[Mesa-dev] [PATCH] intel/fs: Optimize and simplify the copy propagation dataflow logic.

[Eero Tamminen]

Hi,

I got unexpected results with this, when testing it on BXT & SKL GT2.

While performance in GpuTest Volplosion and GfxBench v4 Tessellation 
test improved slightly, performance in SynMark v7 CSDof and GpuTest v0.7 
Piano dropped clearly.

Piano dropped only on SKL, but there the drop was >10%.  There was also 
a peculiar change in its CPU vs. GPU utilization at the start of the 
test.  With this patch, there's 5-10s of 100% CPU usage before the test 
starts, which extra CPU usage happens only with this patch.

Attached is Callgrind output of where the extra time at startup goes.

(I didn't see that issue with the other tests which perf changed.)


	- Eero

On 20.12.2017 04:19, Francisco Jerez wrote:
> Previously the dataflow propagation algorithm would calculate the ACP
> live-in and -out sets in a two-pass fixed-point algorithm.  The first
> pass would update the live-out sets of all basic blocks of the program
> based on their live-in sets, while the second pass would update the
> live-in sets based on the live-out sets.  This is incredibly
> inefficient in the typical case where the CFG of the program is
> approximately acyclic, because it can take up to 2*n passes for an ACP
> entry introduced at the top of the program to reach the bottom (where
> n is the number of basic blocks in the program), until which point the
> algorithm won't be able to reach a fixed point.
> 
> The same effect can be achieved in a single pass by computing the
> live-in and -out sets in lock-step, because that makes sure that
> processing of any basic block will pick up the updated live-out sets
> of the lexically preceding blocks.  This gives the dataflow
> propagation algorithm effectively O(n) run-time instead of O(n^2) in
> the acyclic case.
> 
> The time spent in dataflow propagation is reduced by 30x in the
> GLES31.functional.ssbo.layout.random.all_shared_buffer.5 dEQP
> test-case on my CHV system (the improvement is likely to be of the
> same order of magnitude on other platforms).  This more than reverses
> an apparent run-time regression in this test-case from my previous
> copy-propagation undefined-value handling patch, which was ultimately
> caused by the additional work introduced in that commit to account for
> undefined values being multiplied by a huge quadratic factor.
> 
> According to Chad this test was failing on CHV due to a 30s time-out
> imposed by the Android CTS (this was the case regardless of my
> undefined-value handling patch, even though my patch substantially
> exacerbated the issue).  On my CHV system this patch reduces the
> overall run-time of the test by approximately 12x, getting us to
> around 13s, well below the time-out.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104271
> Reported-by: Chad Versace <chadversary at chromium.org>
> ---
>   src/intel/compiler/brw_fs_copy_propagation.cpp | 30 ++++++++------------------
>   1 file changed, 9 insertions(+), 21 deletions(-)
> 
> diff --git a/src/intel/compiler/brw_fs_copy_propagation.cpp b/src/intel/compiler/brw_fs_copy_propagation.cpp
> index af5635eacef..8d6ab18cd13 100644
> --- a/src/intel/compiler/brw_fs_copy_propagation.cpp
> +++ b/src/intel/compiler/brw_fs_copy_propagation.cpp
> @@ -228,34 +228,17 @@ fs_copy_prop_dataflow::run()
>      do {
>         progress = false;
>   
> -      /* Update liveout for all blocks. */
>         foreach_block (block, cfg) {
>            if (block->parents.is_empty())
>               continue;
>   
>            for (int i = 0; i < bitset_words; i++) {
>               const BITSET_WORD old_liveout = bd[block->num].liveout[i];
> -
> -            bd[block->num].liveout[i] =
> -               bd[block->num].copy[i] | (bd[block->num].livein[i] &
> -                                         ~bd[block->num].kill[i]);
> -
> -            if (old_liveout != bd[block->num].liveout[i])
> -               progress = true;
> -         }
> -      }
> -
> -      /* Update livein for all blocks.  If a copy is live out of all parent
> -       * blocks, it's live coming in to this block.
> -       */
> -      foreach_block (block, cfg) {
> -         if (block->parents.is_empty())
> -            continue;
> -
> -         for (int i = 0; i < bitset_words; i++) {
> -            const BITSET_WORD old_livein = bd[block->num].livein[i];
>               BITSET_WORD livein_from_any_block = 0;
>   
> +            /* Update livein for this block.  If a copy is live out of all
> +             * parent blocks, it's live coming in to this block.
> +             */
>               bd[block->num].livein[i] = ~0u;
>               foreach_list_typed(bblock_link, parent_link, link, &block->parents) {
>                  bblock_t *parent = parent_link->block;
> @@ -278,7 +261,12 @@ fs_copy_prop_dataflow::run()
>                */
>               bd[block->num].livein[i] &= livein_from_any_block;
>   
> -            if (old_livein != bd[block->num].livein[i])
> +            /* Update liveout for this block. */
> +            bd[block->num].liveout[i] =
> +               bd[block->num].copy[i] | (bd[block->num].livein[i] &
> +                                         ~bd[block->num].kill[i]);
> +
> +            if (old_liveout != bd[block->num].liveout[i])
>                  progress = true;
>            }
>         }
> 

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