Some assembler-level optimizations on ARM.

src/arm/macro-assembler-arm.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1131 matching lines @@
     mov(reg, scratch);
   }
 
   void SmiUntag(Register reg, SBit s = LeaveCC) {
     mov(reg, Operand(reg, ASR, kSmiTagSize), s);
   }
   void SmiUntag(Register dst, Register src, SBit s = LeaveCC) {
     mov(dst, Operand(src, ASR, kSmiTagSize), s);
   }
 
+  // Tentatively untag and jump if the value has been a smi.

[ulan, 2012/01/27 15:24:35]

"Tentatively" is a bit misleading, since the function doesn't restore the
original values. 
Consider something along this lines:
Untag the source register to the destination register and jump if the source
register is a smi. The source and the destination registers can be the same.

+  void UntagAndJumpIfSmi(Register dst, Register src, Label* smi_case);
+
+  // Tentatively untag and jump if the value has not been a smi.
+  void UntagAndJumpIfNotSmi(Register dst, Register src, Label* smi_case);
+
   // Jump the register contains a smi.
   inline void JumpIfSmi(Register value, Label* smi_label) {
     tst(value, Operand(kSmiTagMask));
     b(eq, smi_label);
   }
   // Jump if either of the registers contain a non-smi.
   inline void JumpIfNotSmi(Register value, Label* not_smi_label) {
     tst(value, Operand(kSmiTagMask));
     b(ne, not_smi_label);
   }
@@ skipping 189 matching lines @@
 #define __FILE_LINE__ __FILE__ ":" CODE_COVERAGE_TOSTRING(__LINE__)
 #define ACCESS_MASM(masm) masm->stop(__FILE_LINE__); masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_ARM_MACRO_ASSEMBLER_ARM_H_