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Date: Mon, 16 Dec 2019 10:56:22 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: max_align_t mess on i386
On Mon, Dec 16, 2019 at 10:30:30AM -0500, Jeffrey Walton wrote:
> On Sun, Dec 15, 2019 at 1:22 PM Rich Felker <dalias@...c.org> wrote:
> >
> > 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:
> >
> > Generally, you should look to the C11 or POSIX (man 3p) specifications
> > for the functions rather than the "man 3" ones, but here it's pretty
> > close to the same, just imprecisely worded:
> >
> > > The malloc() and calloc() functions return a pointer to the
> > > allocated memory that is suitably aligned for any kind of variable.
> > >
> > > I expect to be able to use a pointer returned by malloc (and friends)
> > > in MMX, SSE and AVX functions.
> >
> > "Any kind of variable" isn't "any kind of load/store instruction". For
> > example you most certainly will not get 32- or 64-byte alignment that
> > you may want for AVX-256 or AVX-512 without memalign.
>
> GCC tells us the largest alignment that we can expect:
>
> $ gcc -dM -E - </dev/null | grep -i align
> #define __BIGGEST_ALIGNMENT__ 16
>
> Because __BIGGEST_ALIGNMENT__ is 16, I don't expect to get 32-byte or
> 64-byte aligned buffers.
I wasn't aware of this gcc feature. Do you know if it's documented and
what it's derived from? It seems to match what max_align_t is expected
to be, including on i386 (16) and powerpc (16) and indeed it's only 4
on a few 32-bit archs and even 2 on m68k.
> > A max_align_t
> > (and corresponding malloc alignment constraint) that heavily aligned
> > would be awful to use, with memory waste possibly exceeding 1000% and
> > over 500% likely for real-world data structures. Over-alignment also
> > weakens hardening properties by making pointers more predictable.
>
> It sounds like you are moving the fragmentation problem from the
> runtime library to the application. (When fragmentation is a problem).
I don't understand what you mean.
Rich
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