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

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

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Dmitry Antipov wrote:
> I don't know how to use VLAs in the way similar to alloca

For vectors the macro needs to generate a series of declarations and 
statements rather than an expression.  It's less clean, but it's good 
enough.  Something like the attached, say.

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/* Placeholders for what's in lisp.h already.  */

#include <stddef.h>
#include <stdbool.h>
#include <inttypes.h>

typedef long Lisp_Object;
enum { word_size = sizeof (Lisp_Object) };
enum { MAX_ALLOCA = 16 * 1024 };
#define USE_SAFE_ALLOCA bool sa_must_free = false
#define SAFE_FREE() (sa_must_free ? magic_freer () : (void) 0)

#define min(a, b) ((a) < (b) ? (a) : (b))

extern void *xnmalloc (size_t, size_t);
extern Lisp_Object make_save_memory (Lisp_Object *, ptrdiff_t);
extern void magic_freer (void);
extern void record_unwind_protect (void (*) (Lisp_Object), Lisp_Object);
extern void free_save_value (Lisp_Object);
extern _Noreturn void memory_full (size_t);


/* The new macro.  */

#ifdef __STDC_NO_VLA__
# define SAFE_LISP_ARRAY(name, elems)	\
    Lisp_Object *name;			\
    SAFE_ALLOCA_LISP (name, elems)
#else
# define SAFE_LISP_ARRAY(name, elems)					\
    ptrdiff_t name##n = elems;						\
    bool name##issmall = name##n <= MAX_ALLOCA / word_size;		\
    Lisp_Object name##vec[name##issmall && name##n ? name##n : 1];	\
    Lisp_Object *name;							\
    if (name##issmall)							\
      name = name##vec;							\
    else								\
      {									\
	name = xnmalloc (name##n, word_size);				\
	record_unwind_protect (free_save_value,				\
			       make_save_memory (name, name##n));	\
	sa_must_free = true;						\
      }
#endif


/* Example use.  */

Lisp_Object
foo (ptrdiff_t n)
{
  USE_SAFE_ALLOCA;
  SAFE_LISP_ARRAY (foo, n);
  for (ptrdiff_t i = 0; i < n; i++)
    foo[i] = i;
  Lisp_Object x = 0;
  for (ptrdiff_t i = 0; i < n; i++)
    x ^= foo[0];
  SAFE_FREE ();
  return x;
}