Re: max_align_t mess on i386

[Jeffrey Walton <noloader@gmail.com>]

On Sun, Dec 15, 2019 at 1:22 PM Rich Felker <dalias@libc.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@libc.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.

> 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).

Jeff