[x64] Use smaller code sequence when pushing SMIs

src/x64/macro-assembler-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/base/bits.h"
 #include "src/base/division-by-constant.h"
 #include "src/bootstrapper.h"
 #include "src/codegen.h"
@@ skipping 2447 matching lines @@
     SmiToInteger32(kScratchRegister, src);
     addl(dst, kScratchRegister);
   }
 }
 
 
 void MacroAssembler::Push(Smi* source) {
   intptr_t smi = reinterpret_cast<intptr_t>(source);
   if (is_int32(smi)) {
     Push(Immediate(static_cast<int32_t>(smi)));
-  } else {
-    Register constant = GetSmiConstant(source);
-    Push(constant);
+    return;
   }
+  int first_byte_set = base::bits::CountTrailingZeros64(smi) / 8;
+  int last_byte_set = (63 - base::bits::CountLeadingZeros64(smi)) / 8;
+  if (first_byte_set == last_byte_set && kPointerSize == kInt64Size) {
+    // This sequence has only 7 bytes, compared to the 12 bytes below.
+    Push(Immediate(0));
+    movb(Operand(rsp, first_byte_set),
+         Immediate(static_cast<int8_t>(smi >> (8 * first_byte_set))));
+    return;
+  }
+  Register constant = GetSmiConstant(source);
+  Push(constant);
 }
 
 
 void MacroAssembler::PushRegisterAsTwoSmis(Register src, Register scratch) {
   DCHECK(!src.is(scratch));
   movp(scratch, src);
   // High bits.
   shrp(src, Immediate(kPointerSize * kBitsPerByte - kSmiShift));
   shlp(src, Immediate(kSmiShift));
   Push(src);
@@ skipping 2799 matching lines @@
   movl(rax, dividend);
   shrl(rax, Immediate(31));
   addl(rdx, rax);
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64