ARM: Try to avoid VMSR instruction and drop redundant VCVT

ARM: Try to avoid VMSR instruction and drop redundant VCVT

We were doing a redundant VCVT operation in MacroAssembler::EmitECMATruncate.
Also, setting the FPSCR exception flags is expensive on some CPUs, wo we should
try to avoid it if we can.

Thanks to Rodolph Perfetta for the input on this!

Committed: https://code.google.com/p/v8/source/detail?r=13601

[hans, 2013-02-05 15:48:28]

Rodolph, Ulan: please take a look.

[Rodolph Perfetta, 2013-02-05 16:14:12]

https://codereview.chromium.org/12217014/diff/1/src/arm/macro-assembler-arm.cc
File src/arm/macro-assembler-arm.cc (right):

https://codereview.chromium.org/12217014/diff/1/src/arm/macro-assembler-arm.c...
src/arm/macro-assembler-arm.cc:2691: vcmp(double_input, double_scratch);
VFPCompareAndSetFlags would be better.

https://codereview.chromium.org/12217014/diff/1/src/arm/macro-assembler-arm.c...
src/arm/macro-assembler-arm.cc:2709: vmov(result, double_scratch.low());
This is not needed, if the conversion is valid result already holds the
truncated value (see line 2688).

[hans, 2013-02-05 16:24:47]

https://codereview.chromium.org/12217014/diff/1/src/arm/macro-assembler-arm.cc
File src/arm/macro-assembler-arm.cc (right):

https://codereview.chromium.org/12217014/diff/1/src/arm/macro-assembler-arm.c...
src/arm/macro-assembler-arm.cc:2691: vcmp(double_input, double_scratch);
On 2013/02/05 16:14:12, Rodolph Perfetta wrote:
> VFPCompareAndSetFlags would be better.

Done.

https://codereview.chromium.org/12217014/diff/1/src/arm/macro-assembler-arm.c...
src/arm/macro-assembler-arm.cc:2709: vmov(result, double_scratch.low());
On 2013/02/05 16:14:12, Rodolph Perfetta wrote:
> This is not needed, if the conversion is valid result already holds the
> truncated value (see line 2688).

Ah, right. Done.

[Rodolph Perfetta, 2013-02-05 16:39:29]

lgtm

[ulan, 2013-02-05 17:31:51]

LGTM

[ulan, 2013-02-06 10:35:15]

Committed manually as r13601 (presubmit successful).