[llvm] [SelectionDAG] Fold VECREDUCE_ADD of a constant BUILD_VECTOR (PR #207560)

[Simon Pilgrim via llvm-commits]

================
@@ -0,0 +1,53 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=riscv64 -mattr=+v -verify-machineinstrs < %s | FileCheck %s
+
+; Case 1: all-constant vector -> should fold to a plain constant (10)
+define i32 @test_const() {
+; CHECK-LABEL: test_const:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    li a0, 10
+; CHECK-NEXT:    ret
+  %r = call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> <i32 1, i32 2, i32 3, i32 4>)
+  ret i32 %r
+}
+
+; Case 2: vector containing an undef element -> should NOT fold
+define i32 @test_undef() {
+; CHECK-LABEL: test_undef:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetivli zero, 4, e32, m1, ta, ma
+; CHECK-NEXT:    vmv.s.x v8, zero
+; CHECK-NEXT:    vid.v v9
+; CHECK-NEXT:    vadd.vi v9, v9, 1
+; CHECK-NEXT:    vredsum.vs v8, v9, v8
+; CHECK-NEXT:    vmv.x.s a0, v8
+; CHECK-NEXT:    ret
+  %r = call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> <i32 1, i32 2, i32 undef, i32 4>)
+  ret i32 %r
+}
----------------
RKSimon wrote:

I'd prefer we add both tests tbh - the undef checker is being melodramatic

https://github.com/llvm/llvm-project/pull/207560