git: DragonFly_RELEASE_4_6 libc - Fix malloc() alignment for small allocations

Matthew Dillon dillon at
Sat Sep 3 10:32:06 PDT 2016

commit 322960dc2f43c50a9116ea9ecdca6e231e6e77f7
Author: Matthew Dillon <dillon at>
Date:   Sat Sep 3 10:24:56 2016 -0700

    libc - Fix malloc() alignment for small allocations
    * malloc()'s slab allocator was set to use 8-byte alignment
      for any allocation < 128 bytes that was not otherwise on
      an integral alignment boundary.  This breaks GCC-7 which assumes
      16-byte alignment for non-16-integral sizes < 128 bytes.  e.g.
      if 18 bytes is allocated, GCC-7 assumes the resulting pointer will
      be 16-byte-aligned.
    * The standard is somewhat deficient in its characterization of what the
      required alignment should be, because there are already instructions
      which prefer 32 and 64 byte alignments, but are relaxed on Intel to
      only require 16-byte alignments (aka %ymm and %zmm registers in the
      vector extensions), and its stupid to enforce even larger alignments
      for tiny allocations.
    * But generally speaking it makes sense to enforce a 16-byte alignment
      for any allocations >= 16 bytes, regardless of the size being passed-in
      not being 16-byte aligned, and this change does that.  Allocations of
      less than 16 bytes will still be 8-byte aligned because it is phenominally
      wasteful for them not to be.
    Reported-by: marino

Summary of changes:
 lib/libc/stdlib/nmalloc.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

DragonFly BSD source repository

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