[AMDGPU] Delete redundant s_or_b32

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

@@ -10160,7 +10160,7 @@ 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;
@@ -10689,6 +10689,25 @@ bool SIInstrInfo::optimizeCompareInstr(MachineInstr &CmpInstr, Register SrcReg,
     if (!optimizeSCC(Def, &CmpInstr, RI))
       return false;
 
+    // If s_or_b32 result is unused (i.e. it is effectively a 64-bit s_cmp_lg of
+    // a register pair) and the input is a 64-bit foldableSelect then transform:
+    //
+    //   (s_or_b32 (S_CSELECT_B64 (non-zero imm), 0), 0 => (S_CSELECT_B64
+    //   (non-zero
+    //     imm), 0)
+    if (Def->getOpcode() == AMDGPU::S_OR_B32 &&
+        MRI->use_nodbg_empty(Def->getOperand(0).getReg())) {
+      MachineOperand OrOpnd1 = Def->getOperand(1);
+      MachineOperand OrOpnd2 = Def->getOperand(2);
+
+      if (OrOpnd1.isReg() && OrOpnd2.isReg() &&
+          OrOpnd1.getReg() != OrOpnd2.getReg()) {
+        auto *Def1 = getVRegSubRegDef(getRegSubRegPair(OrOpnd1), *MRI);
+        auto *Def2 = getVRegSubRegDef(getRegSubRegPair(OrOpnd2), *MRI);
+        if (Def1 == Def2 && foldableSelect(*Def1))
+          optimizeSCC(Def1, Def, RI);
+      }
+    }
     return true;
   };
 

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