[SelectionDAG] Flags are dropped when creating a new FMUL (#66701)

While simplifying some vector operators in DAG combine, we may need to
create new instructions for simplified vectors. At that time, we need to
make sure that all the flags of the new instruction are copied/modified
from the old instruction.

If "contract" is dropped from an instruction like FMUL, it may not
generate FMA instruction which would impact performance.

Here's an example where "contract" flag is dropped when FMUL is created.

Replacing.2 t42: v2f32 = fmul contract t41, t38
With: t48: v2f32 = fmul t38, t38

Co-authored-by: Sirish Pande <[email protected]>

Author: srpande

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -2990,8 +2990,9 @@ bool TargetLowering::SimplifyDemandedVectorElts(
     SDValue NewOp1 = SimplifyMultipleUseDemandedVectorElts(Op1, DemandedElts,
                                                            TLO.DAG, Depth + 1);
     if (NewOp0 || NewOp1) {
-      SDValue NewOp = TLO.DAG.getNode(
-          Opcode, SDLoc(Op), VT, NewOp0 ? NewOp0 : Op0, NewOp1 ? NewOp1 : Op1);
+      SDValue NewOp =
+          TLO.DAG.getNode(Opcode, SDLoc(Op), VT, NewOp0 ? NewOp0 : Op0,
+                          NewOp1 ? NewOp1 : Op1, Op->getFlags());
       return TLO.CombineTo(Op, NewOp);
     }
     return false;

llvm/test/CodeGen/AMDGPU/fma.ll

@@ -159,15 +159,14 @@ define float @fold_fmul_distributive(float %x, float %y) {
 define amdgpu_kernel void @vec_mul_scalar_add_fma(<2 x float> %a, <2 x float> %b, float %c1, ptr addrspace(1) %inptr) {
 ; GFX906-LABEL: vec_mul_scalar_add_fma:
 ; GFX906:       ; %bb.0:
+; GFX906-NEXT:    s_load_dword s8, s[0:1], 0x34
 ; GFX906-NEXT:    s_load_dwordx4 s[4:7], s[0:1], 0x24
-; GFX906-NEXT:    s_waitcnt lgkmcnt(0)
-; GFX906-NEXT:    s_load_dword s5, s[0:1], 0x34
 ; GFX906-NEXT:    s_load_dwordx2 s[2:3], s[0:1], 0x3c
 ; GFX906-NEXT:    v_mov_b32_e32 v0, 0
-; GFX906-NEXT:    v_mov_b32_e32 v1, s6
-; GFX906-NEXT:    v_mul_f32_e32 v1, s4, v1
 ; GFX906-NEXT:    s_waitcnt lgkmcnt(0)
-; GFX906-NEXT:    v_add_f32_e32 v1, s5, v1
+; GFX906-NEXT:    v_mov_b32_e32 v1, s8
+; GFX906-NEXT:    v_mov_b32_e32 v2, s6
+; GFX906-NEXT:    v_fmac_f32_e32 v1, s4, v2
 ; GFX906-NEXT:    global_store_dword v0, v1, s[2:3] offset:4
 ; GFX906-NEXT:    s_endpgm
   %gep = getelementptr float, ptr addrspace(1) %inptr, i32 1