simplify compare-to-branch sequences

src/mir/passes/Peephole.cpp

@@ -46,11 +46,62 @@ bool IsFloatReg(PhysReg reg) {
   return reg >= PhysReg::S0 && reg <= PhysReg::S15;
 }
 
+bool SameReg(const Operand& lhs, const Operand& rhs) {
+  return lhs.GetKind() == Operand::Kind::Reg &&
+         rhs.GetKind() == Operand::Kind::Reg &&
+         NormalizeReg(lhs.GetReg()) == NormalizeReg(rhs.GetReg());
+}
+
+bool IsZeroImm(const Operand& operand) {
+  return operand.GetKind() == Operand::Kind::Imm && operand.GetImm() == 0;
+}
+
+std::vector<MachineInstr> SimplifyCompareToBranch(
+    const std::vector<MachineInstr>& insts) {
+  std::vector<MachineInstr> simplified;
+
+  for (size_t i = 0; i < insts.size();) {
+    if (i + 4 < insts.size() &&
+        (insts[i].GetOpcode() == Opcode::CmpRR ||
+         insts[i].GetOpcode() == Opcode::FCmpRR) &&
+        insts[i + 1].GetOpcode() == Opcode::Cset &&
+        insts[i + 2].GetOpcode() == Opcode::MovImm &&
+        insts[i + 3].GetOpcode() == Opcode::CmpRR &&
+        insts[i + 4].GetOpcode() == Opcode::BCond) {
+      const auto& cset_ops = insts[i + 1].GetOperands();
+      const auto& mov_ops = insts[i + 2].GetOperands();
+      const auto& cmp2_ops = insts[i + 3].GetOperands();
+      const auto& br_ops = insts[i + 4].GetOperands();
+
+      if (mov_ops.size() == 2 && cmp2_ops.size() == 2 && br_ops.size() == 2 &&
+          SameReg(cset_ops.at(0), cmp2_ops.at(0)) &&
+          SameReg(mov_ops.at(0), cmp2_ops.at(1)) &&
+          IsZeroImm(mov_ops.at(1)) &&
+          br_ops.at(0).GetKind() == Operand::Kind::Cond &&
+          br_ops.at(0).GetCondCode() == "ne") {
+        simplified.push_back(insts[i]);
+        simplified.emplace_back(Opcode::BCond,
+                                std::vector<Operand>{
+                                    Operand::Cond(cset_ops.at(1).GetCondCode()),
+                                    br_ops.at(1)});
+        i += 5;
+        continue;
+      }
+    }
+
+    simplified.push_back(insts[i]);
+    i++;
+  }
+
+  return simplified;
+}
+
 } // namespace
 
 void RunPeephole(MachineFunction& function) {
   for (auto& block : function.GetBlocks()) {
     auto& insts = block.GetInstructions();
+    insts = SimplifyCompareToBranch(insts);
     std::vector<MachineInstr> optimized;
     
     // Map from FrameIndex to the normalized physical register that currently holds its value