On Thu, Jan 12, 2017 at 08:37:18PM -0800, Linus Torvalds wrote:
> So then the compiler actually needs to start adding useless instructions
> just to keep the stack 16-byte aligned.

Which it does.  Of course most of the time no extra instructions
are required because there are stack variables, so it's just matter
of adding 8 to the value you're subtracting from rsp.  But it is
probably why gcc assumes that the stack is 16-byte aligned which
triggered my original crash.

Here is an example from the function that was involved in the crash,
without frame pointers:

00000000000001b0 <chacha20_simd>:
 1b0:   41 54                   push   %r12
 1b2:   55                      push   %rbp
 1b3:   48 81 ec f8 00 00 00    sub    $0xf8,%rsp

Email: Herbert Xu <herb...@gondor.apana.org.au>
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