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Date: Sun, 6 Sep 2015 17:17:03 +0300
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: Large stack alignment
On Sun, Sep 06, 2015 at 03:15:11PM +0200, magnum wrote:
> On 2015-09-06 13:20, magnum wrote:
> >So Lei reminded me of this:
> >http://www.openwall.com/lists/john-dev/2015/08/24/8
> >
> >We have an issue for changing all stack allocs using MEM_ALIGN_SIMD to
> >align ourselves.
>
> This is fixed in 1bd8d9d.
I think we (including me) still don't have adequate understanding of
the problem. This commit fixes the instances where we were using gcc's
alignment attributes, but it does not touch explicit vector variables on
the stack such as SIMDmd5body()'s:
vtype w[16*SIMD_PARA_MD5];
vtype a[SIMD_PARA_MD5];
vtype b[SIMD_PARA_MD5];
vtype c[SIMD_PARA_MD5];
vtype d[SIMD_PARA_MD5];
vtype tmp[SIMD_PARA_MD5];
vtype tmp2[SIMD_PARA_MD5];
vtype mask;
We're hoping these will be in registers, but when not will they be
properly aligned for AVX2? I guess this is just as dependent on stack
pointer alignment and gcc's capabilities as uses of gcc's alignment
attribute were. (And if so, the 1bd8d9d commit makes little sense.)
Also, when gcc spills AVX2 registers to stack, does it ensure proper
alignment? Or does it use unaligned-capable instructions? Or neither?
We need to figure all of this out.
A related issue are library callbacks when our code is called by a
library that was compiled with a smaller -mpreferred-stack-boundary or
with non-gcc (and for an ABI permitting a smaller stack alignment than
we need). This may be the cause of some of the problems, such as what
Lei and Jim saw with OpenMP (the threads are started via OpenMP runtime
library), even when the program initially started with proper stack
alignment (which is also dependent not only on gcc, but on dynamic
linker and libc startup code).
We might want to consider reading up on and using -mstackrealign or/and
-mincoming-stack-boundary. And maybe we'd be able to revert the
explicit alignment commits then, which as I suggested above are likely
not a complete solution anyway. I expect that forcing gcc to realign
the stack would have performance impact, though. And then there are
non-gcc compilers.
It's a mess.
Alexander
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