[PATCH] malloc: Improve memalign alignment handling

[Paul Eggert]

On 2026-02-23 05:35, Wilco Dijkstra wrote:
> -              + __builtin_clz (MAX_TCACHE_SMALL_SIZE)
> -              - __builtin_clz (nb);
> +              + stdc_count_zeros ((size_t) MAX_TCACHE_SMALL_SIZE)
> +              - stdc_count_zeros (nb);

Surely you meant stdc_leading_zeros not stdc_count_zeros, as the latter 
is a population count. Similarly elsewhere in the patch.

Given that the patch passed regression tests, that argues the code isn't 
being tested enough....

Less important: why is that (size_t) cast needed? MAX_TCACHE_SMALL_SIZE 
is already of type size_t, no?


+      alignment = (size_t) 2 << (stdc_count_zeros ((size_t) 1)
+                                - stdc_count_zeros (alignment));

Simpler and clearer (and more correct) is the following, which works 
because ALIGNMENT is nonzero and is not a power of 2:

   alignment = (size_t) 2 << (stdc_bit_width (alignment) - 1);

> -  if (alignment % sizeof (void *) != 0
> -      || !powerof2 (alignment / sizeof (void *))
> -      || alignment == 0)
> +  if (alignment < sizeof (void *) || !powerof2 (alignment))

This assumes sizeof (void *) is a power of 2, which is a reasonable 
assumption but that should be stated by adding this:

    static_assert (powerof2 (sizeof (void *)));