> <i32 poison, i32 1, i32 1, i32 1>
+; CHECK-NEXT:    [[TMP2:%.*]] = extractelement <4 x float> [[TMP1]], i64 %idx
+; CHECK-NEXT:    ret float [[TMP2]]
+;
+  %3 = shufflevector <2 x float> %1, <2 x float> poison, <4 x i32> <i32 poison, i32 1, i32 1, i32 1>
+  %4 = extractelement <4 x float> %3, i64 %idx
+  ret float %4
+}
----------------
nikic wrote:

It's worth noting that this case is also safe to fold to extract 1. You can ignore the poison elements. But possibly not worth bothering with.

https://github.com/llvm/llvm-project/pull/143380