Re: New optimized string routines for Intel and alignment of stack.

[Florian Weimer <fweimer at redhat dot com>]

On 06/07/2016 07:56 AM, Carlos O'Donell wrote:
> H.J.,
>
> We have had several users that have built legacy applications
> for 32-bit x86 with stack alignment that does not match the
> ABI.

Let's say the GNU project broke the i386 ABI, which is more accurate. 
The stack pointer alignment requirement is a recent change.

> In all of these cases it has to do with the application
> having been compiled with -falign-stack=assume-4-byte which
> violates the ABI, usually with icc. However, if you're careful
> it all just works.

It will get worse with increased vectorization and GCC 6.  We already 
saw this on x86_64 with the non-compliant malloc in tcsh, where GCC 6 
used vector instructions to copy a struct dirstream object.  I assume 
this could easily happen with any stack-to-stack copy with SSE2 enabled.

Currently, GCC does not seem to exploit the fact that it knows the 
alignment of stack objects.  I played with this:

struct fields
{
  double a, b;
};

struct fields get (void);
void put (struct fields *, struct fields *);

void
copy (void)
{
  struct fields f1 = get ();
  struct fields f2 = f1;
  put (&f1, &f2);
}

And: gcc -m32 -O3 -msse2 -march=westmere -mtune=westmere -o- -S 
stack-align.c

I expected to see an SSE load/store for the copy, but that's not what I got.

I think we need to decide if we want to roll back the ABI change before 
GCC learns about this optimization because eventually, it will not just 
be a matter of string routines.  Any glibc code optimized for 32-bit x86 
CPUs with SSE2 enabled could be affected.

Florian