Ok ... I'll just make it a blanket "#define PERL_NO_GET_CONTEXT" ... no ifs and buts ;-)

I mainly use Inline::C to autogenerate my XS files.
The appealing aspect of using dTHX is that Inline::C already accommodates that approach. That is, this simple Inline::C demo works fine:

use strict;
use warnings;

use Inline C => Config =>
  BUILD_NOISY => 1,
  PRE_HEAD => "#define PERL_NO_GET_CONTEXT",
  ;

use Inline C => <<'EOC';

SV * dubble(SV * in) {
 dTHX;
 return newSViv(SvIV(in) * 2);
}

EOC

my $x = dubble(23);
print $x; # 46
[download]

But handling pTHX_ and aTHX_ is way beyond Inline's current capabilities, and the XS file therefore needs to be written by hand.

I'm not even sure what the XS file should look like when we use pTHX_ and aTHX_. The following seems to work:

#define PERL_NO_GET_CONTEXT

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

SV * dubble(pTHX_ SV * in) {
 return newSViv(SvIV(in) * 2);
}

MODULE = FOO    PACKAGE = FOO    

SV *
dubble (in)
    SV *    in
CODE:
  RETVAL = dubble(aTHX_ in);
OUTPUT:
  RETVAL
[download]

But I wouldn't like to guarantee that it's not doing something wrong and/or unnecessary.
The example in the docs to which you linked, invokes a wrapper function called my_xsub - but I've written it so that dubble() wraps itself.
Is that a valid technique ? Is there something better ?

Thanks Dave. Good question halfcountplus.

Cheers,
Rob