[Mesa-dev] [PATCH 2/2] util/ralloc: Make sizeof(linear_header) a multiple of 8

[Emil Velikov]

Hi Matt,

On Mon, 12 Nov 2018 at 21:26, Matt Turner <mattst88 at gmail.com> wrote:
>
> Prior to this patch sizeof(linear_header) was 20 bytes in a
> non-debug build on 32-bit platforms. We do some pointer arithmetic to
> calculate the next available location with
>
>    ptr = (linear_size_chunk *)((char *)&latest[1] + latest->offset);
>
> in linear_alloc_child(). The &latest[1] adds 20 bytes, so an allocation
> would only be 4-byte aligned.
>
> On 32-bit SPARC a 'sttw' instruction (which stores a consecutive pair of
> 4-byte registers to memory) requires an 8-byte aligned address. Such an
> instruction is used to store to an 8-byte integer type, like intmax_t
> which is used in glcpp's expression_value_t struct.
>
> As a result of the 4-byte alignment returned by linear_alloc_child() we
> would generate a SIGBUS (unaligned exception) on SPARC.
>
> According to the GNU libc manual malloc() always returns memory that has
> at least an alignment of 8-bytes [1]. I think our allocator should do
> the same.
>
> So, simple fix with two parts:
>
>    (1) Increase SUBALLOC_ALIGNMENT to 8 unconditionally.
>    (2) Mark linear_header with an aligned attribute, which will cause
>        its sizeof to be rounded up to that alignment. (We already do
>        this for ralloc_header)
>
> With this done, all Mesa's unit tests now pass on SPARC.
>
> [1] https://www.gnu.org/software/libc/manual/html_node/Aligned-Memory-Blocks.html
>
> Fixes: 47e17586924f ("glcpp: use the linear allocator for most objects")
> Bug: https://bugs.gentoo.org/636326

This patch didn't apply cleanly on 18.3, so I've resolved the trivial conflict.

Fixed patch is at

[1] https://cgit.freedesktop.org/mesa/mesa/commit/?h=staging/18.3&id=44dfbdf989956095e1610ce152e838fd9eeb089f

Emil