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

[Eric Anholt]

Matt Turner <mattst88 at gmail.com> writes:

> 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
> ---
>  src/util/ralloc.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/src/util/ralloc.c b/src/util/ralloc.c
> index 745b4cf1226..fc35661996d 100644
> --- a/src/util/ralloc.c
> +++ b/src/util/ralloc.c
> @@ -552,10 +552,18 @@ ralloc_vasprintf_rewrite_tail(char **str, size_t *start, const char *fmt,
>   */
>  
>  #define MIN_LINEAR_BUFSIZE 2048
> -#define SUBALLOC_ALIGNMENT sizeof(uintptr_t)
> +#define SUBALLOC_ALIGNMENT 8
>  #define LMAGIC 0x87b9c7d3
>  
> -struct linear_header {
> +struct
> +#ifdef _MSC_VER
> + __declspec(align(8))
> +#elif defined(__LP64__)
> + __attribute__((aligned(16)))
> +#else
> + __attribute__((aligned(8)))
> +#endif
> +   linear_header {
>  #ifndef NDEBUG
>     unsigned magic;   /* for debugging */
>  #endif
> @@ -647,6 +655,8 @@ linear_alloc_child(void *parent, unsigned size)
>     ptr = (linear_size_chunk *)((char*)&latest[1] + latest->offset);
>     ptr->size = size;
>     latest->offset += full_size;
> +
> +   assert((uintptr_t)&ptr[1] % SUBALLOC_ALIGNMENT == 0);
>     return &ptr[1];
>  }

These patches are:

Reviewed-by: Eric Anholt <eric at anholt.net>

However, shouldn't we also bump SUBALLOC_ALIGNMENT to 16 on LP64, too,
if that's what glibc is doing for malloc?
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 832 bytes
Desc: not available
URL: <https://lists.freedesktop.org/archives/mesa-dev/attachments/20181112/6ccf08b8/attachment.sig>