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Date: Mon, 16 Dec 2019 11:36:42 -0500
From: Jeffrey Walton <noloader@...il.com>
To: musl@...ts.openwall.com
Subject: Re: max_align_t mess on i386
On Mon, Dec 16, 2019 at 10:56 AM Rich Felker <dalias@...c.org> wrote:
>
> 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.
I believe it is documented at
https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html.
The linker problem discussed in the same area has bitten me several
times in the past. It usually arises on 32-bit systems. But PowerPC
also got me when using AIX.
> > > 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.
When we can't get properly aligned buffers in userland, then we
(userland) have to over-commit in our allocators and play the pointer
games. For example, if I can only get 8-byte aligned pointers, then I
always have to allocate n+16 bytes, move the pointer 'p' to the right
for a 16 byte alignment, and store the offset at p-1 so I can delete
the base pointer on delete/free.
Those kind of pointer games are usually played out in the runtime
library. I can only says "usually" and not always because we have to
do them on AIX and GNU Hurd (among others).
Jeff
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