[AMDGPU] Translate s_and/s_andn2 to s_mov in vcc optimisation

[critson]

Summary

When SCC is dead, but VCC is required then replace s_and / s_andn2
with s_mov into VCC when mask value is 0 or -1.

[arsenm]

llvm/lib/Target/AMDGPU/SIPreEmitPeephole.cpp

140–141

Can we do this earlier? Removing the SCC def earlier would be more useful

[rampitec]

It seems wave32 failure is real.

[critson]

Add missing test diffs.

[critson]

llvm/lib/Target/AMDGPU/SIPreEmitPeephole.cpp

140–141

To the best of my understanding, the earliest this optimisation becomes available is after "Branch Probability Basic Block Placement". Which is not much earlier.

[rampitec]

LGTM

llvm/lib/Target/AMDGPU/SIPreEmitPeephole.cpp

bool SIPreEmitPeephole::optimizeVccBranch(MachineInstr &MI) const {
   bool Changed = false;
   MachineBasicBlock &MBB = *MI.getParent();
   const GCNSubtarget &ST = MBB.getParent()->getSubtarget<GCNSubtarget>();
   const bool IsWave32 = ST.isWave32();
   const unsigned CondReg = TRI->getVCC();
   const unsigned ExecReg = IsWave32 ? AMDGPU::EXEC_LO : AMDGPU::EXEC;
   const unsigned And = IsWave32 ? AMDGPU::S_AND_B32 : AMDGPU::S_AND_B64;
   const unsigned AndN2 = IsWave32 ? AMDGPU::S_ANDN2_B32 : AMDGPU::S_ANDN2_B64;
+  const unsigned Mov = IsWave32 ? AMDGPU::S_MOV_B32 : AMDGPU::S_MOV_B64;
 
   MachineBasicBlock::reverse_iterator A = MI.getReverseIterator(),
                                       E = MBB.rend();
   bool ReadsCond = false;
   unsigned Threshold = 5;
   for (++A; A != E; ++A) {
     if (!--Threshold)
       return false;
if (Op2.isReg()) {
     llvm_unreachable("Op2 must be register or immediate");
   }
 
   // Invert mask for s_andn2
   assert(MaskValue == 0 || MaskValue == -1);
   if (A->getOpcode() == AndN2)
     MaskValue = ~MaskValue;
 
-  if (!ReadsCond && A->registerDefIsDead(AMDGPU::SCC) &&
-      MI.killsRegister(CondReg, TRI))
+  if (!ReadsCond && A->registerDefIsDead(AMDGPU::SCC)) {
+    if (!MI.killsRegister(CondReg, TRI)) {
+      // Replace AND with MOV
+      if (MaskValue == 0) {
+        BuildMI(*A->getParent(), *A, A->getDebugLoc(), TII->get(Mov), CondReg)
+            .addImm(0);
+      } else {
+        BuildMI(*A->getParent(), *A, A->getDebugLoc(), TII->get(Mov), CondReg)
+            .addReg(ExecReg);
+      }
+    }
+    // Remove AND instruction
     A->eraseFromParent();
+  }
 
   bool IsVCCZ = MI.getOpcode() == AMDGPU::S_CBRANCH_VCCZ;
   if (SReg == ExecReg) {
     // EXEC is updated directly
     if (IsVCCZ) {
       MI.eraseFromParent();
       return true;
     }

llvm/test/CodeGen/AMDGPU/infinite-loop.ll

 ; SI-NEXT:    v_mov_b32_e32 v0, 0x3e7
 ; SI-NEXT:    s_waitcnt lgkmcnt(0)
 ; SI-NEXT:    buffer_store_dword v0, off, s[4:7], 0
 ; SI-NEXT:    s_andn2_b64 exec, exec, s[2:3]
 ; SI-NEXT:    s_cbranch_execnz BB3_3
 ; SI-NEXT:  ; %bb.4: ; %loop.exit.guard
 ; SI-NEXT:    ; in Loop: Header=BB3_2 Depth=1
 ; SI-NEXT:    s_or_b64 exec, exec, s[2:3]
-; SI-NEXT:    s_and_b64 vcc, exec, 0
+; SI-NEXT:    s_mov_b64 vcc, 0
 ; SI-NEXT:    s_branch BB3_2
 ; SI-NEXT:  BB3_5: ; %UnifiedReturnBlock
 ; SI-NEXT:    s_endpgm
 ; IR-LABEL: @infinite_loop_nest_ret(
 ; IR-NEXT:  entry:
 ; IR-NEXT:    [[TMP:%.*]] = tail call i32 @llvm.amdgcn.workitem.id.x()
 ; IR-NEXT:    [[COND1:%.*]] = icmp eq i32 [[TMP]], 1
 ; IR-NEXT:    br i1 [[COND1]], label [[OUTER_LOOP:%.*]], label [[UNIFIEDRETURNBLOCK:%.*]]

