Bug 184909 – Disable usage of fused multiply-add instructions for JSC with compiler flag

• Status: RESOLVED
• Resolution: FIXED
• Component: New Bugs

[Dominik Inführ]

Disable usage of fused multiply-add instructions for JSC with compiler flag

[Dominik Inführ]

Created attachment 338641 [details]
Patch

[Yusuke Suzuki]

Comment on attachment 338641 [details]
Patch

Can you limit the scope of this change? If fused-multiply-add works well, we do not need this flag.

[Filip Pizlo]

Fused multiply-add gives a precision boost that results in double math returning a result that does not match bit-for-bit what you would have gotten without the fusion. Based on this, I think it might be a good idea to disable it for all of JSC. 

I would definitely want to disable it if it’s free to disable it.

[Yusuke Suzuki]

(In reply to Filip Pizlo from comment #3)
> Fused multiply-add gives a precision boost that results in double math
> returning a result that does not match bit-for-bit what you would have
> gotten without the fusion. Based on this, I think it might be a good idea to
> disable it for all of JSC. 
> 
> I would definitely want to disable it if it’s free to disable it.

Interesting. After considering about this, I think, in JSC, we do not have performance sensitive FMAs (I believe). So disabling is OK.
I'm a bit worried about disabling this in the entire WebKit tree due to,

1. performance reason. If we have a non-result-sensitive performance-sensitive FMAs, this can affect on that (like, implementing some graphic filters)
2. coordination with third party libraries.

what do you think of?

[Filip Pizlo]

(In reply to Yusuke Suzuki from comment #4)
> (In reply to Filip Pizlo from comment #3)
> > Fused multiply-add gives a precision boost that results in double math
> > returning a result that does not match bit-for-bit what you would have
> > gotten without the fusion. Based on this, I think it might be a good idea to
> > disable it for all of JSC. 
> > 
> > I would definitely want to disable it if it’s free to disable it.
> 
> Interesting. After considering about this, I think, in JSC, we do not have
> performance sensitive FMAs (I believe). So disabling is OK.
> I'm a bit worried about disabling this in the entire WebKit tree due to,
> 
> 1. performance reason. If we have a non-result-sensitive
> performance-sensitive FMAs, this can affect on that (like, implementing some
> graphic filters)
> 2. coordination with third party libraries.
> 
> what do you think of?

Sounds like a good reason to only disable fused fma in JSC and leave it elsewhere.

[Dominik Inführ]

The patch was already reviewed, is there anything for me left to do?

[WebKit Commit Bot]

Comment on attachment 338641 [details]
Patch

Clearing flags on attachment: 338641

Committed r231301: <https://trac.webkit.org/changeset/231301>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.

[Radar WebKit Bug Importer]

<rdar://problem/39938586>