Re: Benchmarking temporary Lisp objects [Was: Re: [RFC] temporary Lisp_Strings]

[Paul Eggert <eggert@cs.ucla.edu>]

Dmitry Antipov wrote:
> I don't see how to fold this snippet into a macro which can be used as
> an rvalue, just like:

How about something like this?  The cool thing here is that GCC 
optimizes 'foo' away to a function that simply returns 0.

typedef unsigned long uintptr_t;

typedef uintptr_t Lisp_Object;

struct __attribute__ ((aligned (8))) Lisp_Cons
{
   Lisp_Object car;
   Lisp_Object cdr;
};

#define BCONS(a, b) ((Lisp_Object) &(struct Lisp_Cons) { a, b } + 3)
#define UNTAG(x) ((struct Lisp_Cons *) (uintptr_t) ((x) - 3))

int
foo (void)
{
   Lisp_Object a = BCONS (0, 0);
   Lisp_Object b = BCONS (0, a);
   return UNTAG (UNTAG (b) -> cdr) -> car;
}