[Mesa-dev] [PATCH 08/25] SQUASH: i965/fs: Add basic dataflow check to opt_sampler_eot().

[Jason Ekstrand]

On Fri, May 27, 2016 at 7:05 PM, Francisco Jerez <currojerez at riseup.net>
wrote:

> The sampler EOT optimization pass naively assumes that the texturing
> instruction provides all the data used by the FB write just because
> they're standing next to each other.  The least we should be checking
> is whether the source and destination regions of the FB write and
> texturing instructions match.  Without this the previous seemingly
> harmless commit would have caused opt_sampler_eot() to misoptimize a
> shader from dota-2 causing DCE to eliminate all of its 78 instructions
> except for the final sampler EOT message (!).
>

Oh, well that's fun...


> ---
>  src/mesa/drivers/dri/i965/brw_defines.h |  1 +
>  src/mesa/drivers/dri/i965/brw_fs.cpp    | 14 ++++++++++++++
>  2 files changed, 15 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_defines.h
> b/src/mesa/drivers/dri/i965/brw_defines.h
> index 8794d44..60b215d 100644
> --- a/src/mesa/drivers/dri/i965/brw_defines.h
> +++ b/src/mesa/drivers/dri/i965/brw_defines.h
> @@ -1395,6 +1395,7 @@ enum fb_write_logical_srcs {
>     FB_WRITE_LOGICAL_SRC_SRC_STENCIL, /* gl_FragStencilRefARB */
>     FB_WRITE_LOGICAL_SRC_OMASK,       /* Sample Mask (gl_SampleMask) */
>     FB_WRITE_LOGICAL_SRC_COMPONENTS,  /* REQUIRED */
> +   FB_WRITE_LOGICAL_NUM_SRCS
>  };
>
>  enum tex_logical_srcs {
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp
> b/src/mesa/drivers/dri/i965/brw_fs.cpp
> index 6abf776..1574cd9 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
> @@ -2647,6 +2647,20 @@ fs_visitor::opt_sampler_eot()
>     if (tex_inst->prev->is_head_sentinel())
>        return false;
>
> +   /* Check that the FB write sources are fully initialized by the single
> +    * texturing instruction.
> +    */
> +   for (unsigned i = 0; i < FB_WRITE_LOGICAL_NUM_SRCS; i++) {
> +      if (i == FB_WRITE_LOGICAL_SRC_COLOR0) {
> +         if (!fb_write->src[i].equals(tex_inst->dst) ||
> +             fb_write->regs_read(i) != tex_inst->regs_written)
> +         return false;
> +      } else if (i != FB_WRITE_LOGICAL_SRC_COMPONENTS) {
> +         if (fb_write->src[i].file != BAD_FILE)
> +            return false;
> +      }
> +   }
> +
>     assert(!tex_inst->eot); /* We can't get here twice */
>     assert((tex_inst->offset & (0xff << 24)) == 0);
>
> --
> 2.7.3
>
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