Bug 182226 – LargeAllocation should do the same distancing as MarkedBlock

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Filip Pizlo]

Patch forthcoming.

[Radar WebKit Bug Importer]

<rdar://problem/36968095>

[Filip Pizlo]

Created attachment 332498 [details]
the patch

[Saam Barati]

Comment on attachment 332498 [details]
the patch

View in context: https://bugs.webkit.org/attachment.cgi?id=332498&action=review

> Source/JavaScriptCore/heap/LargeAllocation.cpp:41
> +    size_t sizeIncludingDistancing = sizeBeforeDistancing + distancing;

We don’t have any JIT code that needs to be updated to also do this? Do we not inline large allocations in the JIT?

[Filip Pizlo]

(In reply to Saam Barati from comment #3)
> Comment on attachment 332498 [details]
> the patch
> 
> View in context:
> https://bugs.webkit.org/attachment.cgi?id=332498&action=review
> 
> > Source/JavaScriptCore/heap/LargeAllocation.cpp:41
> > +    size_t sizeIncludingDistancing = sizeBeforeDistancing + distancing;
> 
> We don’t have any JIT code that needs to be updated to also do this? Do we
> not inline large allocations in the JIT?

Only small allocations are inlined.  Large allocations are very complicated to do.

Here's a GC thinking trick: the cost of an allocation "hides behind" the cost of initializing every element. Because any not-totally-dumb allocation would be followed by at least an initialization of every element.

Large allocations mean allocating at least 8000 bytes.

Whether or not you inline a function call has infinitesimal cost compared to the cost of initializing 8000 bytes.

Therefore, we don't inline large allocations.

[Filip Pizlo]

Landed in Landed in https://trac.webkit.org/changeset/227721/webkit

[Saam Barati]

(In reply to Filip Pizlo from comment #4)
> (In reply to Saam Barati from comment #3)
> > Comment on attachment 332498 [details]
> > the patch
> > 
> > View in context:
> > https://bugs.webkit.org/attachment.cgi?id=332498&action=review
> > 
> > > Source/JavaScriptCore/heap/LargeAllocation.cpp:41
> > > +    size_t sizeIncludingDistancing = sizeBeforeDistancing + distancing;
> > 
> > We don’t have any JIT code that needs to be updated to also do this? Do we
> > not inline large allocations in the JIT?
> 
> Only small allocations are inlined.  Large allocations are very complicated
> to do.
> 
> Here's a GC thinking trick: the cost of an allocation "hides behind" the
> cost of initializing every element. Because any not-totally-dumb allocation
> would be followed by at least an initialization of every element.
> 
> Large allocations mean allocating at least 8000 bytes.
> 
> Whether or not you inline a function call has infinitesimal cost compared to
> the cost of initializing 8000 bytes.
> 
> Therefore, we don't inline large allocations.

Makes sense.