__memcpy_sse2_unaligned on non-8-byte aligned pointer

[Stephan Bergmann <sbergman at redhat dot com>]

<https://bugzilla.redhat.com/attachment.cgi?id=1048694> is a SIGSEGV 
backtrace where apparently glibc's __memcpy_sse2_unaligned 
(sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S) is trying to write to 
an address that is not 8-byte aligned,

> Dump of assembler code for function __memcpy_sse2_unaligned:
>    0x00007fcf46f35b90 <+0>:	mov    %rsi,%rax
>    0x00007fcf46f35b93 <+3>:	lea    (%rdx,%rdx,1),%rcx
>    0x00007fcf46f35b97 <+7>:	sub    %rdi,%rax
>    0x00007fcf46f35b9a <+10>:	sub    %rdx,%rax
>    0x00007fcf46f35b9d <+13>:	cmp    %rcx,%rax
>    0x00007fcf46f35ba0 <+16>:	jb     0x7fcf46f35cad <__memcpy_sse2_unaligned+285>
>    0x00007fcf46f35ba6 <+22>:	cmp    $0x10,%rdx
>    0x00007fcf46f35baa <+26>:	jbe    0x7fcf46f35d3b <__memcpy_sse2_unaligned+427>
>    0x00007fcf46f35bb0 <+32>:	movdqu (%rsi),%xmm8
>    0x00007fcf46f35bb5 <+37>:	cmp    $0x20,%rdx
> => 0x00007fcf46f35bb9 <+41>:	movdqu %xmm8,(%rdi)
[...]

where rdi is 0x7ffc3830b63c (while rsi is 8-byte aligned at 0x7fcf4a314a08).

The code leading up to that memcpy call looks rather unsuspecting, and 
it's unclear to me from the crash report data why it caused a SIGSEGV.

However, the Intel documentation for MOVDQU states:  "If alignment 
checking is enabled (CR0.AM = 1, RFLAGS.AC = 1, and CPL = 3), an 
alignment-check exception (#AC) may or may not be generated (depending 
on processor implementation) when the operand is not aligned on an 
8-byte boundary."

Could it be plausible that (a) those alignment checking conditions are 
met for a Linux user space process, (b) that alignment-check exception 
would translate into a SIGSEGV (instead of a SIGBUS, say), and (c) that 
there actually are processor implementations that would check for 
non-8-byte alignment?

It doesn't sound too likely that that would be true, as it would mean 
that arbitrary calls to memcpy (which unconditionally calls 
__memcpy_sse2_unaligned without checking for pointer alignment, AFAIU) 
could fail, but I thought I'd ask here anyway.