Avoid re-tagging of the index register in optimized byte array access.

Use writable register policy to avoid explicit restoring input register after untagging.

This CL produces more compact code if the writable input
register is the last use by avoiding unnecessary re-tagging.
In that case no extra temp generated by the register allocator.
Committed: https://code.google.com/p/dart/source/detail?r=18515

[Florian Schneider, 2013-02-13 15:48:47]

[+vm-dev]

I also changed UnboxDouble and UnboxInteger to use writable registers instead of
restoring the input.

[Vyacheslav Egorov (Google), 2013-02-13 17:43:04]

lgtm

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/intermediate_l...
File runtime/vm/intermediate_language_ia32.cc (right):

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/intermediate_l...
runtime/vm/intermediate_language_ia32.cc:1332: ?
Location::WritableRegisterOrSmiConstant(index())
I think if CanBeImmediateIndex returns true then the value is guaranteed to be
constant. 

So you don't need WriableReisterOrSmiConstant at this place.

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/intermediate_l...
runtime/vm/intermediate_language_ia32.cc:1336: ?
Location::RegisterOrSmiConstant(index())
ditto

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/intermediate_l...
File runtime/vm/intermediate_language_x64.cc (right):

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/intermediate_l...
runtime/vm/intermediate_language_x64.cc:1027: ?
Location::WritableRegisterOrSmiConstant(index())
ditto

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/intermediate_l...
runtime/vm/intermediate_language_x64.cc:1031: ?
Location::RegisterOrSmiConstant(index())
ditto

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/intermediate_l...
runtime/vm/intermediate_language_x64.cc:1168: ?
Location::WritableRegisterOrSmiConstant(index())
ditto

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/intermediate_l...
runtime/vm/intermediate_language_x64.cc:1172: ?
Location::RegisterOrSmiConstant(index())
ditto

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/locations.h
File runtime/vm/locations.h (right):

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/locations.h#ne...
runtime/vm/locations.h:280: static Location WritableRegisterOrSmiConstant(Value*
value);
I think you don't actually need this.

[Florian Schneider, 2013-02-14 10:35:01]

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/locations.h
File runtime/vm/locations.h (right):

https://codereview.chromium.org/12114008/diff/10002/runtime/vm/locations.h#ne...
runtime/vm/locations.h:280: static Location WritableRegisterOrSmiConstant(Value*
value);
On 2013/02/13 17:43:04, Vyacheslav Egorov (Google) wrote:
> I think you don't actually need this.

Done.