[webkit-dev] Removing webkitSlice (was Re: Existing metrics for deprecated features)

Maciej Stachowiak mjs at apple.com
Thu Sep 13 21:05:41 PDT 2012

On Sep 13, 2012, at 11:15 PM, Darin Fisher <darin at google.com> wrote:

> On Thu, Sep 13, 2012 at 6:16 PM, Adam Barth <abarth at webkit.org> wrote:
> On Thu, Sep 13, 2012 at 5:13 PM, Adam Barth <abarth at webkit.org> wrote:
> > Another metric we have is for Blob.webkitSlice:
> >
> > Ratio of Blob.webkitSlice calls to Blob.slice: 14.87%
> > Ratio of Blob.webkitSlice calls to Document creation: 0.0053%
> >
> > It's difficult to know how to interpret this data because we don't
> > actually correlate calls to webkitSlice with Documents or Pages.
> > Instead, we just count the total number of calls across all Documents.
> >  This gives us an upper bound on how many Documents (or Pages) would
> > be affected by deleting Blob.webkitSlice, but doesn't measure that
> > information as accurately as the data we have for mutation events.
> Based on this data, I've posted a patch for removing Blob.webkitSlice
> in favor of Blob.slice:
> https://bugs.webkit.org/show_bug.cgi?id=96715
> Adam
> So, worst case 53 out of a million pages calls webkitSlice.  But, it is easy to imagine that that upper bound is crazy high, and more likely a couple pages simply call webkitSlice a lot.  Also, given that there are so many more calls to Blob.slice() one could imagine that sites that call webkitSlice probably have fallback to Blob.slice().  Is this the hypothesis?

Adam's data measures webkitSlice() *calls*, not just appearances of the symbol, so these are either sites that have fallback in the wrong direction (trying the prefixed version before the vanilla version), or would break after the removal. I've seen both problems with about similar frequency in the past, so a decent hypothesis is that half those webkitSlice calls will break.

Safari hasn't had Blob for very long at all, so Chrome is probably more impacted. From my own perspective, I think the usage is low enough that it's worth making the change to see the fallout. 

I wonder also how likely it is that some of the webkitSlice uses are on Google-controlled Web properties and therefore could be fixed ahead of time.


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