[Mesa-dev] [PATCH v3 14/48] i965/fs: Extend the live ranges of VGRFs which leave loops

[Iago Toral]

On Wed, 2017-10-25 at 16:25 -0700, Jason Ekstrand 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 take the if, and others will continue."

> will
> +    * continue.  In this case, the next pass through the loop, the 

"...In this case, in the next pass through the loop,..."

> WE_all
> +    * setup of the header register will stomp the disabled channels 

Actually, it would stomp all the channels (and specifically, it would
stomp the channels that were enabled --not disabled-- for the if in the
previous iteration).

> 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.

Right, that sounds like the proper solution, since as soon as we have
gone through a loop once, any variable that was defined inside is
immediately alive at the start of the next loop iteration.

> +    */
> +   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)