On Sep 7, 2008, at 10:20 AM, Paul Pedriana wrote: > I agree that a union is most portable, but there are two problems > with it: > > - The C++ Standard (section 9.5 p1) disallows union members that > have ctors, dtors, or assignment operators. This includes the > case > whereby the class is the only non-POD of the union. > > - It would create an instance of type T. Thus an empty Vector would > construct an instance of T, resizing it would overwrite the > instance of T, > and on destruction it would destruct the T.
Oliver also pointed out the constructor / destructor issue, so yeah, this wouldn't work. We could use double to force 8-byte alignment but that might be wasteful for vector types where wider alignment is not required. - Maciej > > > Paul > > > >> On Sep 4, 2008, at 1:20 AM, Paul Pedriana wrote: >> >>> Well the proposed solution in >>> <https://bugs.webkit.org/show_bug.cgi?id=16925> doesn't work, as VC >>> ++ >>> doesn't accept that syntax. And the solutions in >>> <https://bugs.webkit.org/show_bug.cgi?id=19775> whereby a uint32_t >>> or >>> uint64_t buffer are made don't work for larger types and use more >>> memory >>> than necessary. They might be useful as a fallback for compilers >>> that >>> don't support alignment directives, though the primary >>> implementations >>> of every compiler that I work with (GCC, VC++, CodeWarrior, EDG, SN, >>> IBM) support it. >> >> The buffer should definitely remain char to avoid strict aliasing >> problems. However, I think the most portable way to ensure >> alignment is to union the buffer with a one-element array of the >> template parameter type T. >> >> - Maciej >> >> >>> I'll make a patch and attach it to >>> <https://bugs.webkit.org/show_bug.cgi?id=16925>, if that's OK. >> >> >>> Paul >>> >>>> On Sep 4, 2008, at 2:05 AM, Paul Pedriana wrote: >>>> >>>>> I see that JavaScriptCore/wtf/Vector.h has this: >>>>> >>>>> // FIXME: Nothing guarantees this buffer is appropriately >>>>> aligned to >>>>> hold objects of type T. >>>>> char m_inlineBuffer[m_inlineBufferSize]; >>>> >>>> >>>> We have a bug for this: >>>> <https://bugs.webkit.org/show_bug.cgi?id=16925>. Also, >>>> <https://bugs.webkit.org/show_bug.cgi?id=19775>. >>>> >>>> There are several patches and suggestions in these bugs, which wait >>>> for someone to submit them as clean patches. >>>> >>>> - WBR, Alexey Proskuryakov >>>> >>>> >>>> >>> >>> _______________________________________________ >>> webkit-dev mailing list >>> [email protected] >>> http://lists.webkit.org/mailman/listinfo.cgi/webkit-dev >> >> > _______________________________________________ webkit-dev mailing list [email protected] http://lists.webkit.org/mailman/listinfo.cgi/webkit-dev
