On 07/27/2018 05:10 AM, John Reiser wrote:
Always requiring 16-byte alignment on x86_64 can waste too much space
due to internal fragmentation. The rule should be:
The required alignment is at least min(16, max_p2_divisor_of_size)
where the second argument max_p2_divisor_of_size is the maximum
power of 2
that divides the requested size [when 0!= size].
Thus a request for 6 bytes may be satisfied by a block whose address is
divisible by 2.
But this rule is wrong. Consider this:
struct string
{
size_t length;
char data[];
};
To allocate a one-byte string, malloc would be called with an argument
of sizeof (size_t) + 1. Your rule gives alignment 1, but in reality,
alignment 4 or 8 is required.
(Flexible struct members are not strictly required to implement this, of
course, so it's not a new problem at all.)
Thanks,
Florian
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