Tweak SmiAdd for X64

Tweak SmiAdd for X64

R=danno@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=17074

[haitao.feng, 2013-09-26 14:04:13]

Restart the 31-bit SMI work. Sorry for the delay.

This CL is inspired by the CountOperation implementation and danno's comments
(https://chromiumcodereview.appspot.com/21014003/diff/45001/src/x64/macro-asse...
and
https://chromiumcodereview.appspot.com/21014003/diff/45001/src/x64/macro-asse...)

[danno, 2013-10-02 08:57:07]

This is LGTM for now, but please consider implementing the deferred code
smi-recovery mechanism discussed below.

https://codereview.chromium.org/24754002/diff/4001/src/x64/macro-assembler-x6...
File src/x64/macro-assembler-x64.cc (right):

https://codereview.chromium.org/24754002/diff/4001/src/x64/macro-assembler-x6...
src/x64/macro-assembler-x64.cc:1545: subq(dst, kScratchRegister);
This is probably OK for now, but probably slightly more efficient would be for
the MacroAssembler to keep a list of deferred code (e.g. smi overflow recovery)
items that output at the very end of the generated code. That allows the code
here to be straight-line, only branching away to a code island in the very
unlikely overflow case.

[haitao.feng, 2013-10-02 11:10:23]

danno, thanks for the review. I will commit it as it is and think about your
recommendation.

Could you have a look on https://codereview.chromium.org/22935005/? To make
CountOperation effectively use SmiSubConstant(1), it seems it is necessary to
introduce a mode BAILOUT_ON_NO_OVERFLOW.

[haitao.feng, 2013-10-02 11:46:35]

Committed patchset #2 manually as r17074 (presubmit successful).