feat(peephole): merge adjacent zero-value stack stores\n\nWhen str WZR, fi#N and str WZR, fi#N+1 appear consecutively,\nreplaces them with a single str XZR, fi#N (64-bit zero store).

src/mir/passes/Peephole.cpp

@@ -93,6 +93,34 @@ namespace mir
              s_ops[0].GetReg() != l_ops[0].GetReg();
     }
 
+    static bool TryMergeZeroStores(MachineInstr &first, MachineInstr &second)
+    {
+      if (first.GetOpcode() != Opcode::StoreStack ||
+          second.GetOpcode() != Opcode::StoreStack)
+        return false;
+      const auto &f_ops = first.GetOperands();
+      const auto &s_ops = second.GetOperands();
+      if (f_ops.size() < 2 || s_ops.size() < 2)
+        return false;
+      if (f_ops[0].GetKind() != Operand::Kind::Reg ||
+          s_ops[0].GetKind() != Operand::Kind::Reg)
+        return false;
+      if (f_ops[0].GetReg() != PhysReg::WZR ||
+          s_ops[0].GetReg() != PhysReg::WZR)
+        return false;
+      if (f_ops[1].GetKind() != Operand::Kind::FrameIndex ||
+          s_ops[1].GetKind() != Operand::Kind::FrameIndex)
+        return false;
+      int fi1 = f_ops[1].GetFrameIndex();
+      int fi2 = s_ops[1].GetFrameIndex();
+      if (fi2 != fi1 + 1)
+        return false;
+      first = MachineInstr(Opcode::StoreStack,
+                          {Operand::Reg(PhysReg::XZR),
+                           Operand::FrameIndex(fi1)});
+      return true;
+    }
+
     static void RunPeepholeOnBlock(MachineBasicBlock &block)
     {
       auto &insts = block.GetInstructions();
@@ -123,6 +151,20 @@ namespace mir
           }
         }
 
+        if (!changed)
+        {
+          for (auto it = insts.begin(); it != insts.end(); ++it)
+          {
+            auto next = std::next(it);
+            if (next != insts.end() && TryMergeZeroStores(*it, *next))
+            {
+              next = insts.erase(next);
+              changed = true;
+              break;
+            }
+          }
+        }
+
         if (!changed)
         {
           for (auto it = insts.begin(); it != insts.end(); ++it)