On MIPS, keeps Object::null() in a register.

On MIPS, keeps Object::null() in a register.

On MIPS, we have a lot of registers, and we don't use very many of them.
This change removes the second temporary register TMP2, and instead
dedicates a register to holding Object::null(), which is used frequently
for comparisons, etc..

Also, uses branch delay slot when leaving a dart frame and returning.

R=regis@google.com

Committed: https://code.google.com/p/dart/source/detail?r=22331

[zra, 2013-05-02 17:02:17]

After going over the code, I wanted to try this out. It only reduces instruction
count for "Hello, world!" by ~3%, though, so I'm not sure if it's worth keeping.
What do you think?

[zra, 2013-05-02 17:03:31]

After going over the code, I wanted to try this out. It only reduces instruction
count for "Hello, world!" by ~3%, though, so I'm not sure if it's worth keeping.
What do you think?

[regis, 2013-05-02 17:18:16]

LGTM

I do not know the answer about reserving a register for Object::null(). 3% is
not bad, but that is static code, not executed code, I suppose?

Once we optimize, having one more register may be better. I do not know. I think
it is too early now for such fine tuning, but it is good to keep such ideas in
mind and try them out when we tune performance.

Up to you to keep it in or not.

-- Regis

[zra, 2013-05-02 17:29:33]

On 2013/05/02 17:18:16, regis wrote:
> LGTM
> 
> I do not know the answer about reserving a register for Object::null(). 3% is
> not bad, but that is static code, not executed code, I suppose?
> 
> Once we optimize, having one more register may be better. I do not know. I
think
> it is too early now for such fine tuning, but it is good to keep such ideas in
> mind and try them out when we tune performance.
> 
> Up to you to keep it in or not.
> 
> -- Regis

The 3% reduction is in the executed instruction count. I'll leave it in, for
now, and add a TODO to revisit when we can investigate performance on real
hardware.

Thanks!
Zach

[zra, 2013-05-02 17:53:43]

Committed patchset #2 manually as r22331 (presubmit successful).