[Mesa-dev] [PATCH] llvmpipe: Optimize new fs state setup

[Keith Whitwell]

On Thu, 2011-06-30 at 03:27 -0700, Jose Fonseca wrote:
> 
> ----- Original Message -----
> > On Thu, 2011-06-30 at 03:36 +0200, Roland Scheidegger wrote:
> > > Ok in fact there's a gcc bug about memcmp:
> > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43052
> > > In short gcc's memcmp builtin is totally lame and loses to glibc's
> > > memcmp (including call overhead, no knowledge about alignment etc.)
> > > even
> > > when comparing only very few bytes (and loses BIG time for lots of
> > > bytes
> > > to compare). Oops. Well at least if the strings are the same (I'd
> > > guess
> > > if the first byte is different it's hard to beat the gcc
> > > builtin...).
> > > So this is really a gcc bug. The bug is quite old though with no
> > > fix in
> > > sight apparently so might need to think about some workaround (but
> > > just
> > > not doing the comparison doesn't look like the right idea, since
> > > apparently it would be faster with the comparison if gcc's memcmp
> > > got
> > > fixed).
> > 
> > Looking at the struct again (it's been a while), it seems like it
> > could
> > be rearranged to be variable-sized and on average significantly
> > smaller:
> > 
> > struct lp_rast_state {
> >    struct lp_jit_context jit_context;
> >    struct lp_fragment_shader_variant *variant;
> > };
> > 
> > struct lp_jit_context {
> >    const float *constants;
> >    float alpha_ref_value;
> >    uint32_t stencil_ref_front, stencil_ref_back;
> >    uint8_t *blend_color;
> >    struct lp_jit_texture textures[PIPE_MAX_SAMPLERS];
> > };
> > 
> > If we moved the jit_context part behind "variant", and then hopefully
> > note that most of those lp_jit_texture structs are not in use.  That
> > would save time on the memcmp *and* space in the binned data.
> 
> Yeah, sounds a good idea.
> 
> But there's some subtletly to computing the number of textures: it
>  can't be just the NULL textures, because they may be reffered by the
>  JIT code, which has no NULL checks and  relies on the state setup to
>  provide storage for all textures, or dummy memory if one is not bound.

So it's a property of the variant, right?  We should just store that
information when we generate the llvm variant.

> I think a better idea would be:
> - split the texture/sampler state
> - to make the lp_jit_context::textures an array of pointers, and put the struct lp_jit_texture in the pipe_texture object themselves
> - to make the lp_jit_context::samplers an array of pointers, and put the struct lp_jit_sampler in the pipe_sampler_state CSO

I like this too - it's somewhat more involved of course.

In fact the two are orthogonal -- the struct below can still be shrunk
significantly by knowing how many samplers & textures the variant refers
to.  Interleaving them or packing them would reduce the bytes to be
compared.

Alternatively there could be just a pointer in jit_context to
textures/samplers binned elsewhere.

> struct lp_jit_context {
>     struct lp_jit_texture *textures[PIPE_MAX_SAMPLERS];
>     struct lp_jit_sampler *samplers[PIPE_MAX_SAMPLERS];
> };

The jit context above seems to have lost some of its fields...

The next step might be to split the context into four parts: textures,
samplers, constants, "other", and have jit_context just be a set of
pointers into the binned data:

struct lp_jit_context {
     struct lp_jit_texture **textures;
     struct lp_jit_sampler **samplers;
     const float *constants;
     const struct lp_jit_other *other;   
};

struct lp_jit_other {
   float alpha_ref_value;
   uint32_t stencil_ref_front;
   uint32_t stencil_ref_back;
   uint8_t *blend_color;
};

> struct lp_jit_texture
> {
>    uint32_t width;
>    uint32_t height;
>    uint32_t depth;
>    uint32_t first_level;
>    uint32_t last_level;
>    uint32_t row_stride[LP_MAX_TEXTURE_LEVELS];
>    uint32_t img_stride[LP_MAX_TEXTURE_LEVELS];
>    const void *data[LP_MAX_TEXTURE_LEVELS];
>    /* sampler state, actually */
>    float min_lod;
>    float max_lod;
>    float lod_bias;
>    float border_color[4];
> };
> 
> struct lp_jit_sampler
> {
>    float min_lod;
>    float max_lod;
>    float lod_bias;
>    float border_color[4];
> };
> 
> 
> Jose