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Message-ID: <20191215182314.GB986899@wirbelwind.zhasha.com>
Date: Sun, 15 Dec 2019 19:23:14 +0100
From: Joakim Sindholt <opensource@...sha.com>
To: musl@...ts.openwall.com
Subject: Re: max_align_t mess on i386
On Sun, Dec 15, 2019 at 01:06:29PM -0500, Jeffrey Walton wrote:
> On Sat, Dec 14, 2019 at 10:19 AM Rich Felker <dalias@...c.org> wrote:
> >
> > In reserching how much memory could be saved, and how practical it
> > would be, for the new malloc to align only to 8-byte boundaries
> > instead of 16-byte on archs where alignof(max_align_t) is 8 (pretty
> > much all 32-bit archs), I discovered that GCC quietly changed its
> > idead of i386 max_align_t to 16-byte alignment in GCC 7, to better
> > accommodate the new _Float128 access via SSE. Presumably (I haven't
> > checked) the change is reflected with changes in the psABI document to
> > make it "official".
>
> Be careful with policy changes like this. The malloc (3) man page says:
>
> The malloc() and calloc() functions return a pointer to the
> allocated memory that is suitably aligned for any kind of variable.
Your man pages are not the standard, but the standard does have this to
say:
> The pointer returned if the allocation succeeds shall be suitably
> aligned so that it may be assigned to a pointer to any type of object
> and then used to access such an object in the space allocated (until the
> space is explicitly freed or reallocated).
To me this sounds like my next suggestion is technically disallowed.
> I expect to be able to use a pointer returned by malloc (and friends)
> in MMX, SSE and AVX functions.
I might agree, but would it not be feasible to have the alignment of the
returned pointer be dependent on the size of the allocation? That way,
if you allocate <16 bytes you can get 8 byte alignment. You might even
be able to go all the way down to 4 byte alignment for <8 byte
allocations.
It might violate the standard technically speaking, but I don't know of
any examples of types smaller than 16 bytes that require 16 byte
alignment.
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