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Date: Mon, 24 Aug 2015 08:38:19 +0300
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: Formats using non-SIMD SHA2 implementations

On Mon, Aug 24, 2015 at 11:43:50AM +0800, Lei Zhang wrote:
> BTW, I encountered an issue testing with OpenMP. I used some stack arrays as vector buffers, but gcc-5 cannot align them properly when OpenMP is enabled (thus giving me segfaults). This seems to be a known issue in some older gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=16660

This isn't OpenMP-related, and it isn't an issue in some older gcc -
even though (I am surprised) they're keeping the "bug" open - I guess
they're still considering enhancing gcc to deal even with that.

The stack is only guaranteed to be 16-byte aligned (and on 32-bit x86
even this wasn't the case until gcc 2.95, and some systems didn't
provide this alignment even later).  When you use AVX2, you need 32-byte
alignment, but you can't reliably have that on the stack (unless you do
it on your own: over-allocate space and align the pointer as necessary).

> I also used stack arrays as vector buffers in 7z, but somehow it just works fine.

Luck.  Arguably, that's bad luck (hides a real problem from you).

> If this cannot be solved, I could switch to global arrays anyway.

This can be worked around in a number of ways.  But you really shouldn't
expect the system to just do it for you.

Alexander

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