[llvm] [DAG] Fold trunc(avg(x, y)) for avgceil/floor u/s nodes if they have sufficient leading zero/sign bits (PR #152273)

[Simon Pilgrim via llvm-commits]

================
@@ -0,0 +1,92 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=aarch64-- -O2 -mattr=+neon < %s | FileCheck %s
+
+define <8 x i8> @test_avgceil_u(<8 x i16> %a, <8 x i16> %b) {
+; CHECK-LABEL: test_avgceil_u:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    bic v0.8h, #255, lsl #8
+; CHECK-NEXT:    bic v1.8h, #255, lsl #8
+; CHECK-NEXT:    uhadd v0.8h, v0.8h, v1.8h
+; CHECK-NEXT:    xtn v0.8b, v0.8h
+; CHECK-NEXT:    ret
+  %mask = insertelement <8 x i16> poison, i16 255, i32 0
+  %mask.splat = shufflevector <8 x i16> %mask, <8 x i16> poison, <8 x i32> zeroinitializer
+  %ta16 = and <8 x i16> %a, %mask.splat
+  %tb16 = and <8 x i16> %b, %mask.splat
+  %avg16 = call <8 x i16> @llvm.aarch64.neon.uhadd.v8i16(<8 x i16> %ta16, <8 x i16> %tb16)
+  %res = trunc <8 x i16> %avg16 to <8 x i8>
+  ret <8 x i8> %res
+}
+
+define <8 x i8> @test_avgceil_s(<8 x i16> %a, <8 x i16> %b) {
+; CHECK-LABEL: test_avgceil_s:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    movi v2.8h, #127
+; CHECK-NEXT:    mvni v3.8h, #127
+; CHECK-NEXT:    smin v0.8h, v0.8h, v2.8h
+; CHECK-NEXT:    smin v1.8h, v1.8h, v2.8h
+; CHECK-NEXT:    smax v0.8h, v0.8h, v3.8h
+; CHECK-NEXT:    smax v1.8h, v1.8h, v3.8h
+; CHECK-NEXT:    shadd v0.8h, v0.8h, v1.8h
+; CHECK-NEXT:    xtn v0.8b, v0.8h
+; CHECK-NEXT:    ret
+  %min = insertelement <8 x i16> poison, i16 -128, i32 0
+  %min.splat = shufflevector <8 x i16> %min, <8 x i16> poison, <8 x i32> zeroinitializer
+  %max = insertelement <8 x i16> poison, i16 127, i32 0
+  %max.splat = shufflevector <8 x i16> %max, <8 x i16> poison, <8 x i32> zeroinitializer
+  %ta16 = call <8 x i16> @llvm.smin.v8i16(<8 x i16> %a, <8 x i16> %max.splat)
+  %ta16.clamped = call <8 x i16> @llvm.smax.v8i16(<8 x i16> %ta16, <8 x i16> %min.splat)
+  %tb16 = call <8 x i16> @llvm.smin.v8i16(<8 x i16> %b, <8 x i16> %max.splat)
+  %tb16.clamped = call <8 x i16> @llvm.smax.v8i16(<8 x i16> %tb16, <8 x i16> %min.splat)
+  %avg16 = call <8 x i16> @llvm.aarch64.neon.shadd.v8i16(<8 x i16> %ta16.clamped, <8 x i16> %tb16.clamped)
+  %res = trunc <8 x i16> %avg16 to <8 x i8>
+  ret <8 x i8> %res
+}
+
+define <8 x i8> @test_avgfloor_u(<8 x i16> %a, <8 x i16> %b) {
+; CHECK-LABEL: test_avgfloor_u:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    bic v0.8h, #255, lsl #8
+; CHECK-NEXT:    bic v1.8h, #255, lsl #8
+; CHECK-NEXT:    uhadd v0.8h, v0.8h, v1.8h
+; CHECK-NEXT:    xtn v0.8b, v0.8h
+; CHECK-NEXT:    ret
+  %mask = insertelement <8 x i16> poison, i16 255, i32 0
+  %mask.splat = shufflevector <8 x i16> %mask, <8 x i16> poison, <8 x i32> zeroinitializer
+  %ta16 = and <8 x i16> %a, %mask.splat
+  %tb16 = and <8 x i16> %b, %mask.splat
----------------
RKSimon wrote:

why not use `splat (i16 255)`? it was added to avoid the messy shufflevector(insertelement) pattern

https://github.com/llvm/llvm-project/pull/152273