[llvm] [RISCV][DAGCombiner] Fix potential missed combine in VL->VW extension (PR #168026)

[Kai Lin via llvm-commits]

================
@@ -0,0 +1,54 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc -mtriple=riscv32 -mattr=+v -verify-machineinstrs < %s | FileCheck %s --check-prefixes=CHECK,RV32
+; RUN: llc -mtriple=riscv64 -mattr=+v -verify-machineinstrs < %s | FileCheck %s --check-prefixes=CHECK,RV64
+
+define void @matmul_min(ptr %vptr, ptr %scalars, ptr %acc0_ptr, ptr %acc1_ptr) {
+; CHECK-LABEL: matmul_min:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    li a4, 64
+; CHECK-NEXT:    li a5, 32
+; CHECK-NEXT:    vsetvli zero, a5, e8, m2, ta, ma
+; CHECK-NEXT:    vle8.v v16, (a0)
+; CHECK-NEXT:    lb a0, 0(a1)
+; CHECK-NEXT:    lb a1, 1(a1)
+; CHECK-NEXT:    vsetvli zero, a4, e8, m4, ta, ma
+; CHECK-NEXT:    vle8.v v8, (a2)
+; CHECK-NEXT:    vle8.v v12, (a3)
+; CHECK-NEXT:    vsetvli zero, a5, e8, m2, ta, ma
+; CHECK-NEXT:    vwmacc.vx v8, a0, v16
+; CHECK-NEXT:    vwmacc.vx v12, a1, v16
+; CHECK-NEXT:    vsetvli zero, a4, e8, m4, ta, ma
+; CHECK-NEXT:    vse8.v v8, (a2)
+; CHECK-NEXT:    vse8.v v12, (a3)
+; CHECK-NEXT:    ret
+entry:
+  %acc0 = load <32 x i16>, ptr %acc0_ptr, align 1
+  %acc1 = load <32 x i16>, ptr %acc1_ptr, align 1
+
+  %v8 = load <32 x i8>, ptr %vptr, align 1
+  %v16 = sext <32 x i8> %v8 to <32 x i16>
+
+  %s0_ptr = getelementptr i8, ptr %scalars, i32 0
+  %s0_i8 = load i8, ptr %s0_ptr, align 1
+  %s0_i16 = sext i8 %s0_i8 to i16
+  %tmp0 = insertelement <32 x i16> undef, i16 %s0_i16, i32 0
+  %splat0 = shufflevector <32 x i16> %tmp0, <32 x i16> undef, <32 x i32> zeroinitializer
+  %mul0 = mul <32 x i16> %splat0, %v16
+  %add0 = add <32 x i16> %mul0, %acc0
+
+  %s1_ptr = getelementptr i8, ptr %scalars, i32 1
+  %s1_i8 = load i8, ptr %s1_ptr, align 1
+  %s1_i16 = sext i8 %s1_i8 to i16
+  %tmp1 = insertelement <32 x i16> undef, i16 %s1_i16, i32 0
+  %splat1 = shufflevector <32 x i16> %tmp1, <32 x i16> undef, <32 x i32> zeroinitializer
----------------
OMG-link wrote:

It looks like there were 4 uses of `undef` but only 3 of them were changed in your suggestion. I don't know how to append a new change and commit them together online, so I rebuilt them locally. Please take a look.

```diff
diff --git a/llvm/test/CodeGen/RISCV/rvv/combine-vl-vw-macc.ll b/llvm/test/CodeGen/RISCV/rvv/combine-vl-vw-macc.ll
index 8dd8e07dbb6e..943d8d2409ff 100644
--- a/llvm/test/CodeGen/RISCV/rvv/combine-vl-vw-macc.ll
+++ b/llvm/test/CodeGen/RISCV/rvv/combine-vl-vw-macc.ll
@@ -31,16 +31,16 @@ entry:
   %s0_ptr = getelementptr i8, ptr %scalars, i32 0
   %s0_i8 = load i8, ptr %s0_ptr, align 1
   %s0_i16 = sext i8 %s0_i8 to i16
-  %tmp0 = insertelement <32 x i16> undef, i16 %s0_i16, i32 0
-  %splat0 = shufflevector <32 x i16> %tmp0, <32 x i16> undef, <32 x i32> zeroinitializer
+  %tmp0 = insertelement <32 x i16> poison, i16 %s0_i16, i32 0
+  %splat0 = shufflevector <32 x i16> %tmp0, <32 x i16> poison, <32 x i32> zeroinitializer
   %mul0 = mul <32 x i16> %splat0, %v16
   %add0 = add <32 x i16> %mul0, %acc0
 
   %s1_ptr = getelementptr i8, ptr %scalars, i32 1
   %s1_i8 = load i8, ptr %s1_ptr, align 1
   %s1_i16 = sext i8 %s1_i8 to i16
-  %tmp1 = insertelement <32 x i16> undef, i16 %s1_i16, i32 0
-  %splat1 = shufflevector <32 x i16> %tmp1, <32 x i16> undef, <32 x i32> zeroinitializer
+  %tmp1 = insertelement <32 x i16> poison, i16 %s1_i16, i32 0
+  %splat1 = shufflevector <32 x i16> %tmp1, <32 x i16> poison, <32 x i32> zeroinitializer
   %mul1 = mul <32 x i16> %splat1, %v16
   %add1 = add <32 x i16> %mul1, %acc1
```

https://github.com/llvm/llvm-project/pull/168026