[webkit-dev] PSA: Bit fields won't be packed on Windows if you mix types

[Ryosuke Niwa]

Yup. I'm enabling that for InlineBox on Windows in this bug:
https://bugs.webkit.org/show_bug.cgi?id=82578

- Ryosuke

On Thu, Mar 29, 2012 at 1:38 AM, Eric Seidel <eric at webkit.org> wrote:

> For core classes like these we should COMPILE_ASSERT their size.  We
> do this for many, but not all of these classes.
>
> On Thu, Mar 29, 2012 at 1:21 AM, Ryosuke Niwa <rniwa at webkit.org> wrote:
> > Hi,
> >
> > Unlike gcc and clang, MSVC pads each consecutive member variables of the
> > same type in bitfields. e.g. if you have:
> > struct AB {
> >     unsigned m_1 : 31;
> >     bool m_2 : 1;
> > }
> > then MSVC pads m_1 and allocates sizeof(unsigned) * 2 for AB whereas gcc
> and
> > clang only allocate sizeof(unsigned) * 1 for AB.
> >
> > This is not a compiler bug. It's a spec. compliant behavior, and may in
> fact
> > have a better run-time performance in some situations. However, for core
> > objects like RenderObject and InlineBox, allocating extra 4-8 bytes per
> each
> > object is prohibitory expensive.
> >
> > In such cases, please use the same POD type for all bitfield member
> > variables. (Storage class classifiers and variable qualifiers seem to
> have
> > no effect on how variables are packed; e.g. mutable, const, etc...). For
> > example, MSVC will allocate sizeof(unsigned) * 1 for AB if we rewrite the
> > above code as:
> > struct AB {
> >     unsigned m_1 : 31;
> >     unsigned m_2 : 1;
> > }
> >
> > When you're making this change, be sure to audit all code that assigns a
> > non-boolean value to m_2 because implicit type coercion into boolean is
> no
> > longer in effect. For example,
> >
> > AB ab;
> > ab.m_2 = 2;
> > puts(ab.m_2 ? "true" : "false");
> >
> > will print "true" before the change and will print "false" after the
> change.
> > An easy way to ensure you've audited all such code is to add getters and
> > setters for all bitfield member variables or wrap them in a special
> > structure or class as done
> > in http://trac.webkit.org/changeset/103353 and
> http://trac.webkit.org/changeset/104254.
> >
> > Best regards,
> > Ryosuke Niwa
> > Software Engineer
> > Google Inc.
> >
> >
> >
> > _______________________________________________
> > webkit-dev mailing list
> > webkit-dev at lists.webkit.org
> > http://lists.webkit.org/mailman/listinfo.cgi/webkit-dev
> >
>
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