Paul Eggert <egg...@cs.ucla.edu> writes:
> Mattias Engdegård reported that the Emacs master source currently
> generates the following warning when Emacs is built with mini-gmp.c
> under Clang 13:
>
>> In file included from /Users/mattias/emacs/lib/mini-gmp-gnulib.c:47:
>> /Users/mattias/emacs/lib/mini-gmp.c:1138:2: warning: unused variable '__cy'
>> [-Wunused-variable]
>> gmp_assert_nocarry (mpn_rshift (np, np, dn, shift));
>> ^
>> /Users/mattias/emacs/lib/mini-gmp.c:91:15: note: expanded from macro
>> 'gmp_assert_nocarry'
>> mp_limb_t __cy = (x); \
>
> The problem occurs because assert(X) does not evaluate X when NDEBUG
> is defined. Proposed patch attached.
Thanks for the report. I suspect builds with NDEBUG defined are not that
well tested.
> diff -r d45103d658ca mini-gmp/mini-gmp.c
> --- a/mini-gmp/mini-gmp.c Wed Mar 30 23:16:18 2022 +0200
> +++ b/mini-gmp/mini-gmp.c Fri Apr 08 16:18:06 2022 -0700
> @@ -87,10 +87,11 @@
> #define GMP_MPN_OVERLAP_P(xp, xsize, yp, ysize)
> \
> ((xp) + (xsize) > (yp) && (yp) + (ysize) > (xp))
>
> -#define gmp_assert_nocarry(x) do { \
> - mp_limb_t __cy = (x); \
> - assert (__cy == 0); \
> - } while (0)
> +#ifdef NDEBUG
> +# define gmp_assert_nocarry(x) ((void) (x))
> +#else
> +# define gmp_assert_nocarry(x) assert ((x) == 0)
> +#endif
I have two minor comments on this proposed fix:
1. Is the void cast really needed? Corresponding macros in gmp-impl.h
are defined like
#if WANT_ASSERT
#define ASSERT_CARRY(expr) ASSERT_ALWAYS ((expr) != 0)
#define ASSERT_NOCARRY(expr) ASSERT_ALWAYS ((expr) == 0)
#else
#define ASSERT_CARRY(expr) (expr)
#define ASSERT_NOCARRY(expr) (expr)
#endif
I.e., in builds without run-time asserts, ASSERT_NO_CARRY(X) expands to
just (X).
2. I'm a bit uneasy about side effects in the argument to the standard
assert macro, even with the NDEBUG ifdefs.
Alternative patch:
--- a/mini-gmp/mini-gmp.c Tue Feb 15 09:18:40 2022 +0100
+++ b/mini-gmp/mini-gmp.c Fri Apr 15 07:20:40 2022 +0200
@@ -90,6 +90,7 @@ see https://www.gnu.org/licenses/. */
#define gmp_assert_nocarry(x) do { \
mp_limb_t __cy = (x); \
assert (__cy == 0); \
+ (void) __cy; \
} while (0)
Regards,
/Niels
--
Niels Möller. PGP key CB4962D070D77D7FCB8BA36271D8F1FF368C6677.
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