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RE: [perl #50386] GIMME_V broken with 5.10.0/GCC and XS?

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Jan Dubois
January 30, 2008 01:55
RE: [perl #50386] GIMME_V broken with 5.10.0/GCC and XS?
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On Tue, 29 Jan 2008, Robert May (via RT) wrote:
> -----------------------------------------------------------------
> I'm building the following XS extension with
> gcc 3.4.5 (mingw special) against ActiveState Perl 5.10.0
> (builds 1001 and 1002)
> GIMME_V appears to always return G_VOID (128), regardless of the
> context the XSUB is called in.
> The same code and compiler works fine when built against ActiveState
> Perl 5.8.8 (build 822).  It is also fine when compiled with VC++ 6
> for all 3 perl builds.

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++.

The GIMME_V() function retrieves the context information from the
op_flags byte.  Under GCC the op_flags member fetches one of the
2 filler bytes VC++ inserts into the structure and is therefore

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 would suggest to keep this bug report open until the size of
the BASEOP structure in bleadperl has been reduced for VC++.

For ActivePerl 5.10.x we will probably modify op.h to provide an
alternate definition of BASEOP for GCC that contains 2 filler bytes to
make everything align the same way as it does for VC++ (unless we decide
to break binary compatibility between 5.10 and 5.10.1, which I would not
be too fond of).

I have not yet verified that otherwise the bit mapping between
VC++ and GCC would be compatible though; many details of C bit-field
implementation is really up for the compiler to decide.

The ability to compile extensions with GCC for a Perl compiled
with VC++ is really an ActivePerl feature and not guaranteed
by core Perl.  You have also opened a corresponding ActivePerl bug
that should be used to track that part of the issue:


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