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 <[email protected]> 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 <[email protected]> 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 > > [email protected] > > http://lists.webkit.org/mailman/listinfo.cgi/webkit-dev > > >
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