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;
}

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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?

In reply to Why do I need storing ax over ENTER/SAVETMPS -- FREETMPS/LEAVE? by rehsack

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