[codegen] Use smaller instructions for smi loading on x64

- Use movl which clears the upper 32bits on x64
- Use xorl + movb for Smi.ptr values <= 0xFF, saving one byte over movl

Change-Id: Iacdacfbe397670667e71d1d12ef427a01994481d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2642250
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72282}

src/codegen/x64/macro-assembler-x64.cc

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <cstdint>
 #if V8_TARGET_ARCH_X64
 
 #include "src/base/bits.h"
@@ -1078,17 +1079,7 @@ void TurboAssembler::Set(Operand dst, intptr_t x) {
 // Smi tagging, untagging and tag detection.
 
 Register TurboAssembler::GetSmiConstant(Smi source) {
-  STATIC_ASSERT(kSmiTag == 0);
-  int value = source.value();
-  if (value == 0) {
-    xorl(kScratchRegister, kScratchRegister);
-    return kScratchRegister;
-  }
-  if (SmiValuesAre32Bits()) {
   Move(kScratchRegister, source);
-  } else {
-    movl(kScratchRegister, Immediate(source));
-  }
   return kScratchRegister;
 }
 
@@ -1097,8 +1088,17 @@ void TurboAssembler::Move(Register dst, Smi source) {
   int value = source.value();
   if (value == 0) {
     xorl(dst, dst);
-  } else {
+  } else if (SmiValuesAre32Bits() || value < 0) {
     Move(dst, source.ptr(), RelocInfo::NONE);
+  } else {
+    uint32_t uvalue = static_cast<uint32_t>(source.ptr());
+    if (uvalue <= 0xFF) {
+      // Emit shorter instructions for small Smis
+      xorl(dst, dst);
+      movb(dst, Immediate(uvalue));
+    } else {
+      movl(dst, Immediate(uvalue));
+    }
   }
 }
 

test/cctest/test-macro-assembler-x64.cc

@@ -115,13 +115,20 @@ TEST(SmiMove) {
   TestMoveSmi(masm, &exit, 3, Smi::FromInt(128));
   TestMoveSmi(masm, &exit, 4, Smi::FromInt(255));
   TestMoveSmi(masm, &exit, 5, Smi::FromInt(256));
-  TestMoveSmi(masm, &exit, 6, Smi::FromInt(Smi::kMaxValue));
-  TestMoveSmi(masm, &exit, 7, Smi::FromInt(-1));
-  TestMoveSmi(masm, &exit, 8, Smi::FromInt(-128));
-  TestMoveSmi(masm, &exit, 9, Smi::FromInt(-129));
-  TestMoveSmi(masm, &exit, 10, Smi::FromInt(-256));
-  TestMoveSmi(masm, &exit, 11, Smi::FromInt(-257));
-  TestMoveSmi(masm, &exit, 12, Smi::FromInt(Smi::kMinValue));
+  TestMoveSmi(masm, &exit, 6, Smi::FromInt(0xFFFF - 1));
+  TestMoveSmi(masm, &exit, 7, Smi::FromInt(0xFFFF));
+  TestMoveSmi(masm, &exit, 8, Smi::FromInt(0xFFFF + 1));
+  TestMoveSmi(masm, &exit, 9, Smi::FromInt(Smi::kMaxValue));
+
+  TestMoveSmi(masm, &exit, 10, Smi::FromInt(-1));
+  TestMoveSmi(masm, &exit, 11, Smi::FromInt(-128));
+  TestMoveSmi(masm, &exit, 12, Smi::FromInt(-129));
+  TestMoveSmi(masm, &exit, 13, Smi::FromInt(-256));
+  TestMoveSmi(masm, &exit, 14, Smi::FromInt(-257));
+  TestMoveSmi(masm, &exit, 15, Smi::FromInt(-0xFFFF + 1));
+  TestMoveSmi(masm, &exit, 16, Smi::FromInt(-0xFFFF));
+  TestMoveSmi(masm, &exit, 17, Smi::FromInt(-0xFFFF - 1));
+  TestMoveSmi(masm, &exit, 18, Smi::FromInt(Smi::kMinValue));
 
   __ xorq(rax, rax);  // Success.
   __ bind(&exit);