Refactor the arithmetic instructions for x64 port

Refactor the arithmetic instructions for x64

R=verwaest@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=20329

[haitao.feng, 2014-03-27 14:20:36]

PTAL.

https://codereview.chromium.org/214753002/diff/1/src/x64/assembler-x64.cc
File src/x64/assembler-x64.cc (left):

https://codereview.chromium.org/214753002/diff/1/src/x64/assembler-x64.cc#old...
src/x64/assembler-x64.cc:677: if (!dst.is_byte_register()) {
It seems that we could remove "is_byte_register()" interface as the manual says
"In 64-bit mode, r/m8 can not be encoded to access the following byte registers
if a REX prefix is used: AH, BH, CH, DH.". We are accessing al, bl, cl, dl, it
should not be affected by this restriction.

https://codereview.chromium.org/214753002/diff/1/src/x64/assembler-x64.cc
File src/x64/assembler-x64.cc (right):

https://codereview.chromium.org/214753002/diff/1/src/x64/assembler-x64.cc#new...
src/x64/assembler-x64.cc:531: emit_optional_rex_32(reg, op);
I did not use the "is_byte_register()" interface as the manual says "In 64-bit
mode, r/m8 can not be encoded to access the following byte registers if a REX
prefix is used: AH, BH, CH, DH.". We are accessing al, bl, cl, dl, it seems
"emit_optional_rex_32" is enough.

[Toon Verwaest, 2014-03-27 16:23:07]

lgtm

[haitao.feng, 2014-03-28 02:32:19]

oops, it seems that I mistakenly read the manual. "In 64-bit mode, r/m8 can not
be encoded to access the following byte registers if a REX prefix is used: AH,
BH, CH, DH."

When REX is not used, cmpb could access al, cl, dl, bl, ah, ch, dh, bh. When REX
is sued, cmpb could access al, cl, dl, bl, spl, bpl, sil, dil, r8l, ... r15l. It
is always safe to use REX, that might be the reason the original code use
emit_rex_64 even we do not REX.W. By using the byte_register interface, we could
save one byte if dst register is one of rax, rbx, rcx and rdx.

[Toon Verwaest, 2014-03-28 10:01:22]

Understood. Thanks for fixing this. LGTM

[haitao.feng, 2014-03-28 11:13:30]

Committed patchset #2 manually as r20329 (presubmit successful).