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Postings from May 2008
On Fri, Feb 01, 2008 at 01:40:41PM -0800, Jan Dubois wrote:
> On Wed, 30 Jan 2008, Jan Dubois wrote:
> > This is not really a core Perl bug; the problem is that VC++ and GCC
> > are incompatible as far as bit-fields are concerned. In Perl 5.10
> > the BASEOP structure contains bitfields:
> >
> > unsigned op_type:9; \
> > unsigned op_opt:1; \
> > unsigned op_latefree:1; \
> > unsigned op_latefreed:1; \
> > unsigned op_attached:1; \
> > unsigned op_spare:3; \
> >
> > In VC++ the combined size of the fields above is 4 bytes, whereas
> > in GCC it is only 2. This also pushes the size of the complete
> > structure from 20 to 24 bytes for VC++.
>
> [...]
>
> > VC++ uses the base type (unsigned int, in this case) to determine
> > the alignment requirements and total size of the combined bit-field.
> > You can use the non-standard extension of using smaller integer sizes
> > to force the fields into a smaller size and different alignment.
> > In this case using "unsigned short" for the type of the bit-fields
> > should work.
> >
> > I guess for Perl 5.12 we need a way to let us use the smaller base
> > size for VC++ to keep the size of the OP structure down. I'll create
> > a patch for it eventually, when I get a bit more time. I also need
> > to check if there are other places where Perl is now using bit fields
> > in its structure definitions.
>
> I've attached a patch that defined PERL_BITFIELD8, PERL_BITFIELD16
> and PERL_BITFIELD32 symbols to be used to specify bitfields whose
> base size and alignment requirements are 1, 2 or 4 bytes respectively.
> The "PERL_" prefix is a bit awkward, but these are names that must
> be defined even when PERL_CORE is not.
>
> These symbols are then being redefined for VC (and MinGW GCC) to use the
> corresponding smaller sizes as the base types.
>
> I've also added -mms-bitfields to the default options for MinGW in
> makefile.mk. That way building additional XS modules that interface
> to VC compatible libraries using bit-fields in their external APIs
> should be easier.
Jan, is this patch safe for 5.10.1 - specifically is it binary compatible?
--
A major Starfleet emergency breaks out near the Enterprise, but
fortunately some other ships in the area are able to deal with it to
everyone's satisfaction.
-- Things That Never Happen in "Star Trek" #13
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