[Webkit-unassigned] [Bug 232951] Prevent fused multiply add during ParseInt

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=232951

--- Comment #11 from Yusuke Suzuki <ysuzuki at apple.com> ---
Comment on attachment 443839
  --> https://bugs.webkit.org/attachment.cgi?id=443839
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=443839&action=review

>>>>> Source/JavaScriptCore/runtime/ParseInt.h:113
>>>>> +#pragma GCC optimize ("O0")
>>>> 
>>>> 1. Why not only disabling fp-contract? We do not need to disable all optimizations. (See comment https://bugs.webkit.org/show_bug.cgi?id=232951#c5)
>>>> 2. Why not using function attribute?
>>>> 3. Use it only for GCC. Clang does not need this.
>>> 
>>> I just tried adding `__attribute__((optimize("fp-contract=off")))` locally to the function definition and gcc seems to ignore it, i.e., it generates de fma and the test fails.
>> 
>> Did you try `GCC optimize` pragma and "fp-contract=off" combination?
> 
> yep, I'm passing `-ffp-contract=off` in the command line and tried it with both `__attribute__((optimize("fp-contract=off")))` and `__attribute__((optimize("-ffp-contract=off")))`. Neither worked.
> 
> Maybe it's not working because of the inline attribute? gcc seems to be gracefully ignoring the attribute.
> 
> I only got it to work it either the `volatile` or `__attribute__((optimize("O0")))`.

OK, so at this point, I think the best way is annotating this double with `volatile` *only when it is GCC*. Can you change it? And can you describe the above reasoning in the ChangeLog? (e.g. optimize attribute didn't work etc.).

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