[Mesa-dev] [PATCH] gallium: Fix blend color alignment for 32-bit systems

[Roland Scheidegger]

Am 12.07.2016 um 13:40 schrieb Nicolai Hähnle:
> From: Nicolai Hähnle <nicolai.haehnle at amd.com>
> 
> This fixes a regression introduced by commit d8d6091a8.
> 
> Heap allocations may be only 8-byte aligned on 32-bit system, and so having
> members with 16-byte alignment (such as in the case where pipe_blend_color is
> embedded in radeonsi's si_context) is undefined behavior which indeed causes
> crashes when compiled with gcc -O3.
> 
> Rather than track down and fix all allocation sites where a pipe_blend_color
> may be embedded, assume that the original compiler bug only affects 64-bits.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96835
> Cc: Tim Rowley <timothy.o.rowley at intel.com>
> Cc: Chuck Atkins <chuck.atkins at kitware.com>
> Cc: <mesa-stable at lists.freedesktop.org>
> --
> This should fix the linked bug report. The big question is whether the
> assumption about the original compiler problem is correct?
> ---
>  src/gallium/include/pipe/p_state.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/src/gallium/include/pipe/p_state.h b/src/gallium/include/pipe/p_state.h
> index a73a771..1986495 100644
> --- a/src/gallium/include/pipe/p_state.h
> +++ b/src/gallium/include/pipe/p_state.h
> @@ -335,8 +335,13 @@ struct pipe_blend_color
>      * unaligned accessors resulting in a segfault.  Specifically several
>      * versions of the Intel compiler are known to be affected but it's likely
>      * others are as well.
> +    *
> +    * This only applies on 64-bit architectures, and adding the alignment on
> +    * 32-bit architectures causes bugs because heap allocations are not
> +    * sufficiently aligned.
>      */
> -   PIPE_ALIGN_VAR(16) float color[4];
> +   PIPE_ALIGN_VAR(sizeof(void *) >= 8 ? 16 : 4)
> +   float color[4];
>  };
>  

Honestly, I'd rather get rid of it, this gets really hacky. (If the
compiler bug was happening in the driver, it could easily do alignment
on its own, even if possibly at the cost of a copy).

Note that technically, malloc allocations aren't guaranteed to be 16
byte on a 64bit arch. Rather, malloc() has to honor biggest alignment
for a standard type (and no, __mm128 doesn't count), which on x64 linux
is long double (128bit). No idea if that's the case everywhere (even if
it's not, there's of course some possibility it will return 16 byte
aligned addresses anyway).

Roland