llvm/test/CodeGen/AMDGPU/insert-skip-from-vcc.mir

bb.1:
     S_NOP 0
 
   bb.2:
     $sgpr0_sgpr1 = S_MOV_B64 -1
     $vcc = S_ANDN2_B64 $exec, killed $sgpr0_sgpr1, implicit-def dead $scc
     S_CBRANCH_VCCNZ %bb.1, implicit killed $vcc
     S_ENDPGM 0
 ...
+---
+# GCN-LABEL: name: and_0_mov
+# GCN: bb.2:
+# GCN-NOT: S_AND
+# GCN: $vcc = S_MOV_B64 0
+# GCN-NEXT: S_BRANCH %bb.1
+name:            and_0_mov
+body:             |
+  bb.0:
+    S_NOP 0
+
+  bb.1:
+    S_NOP 0
+
+  bb.2:
+    $sgpr0_sgpr1 = S_MOV_B64 0
+    $vcc = S_AND_B64 $exec, killed $sgpr0_sgpr1, implicit-def dead $scc
+    S_CBRANCH_VCCZ %bb.1, implicit $vcc
+    S_ENDPGM 0
+...
+---
+# GCN-LABEL: name: andn2_m1_mov
+# GCN: bb.2:
+# GCN-NOT: S_ANDN2
+# GCN: $vcc = S_MOV_B64 0
+# GCN-NEXT: S_BRANCH %bb.1
+name:            andn2_m1_mov
+body:             |
+  bb.0:
+    S_NOP 0
+
+  bb.1:
+    S_NOP 0
+
+  bb.2:
+    $sgpr0_sgpr1 = S_MOV_B64 -1
+    $vcc = S_ANDN2_B64 $exec, killed $sgpr0_sgpr1, implicit-def dead $scc
+    S_CBRANCH_VCCZ %bb.1, implicit $vcc
+    S_ENDPGM 0
+...
+---
+# GCN-LABEL: name: and_m1_mov
+# GCN: bb.2:
+# GCN-NOT: S_AND
+# GCN: $vcc = S_MOV_B64 $exec
+# GCN-NEXT: S_CBRANCH_EXECZ %bb.1, implicit $exec
+name:            and_m1_mov
+body:             |
+  bb.0:
+    S_NOP 0
+
+  bb.1:
+    S_NOP 0
+
+  bb.2:
+    $sgpr0_sgpr1 = S_MOV_B64 -1
+    $vcc = S_AND_B64 $exec, killed $sgpr0_sgpr1, implicit-def dead $scc
+    S_CBRANCH_VCCZ %bb.1, implicit $vcc
+    S_ENDPGM 0
+...
+---
+# GCN-LABEL: name: andn2_0_mov
+# GCN: bb.2:
+# GCN-NOT: S_ANDN2
+# GCN: $vcc = S_MOV_B64 $exec
+# GCN-NEXT: S_CBRANCH_EXECZ %bb.1, implicit $exec
+name:            andn2_0_mov
+body:             |
+  bb.0:
+    S_NOP 0
+
+  bb.1:
+    S_NOP 0
+
+  bb.2:
+    $sgpr0_sgpr1 = S_MOV_B64 0
+    $vcc = S_ANDN2_B64 $exec, killed $sgpr0_sgpr1, implicit-def dead $scc
+    S_CBRANCH_VCCZ %bb.1, implicit $vcc
+    S_ENDPGM 0
+...
+---
+# GCN-LABEL: name: and_0_scc_req
+# GCN: bb.2:
+# GCN-NOT: S_MOV_
+# GCN: S_AND_
+# GCN-NEXT: S_BRANCH %bb.1
+name:            and_0_scc_req
+body:             |
+  bb.0:
+    S_NOP 0
+
+  bb.1:
+    S_NOP 0
+
+  bb.2:
+    $sgpr0_sgpr1 = S_MOV_B64 0
+    $vcc = S_AND_B64 $exec, killed $sgpr0_sgpr1, implicit-def $scc
+    S_CBRANCH_VCCZ %bb.1, implicit $vcc
+    S_ENDPGM 0
+...
+---
+# GCN-LABEL: name: andn2_m1_scc_req
+# GCN: bb.2:
+# GCN-NOT: S_MOV_
+# GCN: S_ANDN2_
+# GCN-NEXT: S_BRANCH %bb.1
+name:            andn2_m1_scc_req
+body:             |
+  bb.0:
+    S_NOP 0
+
+  bb.1:
+    S_NOP 0
+
+  bb.2:
+    $sgpr0_sgpr1 = S_MOV_B64 -1
+    $vcc = S_ANDN2_B64 $exec, killed $sgpr0_sgpr1, implicit-def $scc
+    S_CBRANCH_VCCZ %bb.1, implicit $vcc
+    S_ENDPGM 0
+...