Why do I need storing ax over ENTER/SAVETMPS -- FREETMPS/LEAVE?

rehsack has asked for the wisdom of the Perl Monks concerning the following question:

I try to speed up Params::Util using XS. One method the author wrote, allows a method to override the isa() method, so sv_derived_from() doesn't work in any case. I used following code now:

void
_INSTANCE(ref,type)
    SV *ref;
    char *type;
PROTOTYPE: $$
CODE:
{
    STRLEN len;
    if( SvROK(ref) && type && ( ( len = strlen(type) ) > 0 ) )
    {
        if( SvMAGICAL(ref) )
            mg_get(ref);
        if( sv_isobject(ref) )
        {
            if( sv_derived_from(ref,type) )
            {
                ST(0) = ref;
                XSRETURN(1);
            }
            else
            {
                I32 isa_type = 0;
                int count;

                ENTER;
                SAVETMPS;
                PUSHMARK(SP);
                XPUSHs( sv_2mortal( newSVsv( ref ) ) );
                XPUSHs( sv_2mortal( newSVpv( type, len ) ) );
                PUTBACK;

                if( ( count = call_method("isa", G_SCALAR) ) )
                {
                    I32 oldax = ax;
                    SPAGAIN;
                    SP -= count;
                    ax = (SP - PL_stack_base) + 1;
                    isa_type = SvIV(ST(0));
                    ax = oldax;
                }

                PUTBACK;
                FREETMPS;
                LEAVE;

                if( isa_type )
                {
                    ST(0) = ref;
                    XSRETURN(1);
                }
            }
        }
    }
    XSRETURN_UNDEF;
}
[download]

Why is it necessary to save ax and why isn't it documented in perlcall (http://search.cpan.org/~rgarcia/perl-5.10.0/pod/perlcall.pod#Using_Perl_to_dispose_of_temporaries)? Did I understand sth. wrong and made another mistake?

Comment on Why do I need storing ax over ENTER/SAVETMPS -- FREETMPS/LEAVE? Download Code

Replies are listed 'Best First'.
Re: Why do I need storing ax over ENTER/SAVETMPS -- FREETMPS/LEAVE? by almut (Canon) on Oct 22, 2008 at 23:35 UTC
Why is it necessary to save ax As you're in an XSUB, the `ax` variable has automatically been setup (by xsubpp) to indicate the stack base offset, which is used by macros like ST and XSRETURN. It's usually a good idea to restore the variable to its original value after you've been messing with it, or else an ST/XSRETURN call further down might do bad things... Also see section Variables created by "xsubpp"... in perlapi.	[reply] [d/l]
Re: Why do I need storing ax over ENTER/SAVETMPS -- FREETMPS/LEAVE? by Anonymous Monk on Oct 23, 2008 at 00:49 UTC
FYI, I think Params::Validate does something similar http://search.cpan.org/src/DROLSKY/Params-Validate-0.91/Validate.xs	[reply]
Re: Why do I need storing ax over ENTER/SAVETMPS -- FREETMPS/LEAVE? by gone2015 (Deacon) on Oct 22, 2008 at 23:29 UTC
I dunno, really. But in your position I'd try looking at the C preprocessor output to see what all those macros are up to. On `gcc` the `-E` option.	[reply] [d/l] [select]