[RISCV] Fold vfmv.f.s into load from stack

This is the f64/f32 version of llvm#109774.
I've left out f16 and bf16 for now because there's a separate issue where we can't select extract_vector_elt when f16/bf16 is a legal type, see llvm#110126.

Author: lukel97

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

@@ -784,6 +784,26 @@ MachineInstr *RISCVInstrInfo::foldMemoryOperandImpl(
       }
       break;
     }
+    if (RISCV::getRVVMCOpcode(MI.getOpcode()) == RISCV::VFMV_F_S) {
+      unsigned Log2SEW =
+          MI.getOperand(RISCVII::getSEWOpNum(MI.getDesc())).getImm();
+      if (STI.getXLen() < (1 << Log2SEW))
+        return nullptr;
+      switch (Log2SEW) {
+      case 4:
+        // TODO: Support f16/bf16
+        return nullptr;
+      case 5:
+        LoadOpc = RISCV::FLW;
+        break;
+      case 6:
+        LoadOpc = RISCV::FLD;
+        break;
+      default:
+        llvm_unreachable("Unexpected SEW");
+      }
+      break;
+    }
     return nullptr;
   case RISCV::SEXT_H:
     LoadOpc = RISCV::LH;

llvm/test/CodeGen/RISCV/rvv/stack-folding.ll

@@ -206,10 +206,7 @@ define double @f64(<vscale x 1 x double> %v, i1 %c) {
 ; RV64-NEXT:    #NO_APP
 ; RV64-NEXT:    beqz a0, .LBB4_2
 ; RV64-NEXT:  # %bb.1: # %truebb
-; RV64-NEXT:    addi a0, sp, 16
-; RV64-NEXT:    vl1r.v v8, (a0) # Unknown-size Folded Reload
-; RV64-NEXT:    vsetivli zero, 1, e64, m1, ta, ma
-; RV64-NEXT:    vfmv.f.s fa0, v8
+; RV64-NEXT:    fld fa0, 16(sp) # 8-byte Folded Reload
 ; RV64-NEXT:    j .LBB4_3
 ; RV64-NEXT:  .LBB4_2: # %falsebb
 ; RV64-NEXT:    fmv.d.x fa0, zero
@@ -244,10 +241,7 @@ define float @f32(<vscale x 2 x float> %v, i1 %c) {
 ; CHECK-NEXT:    #NO_APP
 ; CHECK-NEXT:    beqz a0, .LBB5_2
 ; CHECK-NEXT:  # %bb.1: # %truebb
-; CHECK-NEXT:    addi a0, sp, 16
-; CHECK-NEXT:    vl1r.v v8, (a0) # Unknown-size Folded Reload
-; CHECK-NEXT:    vsetivli zero, 1, e32, m1, ta, ma
-; CHECK-NEXT:    vfmv.f.s fa0, v8
+; CHECK-NEXT:    flw fa0, 16(sp) # 8-byte Folded Reload
 ; CHECK-NEXT:    j .LBB5_3
 ; CHECK-NEXT:  .LBB5_2: # %falsebb
 ; CHECK-NEXT:    fmv.w.x fa0, zero