ARM: Smi refactoring and improvements.

ARM: Smi refactoring and improvements.

Refactoring:
 * consistent use of SmiTag/Untag
 * added a few Smi macros and helpers
Improvements
 * small optimisations (e.g. merging untag and cmp #0)
 * added fixed point to double conversion instructions for simpler conversions

More on the last point: a Smi can be seen as a fixed point number with the
a one bit fractional part. Fixed to double instructions allow us to convert
a Smi to a double without untagging.

BUG=none
TEST=none

Committed: https://code.google.com/p/v8/source/detail?r=14724

[ulan, 2013-05-17 09:57:14]

LGTM

https://codereview.chromium.org/15085026/diff/1/src/arm/macro-assembler-arm.h
File src/arm/macro-assembler-arm.h (right):

https://codereview.chromium.org/15085026/diff/1/src/arm/macro-assembler-arm.h...
src/arm/macro-assembler-arm.h:1196: void TrySmiTag(Register reg, Register src,
Label* not_a_smi) {
What do you think about passing explicit scratch register instead of using ip
implicitly here and in SmiToDouble?

[Rodolph Perfetta, 2013-05-17 11:44:55]

https://codereview.chromium.org/15085026/diff/1/src/arm/macro-assembler-arm.h
File src/arm/macro-assembler-arm.h (right):

https://codereview.chromium.org/15085026/diff/1/src/arm/macro-assembler-arm.h...
src/arm/macro-assembler-arm.h:1196: void TrySmiTag(Register reg, Register src,
Label* not_a_smi) {
If these macro are used in crankshaft then you may have to ask the register
allocator for an extra temp register and I was worried it could impact
performance.

In modern cores with register renaming, which register you use is irrelevant and
ip should only be used by the macro assembler (though I believe this is not true
in practice so maybe I should push another clean up patch)

If you prefer the flexibility of controlling the scratch register let me know
and I'll upload a new patch.

[ulan, 2013-05-17 11:56:12]

> If you prefer the flexibility of controlling the scratch register let 
> me know and I'll upload a new patch.
I don't have strong preference. I was concerned that ip gets aliased with other
scratch registers of the caller. If only masm can use ip, then it looks good.