On Tue, Jan 12, 2010 at 10:15 AM, Craig Schlenter <
craig.schlen...@chromium.org> wrote:
> On Tue, Jan 12, 2010 at 7:13 PM, Evan Martin <e...@chromium.org> wrote:
> > In this bug
> > http://code.google.com/p/chromium/issues/detail?id=28749
> > It seems we're running afoul of a more finicky compiler not liking
> > some tricks we're playing with objects and memory in LazyInstance.
> > (You can skip down to comment #30 or so to get to the meat of it --
> > above that point we were still trying to figure out what was causing
> > the crash.)
> >
> > I know enough to say that it sounds like the solution to the problem
> > should involve placement new, but that is the extent of my C++ skill.
> > If you are a wizard, could you take a look at the patch Craig posted
> > on the bug (http://codereview.chromium.org/519045/show) and comment
> > there?
>
> [see base/lazy_instance.{h,c}]
>
> One possible way of attacking the problem is to ignore the stuff I
> posted and just figure out how to plumb the return value from the
> placement new in New() through to Pointer() and use that return value
> instead of instance in Pointer() but to do it without bloating up the
> header file with lots more code. It's the reinterpret_cast in
> Pointer() that we want to avoid.
>
> I actually have a patch that does the ugly thing I suggested in the
> last comment in rietveld but my brain is coming unstuck trying to work
> out how to do Darin's suggestion which is likely to be more elegant
> than my attempts.
>
> Any help is welcome :)
>
In theory chars are ok to alias with anything.
It may be completely stupid but... technically, &buff_ is not strictly
speaking a char*, so reinterpret_cast-ing it may confuse the compiler.
&buff_[0] however is a char* - did we try this trivial change ?
Also, from the header description: "It also preallocates the space for Type,
as to avoid allocating the Type instance on the heap. This may help with
the performance of creating the instance, and reducing heap fragmentation.
This requires that Type be a complete type so we can determine the size."
Have we measured it to be significant enough to make it worth banging our
collective heads on this giant hack ?
Antoine
> --Craig
>
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