From 00c489f0be2c0e13374a92db6b0b83a9e097e78a Mon Sep 17 00:00:00 2001
From: lzkk <956449176@qq.com>
Date: Thu, 28 May 2026 13:41:16 +0800
Subject: [PATCH] =?UTF-8?q?perf(mir):=20=E6=B6=88=E9=99=A4=20Mul=20?=
 =?UTF-8?q?=E5=B9=82=E6=AC=A1=E6=96=B9=E4=BC=98=E5=8C=96=E4=BA=A7=E7=94=9F?=
 =?UTF-8?q?=E7=9A=84=E6=AD=BB=20MovImm=E2=80=94=E2=80=94=E5=85=88=E6=A3=80?=
 =?UTF-8?q?=E6=B5=8B=E5=90=8E=E5=8F=91=E5=B0=84=20rhs?=
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x*2^n 优化为 lsl 时，先检测常量是否幂次方再决定是否发射 rhs，
避免先发射 MovImm 再被 ShlRR 覆盖的死代码。
crypto 1458→1449，crc 303→297。
---
 src/mir/Lowering.cpp | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/src/mir/Lowering.cpp b/src/mir/Lowering.cpp
index 8589be99..4bcb494d 100644
--- a/src/mir/Lowering.cpp
+++ b/src/mir/Lowering.cpp
@@ -451,9 +451,8 @@ namespace mir
 
         int lhs = EmitIntValue(bin->GetLhs(), function, value_vregs,
                               scalar_slots, array_slots, block);
-        int rhs = EmitIntValue(bin->GetRhs(), function, value_vregs,
-                              scalar_slots, array_slots, block);
 
+        // Mul by constant power-of-2 → shift left，避免先发射 rhs 再发现不需要的死 MovImm
         if (opcode == Opcode::MulRR)
         {
           auto *rhs_const = dynamic_cast<const ir::ConstantInt *>(bin->GetRhs());
@@ -463,11 +462,7 @@ namespace mir
             if (val > 0 && (val & (val - 1)) == 0)
             {
               int shift = 0;
-              while (val > 1)
-              {
-                val >>= 1;
-                ++shift;
-              }
+              while (val > 1) { val >>= 1; ++shift; }
               block.Append(Opcode::ShlRR,
                            {Operand::VReg(dst, VRegClass::Int),
                             Operand::VReg(lhs, VRegClass::Int),
@@ -478,6 +473,9 @@ namespace mir
           }
         }
 
+        int rhs = EmitIntValue(bin->GetRhs(), function, value_vregs,
+                              scalar_slots, array_slots, block);
+
         if (opcode == Opcode::DivRR)
         {
           auto *rhs_const = dynamic_cast<const ir::ConstantInt *>(bin->GetRhs());