412210 - Spidermonkey faster at -O2 with msvc

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Robert Sayre]

Crowder's claim we should measure this finally got to me, and he was right. Good for a noticeable SunSpider in-browser win.

Comment 1

[Robert Sayre]

Attachment: browser patch

Switch to -O2 -GL. GL doesn't change the numbers, but will allow us to do PGO later.

Comment 2

[Robert Sayre]

Attachment: Makefile.in and config.mk

easy to get the shell at the same time

Comment 3

[Robert Sayre]

Attachment: correct typo

sigh, not my day

Comment 4

[Brendan Eich [:brendan]]

Comment on attachment 296874 [details] [diff] [review]
correct typo

Could you tell me, what does -LTCG do? Maybe worth a comment if interesting.

Feel free to mirror into Makefile.ref. Thanks,

/be

Comment 5

[Robert Sayre]

(In reply to comment #4)
> (From update of attachment 296874 [details] [diff] [review])
> Could you tell me, what does -LTCG do? Maybe worth a comment if interesting.

Paraphrasing MS docs: The -GL option and the linker command -LTCG (Link Time Code Generation) combine to allow whole-program optimzation. Whole program optimization allows the compiler to perform optimizations with information on all modules in the program. Without whole program optimization, optimizations are performed on a per module (compiland) basis. Improvements:

    * Optimize the use of registers across function boundaries.
    * Do a better job of tracking modifications to global data, allowing a 
      reduction in the number of loads and stores.
    * Do a better job of tracking the possible set of items modified by a
      pointer dereference, reducing the numbers of loads and stores.
    * Inline a function in a module even when the function is defined in 
      another module.

LTCG can take flags--LTCG:PGINSTRUMENT and LTCG:PGOPTIMIZE. This works just like a two-pass gcc PGO build. Build once with PGINSTRUMENT, run over some input (talos, ajax test suites), build with PGOPTIMIZE. This allows for better branch prediction.