A64: Improve constraints for StoreNamedField

A64: Improve constraints for StoreNamedField

Improve register constraints for cases that don't need write barriers, and
remove TODOs.

BUG=
R=ulan@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=19807

[ulan, 2014-03-07 15:16:26]

https://codereview.chromium.org/189373006/diff/1/src/a64/lithium-a64.cc
File src/a64/lithium-a64.cc (right):

https://codereview.chromium.org/189373006/diff/1/src/a64/lithium-a64.cc#newco...
src/a64/lithium-a64.cc:2237: LOperand* object = UseRegister(instr->object());
This is difficult to understand and to keep in sync with codegen. I hope it pays
off with performance improvement.

To make reading a little bit easier, could you please rearrange the conditions
to mirror the structure of codegen?

Along these lines:

if (instr->access().IsExternalMemory() ||
instr->field_representation().IsDouble()) {
 ...
} else if (instr->NeedsWriteBarrierForMap() || instr->NeedsWriteBarrier()) {
 ...
} else {
 ...
}

[m.m.capewell, 2014-03-07 17:53:19]

I've tidied up the constraints so they're easier to read. We're trying to avoid
using UseRegisterAndClobber where possible, due to register pressure and the
additional move it may require, so there should be a little performance here.

[ulan, 2014-03-11 11:22:03]

On 2014/03/07 17:53:19, m.m.capewell wrote:
> I've tidied up the constraints so they're easier to read. We're trying to
avoid
> using UseRegisterAndClobber where possible, due to register pressure and the
> additional move it may require, so there should be a little performance here.

Thanks, lgtm!

[m.m.capewell, 2014-03-11 15:46:37]

Committed patchset #2 manually as r19807 (presubmit successful).