[AMDGPU] Delete redundant s_or_b32 (#165261)

Transform sequences like:

```
s_cselect_b64 s[12:13], -1, 0
s_or_b32 s6, s12, s13
```

where s6 is dead to: 

`s_cselect_b64 s[12:13], -1, 0`

---------

Signed-off-by: John Lu <John.Lu@amd.com>

Author: LU-JOHN

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

@@ -10163,7 +10163,7 @@ static bool followSubRegDef(MachineInstr &MI,
 }
 
 MachineInstr *llvm::getVRegSubRegDef(const TargetInstrInfo::RegSubRegPair &P,
-                                     MachineRegisterInfo &MRI) {
+                                     const MachineRegisterInfo &MRI) {
   assert(MRI.isSSA());
   if (!P.Reg.isVirtual())
     return nullptr;
@@ -10628,6 +10628,8 @@ bool SIInstrInfo::analyzeCompare(const MachineInstr &MI, Register &SrcReg,
 static bool optimizeSCC(MachineInstr *SCCValid, MachineInstr *SCCRedefine,
                         const SIRegisterInfo &RI) {
   MachineInstr *KillsSCC = nullptr;
+  if (SCCValid->getParent() != SCCRedefine->getParent())
+    return false;
   for (MachineInstr &MI : make_range(std::next(SCCValid->getIterator()),
                                      SCCRedefine->getIterator())) {
     if (MI.modifiesRegister(AMDGPU::SCC, &RI))
@@ -10672,8 +10674,8 @@ bool SIInstrInfo::optimizeCompareInstr(MachineInstr &CmpInstr, Register SrcReg,
     if (CmpValue != 0)
       return false;
 
-    MachineInstr *Def = MRI->getUniqueVRegDef(SrcReg);
-    if (!Def || Def->getParent() != CmpInstr.getParent())
+    MachineInstr *Def = MRI->getVRegDef(SrcReg);
+    if (!Def)
       return false;
 
     // For S_OP that set SCC = DST!=0, do the transformation
@@ -10692,6 +10694,32 @@ bool SIInstrInfo::optimizeCompareInstr(MachineInstr &CmpInstr, Register SrcReg,
     if (!optimizeSCC(Def, &CmpInstr, RI))
       return false;
 
+    // If s_or_b32 result, sY, is unused (i.e. it is effectively a 64-bit
+    // s_cmp_lg of a register pair) and the inputs are the hi and lo-halves of a
+    // 64-bit foldableSelect then delete s_or_b32 in the sequence:
+    //    sX = s_cselect_b64 (non-zero imm), 0
+    //    sLo = copy sX.sub0
+    //    sHi = copy sX.sub1
+    //    sY = s_or_b32 sLo, sHi
+    if (Def->getOpcode() == AMDGPU::S_OR_B32 &&
+        MRI->use_nodbg_empty(Def->getOperand(0).getReg())) {
+      const MachineOperand &OrOpnd1 = Def->getOperand(1);
+      const MachineOperand &OrOpnd2 = Def->getOperand(2);
+      if (OrOpnd1.isReg() && OrOpnd2.isReg()) {
+        MachineInstr *Def1 = MRI->getVRegDef(OrOpnd1.getReg());
+        MachineInstr *Def2 = MRI->getVRegDef(OrOpnd2.getReg());
+        if (Def1 && Def1->getOpcode() == AMDGPU::COPY && Def2 &&
+            Def2->getOpcode() == AMDGPU::COPY && Def1->getOperand(1).isReg() &&
+            Def2->getOperand(1).isReg() &&
+            Def1->getOperand(1).getSubReg() == AMDGPU::sub0 &&
+            Def2->getOperand(1).getSubReg() == AMDGPU::sub1 &&
+            Def1->getOperand(1).getReg() == Def2->getOperand(1).getReg()) {
+          MachineInstr *Select = MRI->getVRegDef(Def1->getOperand(1).getReg());
+          if (Select && foldableSelect(*Select))
+            optimizeSCC(Select, Def, RI);
+        }
+      }
+    }
     return true;
   };
 
@@ -10721,8 +10749,8 @@ bool SIInstrInfo::optimizeCompareInstr(MachineInstr &CmpInstr, Register SrcReg,
     // s_cmp_lg_i32 (s_and_b32 $src, 1 << n), 1 << n => s_bitcmp0_b32 $src, n
     // s_cmp_lg_u64 (s_and_b64 $src, 1 << n), 1 << n => s_bitcmp0_b64 $src, n
 
-    MachineInstr *Def = MRI->getUniqueVRegDef(SrcReg);
-    if (!Def || Def->getParent() != CmpInstr.getParent())
+    MachineInstr *Def = MRI->getVRegDef(SrcReg);
+    if (!Def)
       return false;
 
     if (Def->getOpcode() != AMDGPU::S_AND_B32 &&

llvm/lib/Target/AMDGPU/SIInstrInfo.h

@@ -1687,7 +1687,7 @@ TargetInstrInfo::RegSubRegPair getRegSequenceSubReg(MachineInstr &MI,
 /// skipping copy like instructions and subreg-manipulation pseudos.
 /// Following another subreg of a reg:subreg isn't supported.
 MachineInstr *getVRegSubRegDef(const TargetInstrInfo::RegSubRegPair &P,
-                               MachineRegisterInfo &MRI);
+                               const MachineRegisterInfo &MRI);
 
 /// \brief Return false if EXEC is not changed between the def of \p VReg at \p
 /// DefMI and the use at \p UseMI. Should be run on SSA. Currently does not