Some assembler-level optimizations on ARM.

src/arm/macro-assembler-arm.h

Index: src/arm/macro-assembler-arm.h
diff --git a/src/arm/macro-assembler-arm.h b/src/arm/macro-assembler-arm.h
index 8e73b32d8906a17846cbbbe85b9ffadecde1e3a8..862cc68c54acf027e7c85d1276ea1e55678f5098 100644
--- a/src/arm/macro-assembler-arm.h
+++ b/src/arm/macro-assembler-arm.h
@@ -1149,6 +1149,12 @@ class MacroAssembler: public Assembler {
     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));