[AMDGPU][SelectionDAG] Use COPY instead of S_MOV_B32 to assign values to M0 (#132957)

This is consistent with what's done on GISel. This allows the register
coalescer to remove the redundant intermediate `s_mov_b32` instructions
by using `m0` directly as the result register.

Author: jmmartinez

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -4691,7 +4691,7 @@ emitLoadM0FromVGPRLoop(const SIInstrInfo *TII, MachineRegisterInfo &MRI,
   } else {
     // Move index from VCC into M0
     if (Offset == 0) {
-      BuildMI(LoopBB, I, DL, TII->get(AMDGPU::S_MOV_B32), AMDGPU::M0)
+      BuildMI(LoopBB, I, DL, TII->get(AMDGPU::COPY), AMDGPU::M0)
           .addReg(CurrentIdxReg, RegState::Kill);
     } else {
       BuildMI(LoopBB, I, DL, TII->get(AMDGPU::S_ADD_I32), AMDGPU::M0)
@@ -4805,7 +4805,7 @@ static void setM0ToIndexFromSGPR(const SIInstrInfo *TII,
 
   if (Offset == 0) {
     // clang-format off
-    BuildMI(*MBB, I, DL, TII->get(AMDGPU::S_MOV_B32), AMDGPU::M0)
+    BuildMI(*MBB, I, DL, TII->get(AMDGPU::COPY), AMDGPU::M0)
         .add(*Idx);
     // clang-format on
   } else {
@@ -5400,9 +5400,11 @@ SITargetLowering::EmitInstrWithCustomInserter(MachineInstr &MI,
     return BB;
   }
   case AMDGPU::SI_INIT_M0: {
+    MachineOperand &M0Init = MI.getOperand(0);
     BuildMI(*BB, MI.getIterator(), MI.getDebugLoc(),
-            TII->get(AMDGPU::S_MOV_B32), AMDGPU::M0)
-        .add(MI.getOperand(0));
+            TII->get(M0Init.isReg() ? AMDGPU::COPY : AMDGPU::S_MOV_B32),
+            AMDGPU::M0)
+        .add(M0Init);
     MI.eraseFromParent();
     return BB;
   }

llvm/test/CodeGen/AMDGPU/extract_vector_dynelt.ll

@@ -301,12 +301,11 @@ define amdgpu_kernel void @double8_extelt(ptr addrspace(1) %out, i32 %sel) {
 ; GCN-NEXT:    s_mov_b32 s10, s0
 ; GCN-NEXT:    s_mov_b32 s12, s0
 ; GCN-NEXT:    s_mov_b32 s14, s0
-; GCN-NEXT:    s_waitcnt lgkmcnt(0)
-; GCN-NEXT:    s_lshl_b32 s18, s18, 1
 ; GCN-NEXT:    v_mov_b32_e32 v0, s0
 ; GCN-NEXT:    v_mov_b32_e32 v1, s1
 ; GCN-NEXT:    v_mov_b32_e32 v15, s15
-; GCN-NEXT:    s_mov_b32 m0, s18
+; GCN-NEXT:    s_waitcnt lgkmcnt(0)
+; GCN-NEXT:    s_lshl_b32 m0, s18, 1
 ; GCN-NEXT:    v_mov_b32_e32 v2, s2
 ; GCN-NEXT:    v_mov_b32_e32 v3, s3
 ; GCN-NEXT:    v_mov_b32_e32 v4, s4
@@ -352,11 +351,10 @@ define amdgpu_kernel void @double7_extelt(ptr addrspace(1) %out, i32 %sel) {
 ; GCN-NEXT:    s_mov_b32 s10, s0
 ; GCN-NEXT:    s_mov_b32 s12, s0
 ; GCN-NEXT:    s_waitcnt lgkmcnt(0)
-; GCN-NEXT:    s_lshl_b32 s16, s16, 1
 ; GCN-NEXT:    v_mov_b32_e32 v0, s0
 ; GCN-NEXT:    v_mov_b32_e32 v1, s1
 ; GCN-NEXT:    v_mov_b32_e32 v15, s15
-; GCN-NEXT:    s_mov_b32 m0, s16
+; GCN-NEXT:    s_lshl_b32 m0, s16, 1
 ; GCN-NEXT:    v_mov_b32_e32 v2, s2
 ; GCN-NEXT:    v_mov_b32_e32 v3, s3
 ; GCN-NEXT:    v_mov_b32_e32 v4, s4
@@ -451,12 +449,11 @@ define amdgpu_kernel void @double15_extelt(ptr addrspace(1) %out, i32 %sel) {
 ; GCN-NEXT:    s_mov_b32 s60, s36
 ; GCN-NEXT:    s_mov_b32 s62, s36
 ; GCN-NEXT:    s_mov_b32 s64, s36
-; GCN-NEXT:    s_waitcnt lgkmcnt(0)
-; GCN-NEXT:    s_lshl_b32 s2, s2, 1
 ; GCN-NEXT:    v_mov_b32_e32 v0, s36
 ; GCN-NEXT:    v_mov_b32_e32 v1, s37
 ; GCN-NEXT:    v_mov_b32_e32 v31, s67
-; GCN-NEXT:    s_mov_b32 m0, s2
+; GCN-NEXT:    s_waitcnt lgkmcnt(0)
+; GCN-NEXT:    s_lshl_b32 m0, s2, 1
 ; GCN-NEXT:    v_mov_b32_e32 v2, s38
 ; GCN-NEXT:    v_mov_b32_e32 v3, s39
 ; GCN-NEXT:    v_mov_b32_e32 v4, s40
@@ -535,12 +532,11 @@ define amdgpu_kernel void @double16_extelt(ptr addrspace(1) %out, i32 %sel) {
 ; GCN-NEXT:    s_mov_b32 s62, s36
 ; GCN-NEXT:    s_mov_b32 s64, s36
 ; GCN-NEXT:    s_mov_b32 s66, s36
-; GCN-NEXT:    s_waitcnt lgkmcnt(0)
-; GCN-NEXT:    s_lshl_b32 s2, s2, 1
 ; GCN-NEXT:    v_mov_b32_e32 v0, s36
 ; GCN-NEXT:    v_mov_b32_e32 v1, s37
 ; GCN-NEXT:    v_mov_b32_e32 v31, s67
-; GCN-NEXT:    s_mov_b32 m0, s2
+; GCN-NEXT:    s_waitcnt lgkmcnt(0)
+; GCN-NEXT:    s_lshl_b32 m0, s2, 1
 ; GCN-NEXT:    v_mov_b32_e32 v2, s38
 ; GCN-NEXT:    v_mov_b32_e32 v3, s39
 ; GCN-NEXT:    v_mov_b32_e32 v4, s40