[patch] Disable store merging in asan_expand_mark_ifn

[Jakub Jelinek]

On Mon, Feb 11, 2019 at 12:31:43PM +0100, Eric Botcazou wrote:
> > So, wouldn't it be better to check for STRICT_ALIGNMENT
> > get_pointer_alignment (base_addr) and do this only if that alignment
> > (shifted right by ASAN_SHADOW_SHIFT) is not sufficient and e.g. if we would
> > know that the shadow is at least 2 byte aligned but not 4 byte aligned, use
> > size = 2 instead of always 1?  E.g. compute this before the loop as
> > max_size and for !STRICT_ALIGNMENT use always max_size 4?
> 
> In practice this makes a difference only for objects aligned on 128-bit or 

No.  64-bit aligned offsets too.  If you know 64-bit alignment of base_addr,
you can use size 2 stores (though not size 4 stores) on the
!STRICT_ALIGNMENT targets.  And that is something still pretty common.

> above boundaries though.  Moreover, don't you need to take into account the 
> offset as well, which can be modified through -fasan-shadow-offset?

No.  If people use a bogus shadow offset, prologues/epilogues will not work
either on strict aligned targets, and -fasan-shadow-offset is for
-fsanitize=kernel-address only.  Only page aligned offsets are something
that is supported/reasonable.

	Jakub