[Mesa-dev] [PATCH] i965/fs: Extend the live ranges of VGRFs which leave loops

[Connor Abbott]

I'm a little nervous about this, because really, the only solution to
this problem is to ignore all non-WE_all definitions of all variables
in liveness analysis. For example, in something like:

vec4 color2 = ...
if (...) {
   color2 = texture();
}

texture() can also overwrite inactive channels of color2. We happen to
get this right because we turn live ranges into live intervals without
holes, but I can't come up with a good reason why that would save us
in all cases except the one in this patch -- which makes me worry that
we'll find yet another case where there's a similar problem. I think
it would be clearer if we what I said above, i.e. ignore all
non-WE_all definitions, which will make things much worse, but then
apply Curro's patch which will return things to pretty much how they
were before, except this case will be fixed and maybe some other cases
we haven't thought of.



On Thu, Oct 5, 2017 at 2:52 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> No Shader-db changes.
>
> Cc: mesa-stable at lists.freedesktop.org
> ---
>  src/intel/compiler/brw_fs_live_variables.cpp | 55 ++++++++++++++++++++++++++++
>  1 file changed, 55 insertions(+)
>
> diff --git a/src/intel/compiler/brw_fs_live_variables.cpp b/src/intel/compiler/brw_fs_live_variables.cpp
> index c449672..380060d 100644
> --- a/src/intel/compiler/brw_fs_live_variables.cpp
> +++ b/src/intel/compiler/brw_fs_live_variables.cpp
> @@ -223,6 +223,61 @@ fs_live_variables::compute_start_end()
>           }
>        }
>     }
> +
> +   /* Due to the explicit way the SIMD data is handled on GEN, we need to be a
> +    * bit more careful with live ranges and loops.  Consider the following
> +    * example:
> +    *
> +    *    vec4 color2;
> +    *    while (1) {
> +    *       vec4 color = texture();
> +    *       if (...) {
> +    *          color2 = color * 2;
> +    *          break;
> +    *       }
> +    *    }
> +    *    gl_FragColor = color2;
> +    *
> +    * In this case, the definition of color2 dominates the use because the
> +    * loop only has the one exit.  This means that the live range interval for
> +    * color2 goes from the statement in the if to it's use below the loop.
> +    * Now suppose that the texture operation has a header register that gets
> +    * assigned one of the registers used for color2.  If the loop condition is
> +    * non-uniform and some of the threads will take the and others will
> +    * continue.  In this case, the next pass through the loop, the WE_all
> +    * setup of the header register will stomp the disabled channels of color2
> +    * and corrupt the value.
> +    *
> +    * This same problem can occur if you have a mix of 64, 32, and 16-bit
> +    * registers because the channels do not line up or if you have a SIMD16
> +    * program and the first half of one value overlaps the second half of the
> +    * other.
> +    *
> +    * To solve this problem, we take any VGRFs whose live ranges cross the
> +    * while instruction of a loop and extend their live ranges to the top of
> +    * the loop.  This more accurately models the hardware because the value in
> +    * the VGRF needs to be carried through subsequent loop iterations in order
> +    * to remain valid when we finally do break.
> +    */
> +   foreach_block (block, cfg) {
> +      if (block->end()->opcode != BRW_OPCODE_WHILE)
> +         continue;
> +
> +      /* This is a WHILE instrution. Find the DO block. */
> +      bblock_t *do_block = NULL;
> +      foreach_list_typed(bblock_link, child_link, link, &block->children) {
> +         if (child_link->block->start_ip < block->end_ip) {
> +            assert(do_block == NULL);
> +            do_block = child_link->block;
> +         }
> +      }
> +      assert(do_block);
> +
> +      for (int i = 0; i < num_vars; i++) {
> +         if (start[i] < block->end_ip && end[i] > block->end_ip)
> +            start[i] = MIN2(start[i], do_block->start_ip);
> +      }
> +   }
>  }
>
>  fs_live_variables::fs_live_variables(fs_visitor *v, const cfg_t *cfg)
> --
> 2.5.0.400.gff86faf
>
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