Some assembler-level optimizations on ARM.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index 76c8443e7c30cc422154eab202c61dba16e5941e..49d95b36602d66cd090e1efef434c3fe57bcc8c1 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -3928,7 +3928,9 @@ void LCodeGen::EmitNumberUntagD(Register input_reg,
   Label load_smi, heap_number, done;
 
   // Smi check.
-  __ JumpIfSmi(input_reg, &load_smi);
+  STATIC_CHECK(kSmiTagSize == 1 && kSmiTag == 0);

[ulan, 2012/01/27 13:19:11]

UntagAndJumpIfSmi

+  __ mov(scratch, Operand(input_reg, ASR, kSmiTagSize), SetCC);
+  __ b(cc, &load_smi);  // Shifter carry is not set for a smi.
 
   // Heap number map check.
   __ ldr(scratch, FieldMemOperand(input_reg, HeapObject::kMapOffset));
@@ -3967,10 +3969,9 @@ void LCodeGen::EmitNumberUntagD(Register input_reg,
 
   // Smi to double register conversion
   __ bind(&load_smi);
-  __ SmiUntag(input_reg);  // Untag smi before converting to float.
-  __ vmov(flt_scratch, input_reg);
+  // scratch: untagged value of input_reg
+  __ vmov(flt_scratch, scratch);
   __ vcvt_f64_s32(result_reg, flt_scratch);
-  __ SmiTag(input_reg);  // Retag smi.
   __ bind(&done);
 }
 
@@ -4256,7 +4257,9 @@ void LCodeGen::DoClampTToUint8(LClampTToUint8* instr) {
   Label is_smi, done, heap_number;
 
   // Both smi and heap number cases are handled.
-  __ JumpIfSmi(input_reg, &is_smi);
+  STATIC_ASSERT(kSmiTagSize == 1 && kSmiTag == 0);
+  __ mov(result_reg, Operand(input_reg, ASR, kSmiTagSize), SetCC);
+  __ b(cc, &is_smi);  // Shifter carry is not set for a smi.
 
   // Check for heap number
   __ ldr(scratch, FieldMemOperand(input_reg, HeapObject::kMapOffset));
@@ -4279,7 +4282,6 @@ void LCodeGen::DoClampTToUint8(LClampTToUint8* instr) {
 
   // smi
   __ bind(&is_smi);
-  __ SmiUntag(result_reg, input_reg);
   __ ClampUint8(result_reg, result_reg);
 
   __ bind(&done);