[Mesa-dev] [PATCH 05/36] i965: old VS: use the VUE map to compute the URB entry size.

[Eric Anholt]

On Fri,  2 Sep 2011 09:06:44 -0700, Paul Berry <stereotype441 at gmail.com> wrote:
> Previously, the old VS backend computed the URB entry size by adding
> the number of vertex shader outputs to the size of the URB header.
> This often produced a larger result than necessary, because some
> vertex shader outputs are stored in the header, so they were being
> double counted.  This patch changes the old VS backend to compute the
> URB entry size directly from the number of slots in the VUE map.

This kind of scares me, if the later stages with similar failure at
computing the size might be trying to read too much data from a
now-smaller VUE.  I'm not sure what happens in that case.  (hopefully
the answer is "read garbage", but what if our vertex happens to be at
the end of URB space?

> ---
>  src/mesa/drivers/dri/i965/brw_vs.h      |    1 -
>  src/mesa/drivers/dri/i965/brw_vs_emit.c |   31 +++++++------------------------
>  2 files changed, 7 insertions(+), 25 deletions(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/brw_vs.h b/src/mesa/drivers/dri/i965/brw_vs.h
> index a02c06d..c31869c 100644
> --- a/src/mesa/drivers/dri/i965/brw_vs.h
> +++ b/src/mesa/drivers/dri/i965/brw_vs.h
> @@ -65,7 +65,6 @@ struct brw_vs_compile {
>  
>     struct brw_vue_map vue_map;
>     GLuint first_output;
> -   GLuint nr_outputs;
>     GLuint last_scratch;
>  
>     GLuint first_tmp;
> diff --git a/src/mesa/drivers/dri/i965/brw_vs_emit.c b/src/mesa/drivers/dri/i965/brw_vs_emit.c
> index e7667c8..7c430ce 100644
> --- a/src/mesa/drivers/dri/i965/brw_vs_emit.c
> +++ b/src/mesa/drivers/dri/i965/brw_vs_emit.c
> @@ -402,36 +402,19 @@ static void brw_vs_alloc_regs( struct brw_vs_compile *c )
>     /* The VS VUEs are shared by VF (outputting our inputs) and VS, so size
>      * them to fit the biggest thing they need to.
>      */
> -   attributes_in_vue = MAX2(c->nr_outputs, c->nr_inputs);
> +   attributes_in_vue = MAX2(c->vue_map.num_slots, c->nr_inputs);

There's a subtle change here, that we're no longer counting header regs
towards the size of the VF input.  Is that appropriate?  I think it is,
because brw_vs_state.c has vs->thread3.urb_entry_read_offset = 0.

This probably deserves mention in the commit message.

> -   /* See emit_vertex_write() for where the VUE's overhead on top of the
> -    * attributes comes from.
> -    */
> -   if (intel->gen >= 7) {
> -      int header_regs = 2;
> -      if (c->key.nr_userclip)
> -	 header_regs += 2;
> -
> -      /* Each attribute is 16 bytes (1 vec4), so dividing by 4 gives us the
> -       * number of 64-byte (512-bit) units.
> -       */
> -      c->prog_data.urb_entry_size = (attributes_in_vue + header_regs + 3) / 4;
> -   } else if (intel->gen == 6) {
> -      int header_regs = 2;
> -      if (c->key.nr_userclip)
> -	 header_regs += 2;
> -
> -      /* Each attribute is 16 bytes (1 vec4), so dividing by 8 gives us the
> +   if (intel->gen == 6) {
> +      /* Each attribute is 32 bytes (2 vec4s), so dividing by 8 gives us the
>         * number of 128-byte (1024-bit) units.
>         */

This comment change here looks wrong to me.  32 bytes * 8 = 256 bytes.
Same below.  An attribute is 1 vec4, and the urb_entry_size is for one
vertex's URB entry, even though a register contains 2 vec4s which get
output to two separate URB entries.

> -      c->prog_data.urb_entry_size = (attributes_in_vue + header_regs + 7) / 8;
> -   } else if (intel->gen == 5)
> +      c->prog_data.urb_entry_size = (attributes_in_vue + 7) / 8;
> +   } else {
>        /* Each attribute is 16 bytes (1 vec4), so dividing by 4 gives us the
>         * number of 64-byte (512-bit) units.
>         */
> -      c->prog_data.urb_entry_size = (attributes_in_vue + 6 + 3) / 4;
> -   else
> -      c->prog_data.urb_entry_size = (attributes_in_vue + 2 + 3) / 4;
> +      c->prog_data.urb_entry_size = (attributes_in_vue + 3) / 4;
> +   }
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