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

[Matt Turner]

On Mon, Nov 12, 2018 at 3:07 PM Eric Anholt <eric at anholt.net> wrote:
>
> 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>

Thanks a bunch! I hope this is useful for you on arm as well.

> However, shouldn't we also bump SUBALLOC_ALIGNMENT to 16 on LP64, too,
> if that's what glibc is doing for malloc?

16-byte alignment is necessary for SSE aligned vector load/store
instructions. I suppose we're not getting any vectorized SSE
load/store instructions to memory allocated by linear_alloc_* and
that's why we haven't seen problems?

Seems reasonable to bump it to 16-bytes.