[Bug malloc/30723] Repeated posix_memalign calls produce long free lists, high fragmentation

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30723

--- Comment #4 from Florian Weimer <fweimer at redhat dot com> ---
First part committed:

commit 542b1105852568c3ebc712225ae78b8c8ba31a78
Author: Florian Weimer <fweimer@redhat.com>
Date:   Fri Aug 11 11:18:17 2023 +0200

    malloc: Enable merging of remainders in memalign (bug 30723)

    Previously, calling _int_free from _int_memalign could put remainders
    into the tcache or into fastbins, where they are invisible to the
    low-level allocator.  This results in missed merge opportunities
    because once these freed chunks become available to the low-level
    allocator, further memalign allocations (even of the same size are)
    likely obstructing merges.

    Furthermore, during forwards merging in _int_memalign, do not
    completely give up when the remainder is too small to serve as a
    chunk on its own.  We can still give it back if it can be merged
    with the following unused chunk.  This makes it more likely that
    memalign calls in a loop achieve a compact memory layout,
    independently of initial heap layout.

    Drop some useless (unsigned long) casts along the way, and tweak
    the style to more closely match GNU on changed lines.

    Reviewed-by: DJ Delorie <dj@redhat.com>

Second part is still under review, I need to send a v2.

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