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author | Paul Eggert <eggert@cs.ucla.edu> | 2016-06-13 11:17:03 -0700 |
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committer | Paul Eggert <eggert@cs.ucla.edu> | 2016-06-13 11:17:44 -0700 |
commit | 097a5928a785bae77ed1255e57b6553dd1912efe (patch) | |
tree | b0a4a3d61167036522f731dec443309068d693aa /lib/xalloc-oversized.h | |
parent | 6dc5c2148c80205f630e2ca19e01cec0d946a949 (diff) | |
download | gnulib-097a5928a785bae77ed1255e57b6553dd1912efe.tar.gz |
xalloc-oversized: port to GCC 7; fewer warnings
GCC 7 will have a better way to deal with integer overflow.
Plus, fix a warnings problem reported by Tim Ruehsen in:
http://lists.gnu.org/archive/html/bug-gnulib/2016-06/msg00022.html
* lib/xalloc-oversized.h (__xalloc_oversized): New macro.
(xalloc_oversized): Use plain __builtin_mul_overflow if GCC 7 or later.
For GCC 5, use __xalloc_oversized if both args are constants,
or if pedantic.
Diffstat (limited to 'lib/xalloc-oversized.h')
-rw-r--r-- | lib/xalloc-oversized.h | 42 |
1 files changed, 33 insertions, 9 deletions
diff --git a/lib/xalloc-oversized.h b/lib/xalloc-oversized.h index d81a8471d3..44f16441c7 100644 --- a/lib/xalloc-oversized.h +++ b/lib/xalloc-oversized.h @@ -20,15 +20,13 @@ #include <stddef.h> +/* Default for (non-Clang) compilers that lack __has_builtin. */ #ifndef __has_builtin # define __has_builtin(x) 0 #endif -/* Return 1 if an array of N objects, each of size S, cannot exist due - to size arithmetic overflow. S must be positive and N must be - nonnegative. This is a macro, not a function, so that it - works correctly even when SIZE_MAX < N. - +/* True if N * S would overflow in a size calculation. + This expands to a constant expression if N and S are both constants. By gnulib convention, SIZE_MAX represents overflow in size calculations, so the conservative dividend to use here is SIZE_MAX - 1, since SIZE_MAX might represent an overflowed value. @@ -36,12 +34,38 @@ sizeof (ptrdiff_t) <= sizeof (size_t), so do not bother to test for exactly-SIZE_MAX allocations on such hosts; this avoids a test and branch when S is known to be 1. */ -#if 5 <= __GNUC__ || __has_builtin (__builtin_mul_overflow) +#define __xalloc_oversized(n, s) \ + ((size_t) (sizeof (ptrdiff_t) <= sizeof (size_t) ? -1 : -2) / (s) < (n)) + + +/* Return 1 if an array of N objects, each of size S, cannot exist due + to size arithmetic overflow. S must be positive and N must be + nonnegative. This is a macro, not a function, so that it + works correctly even when SIZE_MAX < N. */ + +/* GCC 7 __builtin_mul_overflow should easily compute this. See: + https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68120 */ +#if 7 <= __GNUC__ +# define xalloc_oversized(n, s) __builtin_mul_overflow (n, s, (size_t *) NULL) + +/* GCC 5 and Clang __builtin_mul_overflow needs a temporary, and + should be used only for non-constant operands, so that + xalloc_oversized is a constant expression if both arguments are. + Do not use this if pedantic, since pedantic GCC issues a diagnostic + for ({ ... }). */ +#elif ((5 <= __GNUC__ \ + || (__has_builtin (__builtin_mul_overflow) \ + && __has_builtin (__builtin_constant_p))) \ + && !__STRICT_ANSI__) # define xalloc_oversized(n, s) \ - ({ size_t __xalloc_size; __builtin_mul_overflow (n, s, &__xalloc_size); }) + (__builtin_constant_p (n) && __builtin_constant_p (s) \ + ? __xalloc_oversized (n, s) \ + : ({ size_t __xalloc_size; __builtin_mul_overflow (n, s, &__xalloc_size); })) + +/* Other compilers use integer division; this may be slower but is + more portable. */ #else -# define xalloc_oversized(n, s) \ - ((size_t) (sizeof (ptrdiff_t) <= sizeof (size_t) ? -1 : -2) / (s) < (n)) +# define xalloc_oversized(n, s) __xalloc_oversized (n, s) #endif #endif /* !XALLOC_OVERSIZED_H_ */ |