`checking size of mp_limb_t... 0` and `configure: error: Oops, mp_limb_t doesn't seem to work` when compiling the Microchip XC32 compiler from source

[Niels Möller]

Marc Glisse <marc.glisse at inria.fr> writes:

> From acinclude.m4:
>
>
> dnl  GMP_INCLUDE_GMP_H
> dnl  -----------------
> dnl  Expand to the right way to #include gmp-h.in.  This must be used
> dnl  instead of gmp.h, since that file isn't generated until the end
> of the
> dnl  configure.
> dnl
> dnl  Dummy value for GMP_LIMB_BITS is enough
> dnl  for all current configure-time uses of gmp.h.
>
> define(GMP_INCLUDE_GMP_H,
> [[#define __GMP_WITHIN_CONFIGURE 1   /* ignore template stuff */
> #define GMP_NAIL_BITS $GMP_NAIL_BITS
> #define GMP_LIMB_BITS 123
> $DEFN_LONG_LONG_LIMB
> #include "$srcdir/gmp-h.in"]
> ])

Ah, I wasn't aware of that. Seems to be used for four tests, the
AC_CHECK_SIZEOF for mp_limb_t (that's the one failing in this bug
report), GMP_FUNC_ALLOCA, GMP_H_EXTERN_INLINE, and GMP_H_HAVE_FILE.

I wonder if it would be more robust to temporarily(?) add -I$srcdir to
CFLAGS?

I don't quite get how this part of the configure works, e.g., I'm
looking closer at

  AC_CHECK_SIZEOF(mp_limb_t, , GMP_INCLUDE_GMP_H)

which is used for define GMP_LIMB_BITS,

  AC_SUBST(GMP_LIMB_BITS, `expr 8 \* $ac_cv_sizeof_mp_limb_t`)

The definition of mp_limb_t in gmp-h.in depends on two preprocessor
symbols, __GMP_SHORT_LIMB (not defined by configure, so presumably set
by the user, like CFLAGS=-D__GMP_SHORT_LIMB?), and _LONG_LONG_LIMB.

But the latter depends on the substitution 

  @DEFN_LONG_LONG_LIMB@

which is skipped during the configure tests, but defined by configure
based on

  case $limb_chosen in
  longlong) DEFN_LONG_LONG_LIMB="#define _LONG_LONG_LIMB 1"    ;;
  *)        DEFN_LONG_LONG_LIMB="/* #undef _LONG_LONG_LIMB */" ;;
  esac
  AC_SUBST(DEFN_LONG_LONG_LIMB)

So not clear to me if/how GMP_LIMB_BITS is set correctly in a long long
configuration?

Regards,
/Niels

-- 
Niels Möller. PGP key CB4962D070D77D7FCB8BA36271D8F1FF368C6677.
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