[llvm] [RISCV] Vectorize phi for loop carried @llvm.vector.reduce.fadd (PR #78244)

[Yingwei Zheng via llvm-commits]

================
@@ -103,6 +105,61 @@ bool RISCVCodeGenPrepare::visitAnd(BinaryOperator &BO) {
   return true;
 }
 
+// LLVM vector reduction intrinsics return a scalar result, but on RISC-V vector
+// reduction instructions write the result in the first element of a vector
+// register. So when a reduction in a loop uses a scalar phi, we end up with
+// unnecessary scalar moves:
+//
+// loop:
+// vfmv.s.f v10, fa0
+// vfredosum.vs v8, v8, v10
+// vfmv.f.s fa0, v8
+//
+// This mainly affects ordered fadd reductions, since other types of reduction
+// typically use element-wise vectorisation in the loop body. This tries to
+// vectorize any scalar phis that feed into a fadd reduction:
+//
+// loop:
+// %phi = phi <float> [ ..., %entry ], [ %acc, %loop]
+// %acc = call float @llvm.vector.reduce.fadd.nxv4f32(float %phi, <vscale x 2 x
+// float> %vec)
+//
+// ->
+//
+// loop:
+// %phi = phi <vscale x 2 x float> [ ..., %entry ], [ %acc.vec, %loop]
+// %phi.scalar = extractelement <vscale x 2 x float> %phi, i64 0
+// %acc = call float @llvm.vector.reduce.fadd.nxv4f32(float %x, <vscale x 2 x
+// float> %vec) %acc.vec = insertelement <vscale x 2 x float> poison, float
+// %acc.next, i64 0
----------------
dtcxzyw wrote:

```suggestion
// float> %vec)
// %acc.vec = insertelement <vscale x 2 x float> poison, float %acc.next, i64 0
```

https://github.com/llvm/llvm-project/pull/78244