diff --git a/src/mir/Lowering.cpp b/src/mir/Lowering.cpp
index b47e11ec..2efce285 100644
--- a/src/mir/Lowering.cpp
+++ b/src/mir/Lowering.cpp
@@ -599,61 +599,55 @@ namespace mir
             }
             if (val < 0 && (-val & (-val - 1)) == 0 && val != -1)
             {
+              // x % -2^n → 同 x % 2^n: (x & (2^n-1)) + sign fixup
               int abs_val = -val;
-              int bias = abs_val - 1;
-              int biased = function.CreateVReg(VRegClass::Int);
-              if (bias <= 4095)
+              int mask = abs_val - 1;
+              int masked = function.CreateVReg(VRegClass::Int);
+              if (mask <= 4095)
               {
-                block.Append(Opcode::AddRR,
-                             {Operand::VReg(biased, VRegClass::Int),
+                block.Append(Opcode::AndRR,
+                             {Operand::VReg(masked, VRegClass::Int),
                               Operand::VReg(lhs, VRegClass::Int),
-                              Operand::Imm(bias)});
+                              Operand::Imm(mask)});
               }
               else
               {
-                int bias_reg = function.CreateVReg(VRegClass::Int);
+                int mask_reg = function.CreateVReg(VRegClass::Int);
                 block.Append(Opcode::MovImm,
-                             {Operand::VReg(bias_reg, VRegClass::Int),
-                              Operand::Imm(bias)}).SetRematerializable(true).SetRematImm(bias);
-                block.Append(Opcode::AddRR,
-                             {Operand::VReg(biased, VRegClass::Int),
+                             {Operand::VReg(mask_reg, VRegClass::Int),
+                              Operand::Imm(mask)}).SetRematerializable(true).SetRematImm(mask);
+                block.Append(Opcode::AndRR,
+                             {Operand::VReg(masked, VRegClass::Int),
                               Operand::VReg(lhs, VRegClass::Int),
-                              Operand::VReg(bias_reg, VRegClass::Int)});
-              }
-              int shift = 0;
-              int tmp = abs_val;
-              while (tmp > 1)
-              {
-                tmp >>= 1;
-                ++shift;
+                              Operand::VReg(mask_reg, VRegClass::Int)});
               }
               block.Append(Opcode::CmpImm,
                            {Operand::VReg(lhs, VRegClass::Int),
                             Operand::Imm(0)});
-              int selected = function.CreateVReg(VRegClass::Int);
+              int neg_fixup = function.CreateVReg(VRegClass::Int);
+              if (static_cast<unsigned int>(abs_val) <= 4095)
+              {
+                block.Append(Opcode::SubRR,
+                             {Operand::VReg(neg_fixup, VRegClass::Int),
+                              Operand::VReg(masked, VRegClass::Int),
+                              Operand::Imm(abs_val)});
+              }
+              else
+              {
+                int val_reg = function.CreateVReg(VRegClass::Int);
+                block.Append(Opcode::MovImm,
+                             {Operand::VReg(val_reg, VRegClass::Int),
+                              Operand::Imm(abs_val)}).SetRematerializable(true).SetRematImm(abs_val);
+                block.Append(Opcode::SubRR,
+                             {Operand::VReg(neg_fixup, VRegClass::Int),
+                              Operand::VReg(masked, VRegClass::Int),
+                              Operand::VReg(val_reg, VRegClass::Int)});
+              }
               block.Append(Opcode::Csel,
-                           {Operand::VReg(selected, VRegClass::Int),
-                            Operand::VReg(biased, VRegClass::Int),
-                            Operand::VReg(lhs, VRegClass::Int),
-                            Operand::Imm(static_cast<int>(CondCode::LT))});
-              int asr_result = function.CreateVReg(VRegClass::Int);
-              block.Append(Opcode::AsrRR,
-                           {Operand::VReg(asr_result, VRegClass::Int),
-                            Operand::VReg(selected, VRegClass::Int),
-                            Operand::Imm(shift)});
-              int q_dst = function.CreateVReg(VRegClass::Int);
-              block.Append(Opcode::NegRR,
-                           {Operand::VReg(q_dst, VRegClass::Int),
-                            Operand::VReg(asr_result, VRegClass::Int)});
-              int d_reg = function.CreateVReg(VRegClass::Int);
-              block.Append(Opcode::MovImm,
-                           {Operand::VReg(d_reg, VRegClass::Int),
-                            Operand::Imm(val)}).SetRematerializable(true).SetRematImm(val);
-              block.Append(Opcode::Msub,
                            {Operand::VReg(dst, VRegClass::Int),
-                            Operand::VReg(q_dst, VRegClass::Int),
-                            Operand::VReg(d_reg, VRegClass::Int),
-                            Operand::VReg(lhs, VRegClass::Int)});
+                            Operand::VReg(neg_fixup, VRegClass::Int),
+                            Operand::VReg(masked, VRegClass::Int),
+                            Operand::Imm(static_cast<int>(CondCode::LT))});
               value_vregs[value] = dst;
               return dst;
             }