Op 2016-04-07 om 06:23 schreef Edward Tomasz Napierala:
+static uint64_t
+xmul(uint64_t a, uint64_t b)
+{
+ uint64_t c;
+
+ if (a == 0 || b == 0)
+ return (0);
+
+ c = a * b;
+
+ if (c < a || c < b)
+ return (UINT64_MAX);
If the intent is to check for overflow, then this check is insufficient.
It fails for example if a = 2^32+1 and b = 2^32.
This works for all cases, assuming a != 0:
if(UINT64_MAX / a > b)
return (UINT64_MAX);
If the extra division is too expensive, GCC and clang provide
__builtin_mul_overflow().
--
Pieter de Goeje
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