[Mesa-dev] [PATCH] nir/remove_phis: handle trivial back-edges

[Matt Turner]

On Fri, Oct 2, 2015 at 9:28 AM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> Some loops may have phi nodes that look like:
>
> foo = ...
> loop {
>     bar = phi(foo, bar)
>     ...
> }
>
> in which case we can remove the phi node and replace all uses of 'bar'
> with 'foo'. In particular, there are some L4D2 vertex shaders with loops
> that, after optimization, look like:
>
>         /* succs: block_1 */
>         loop {
>                 block block_1:
>                 /* preds: block_0 block_4 */
>                 vec1 ssa_2195 = phi block_0: ssa_2136, block_4: ssa_994
>                 vec1 ssa_7321 = phi block_0: ssa_8195, block_4: ssa_7321
>                 vec1 ssa_7324 = phi block_0: ssa_8198, block_4: ssa_7324
>                 vec1 ssa_7327 = phi block_0: ssa_8174, block_4: ssa_7327
>                 vec1 ssa_8139 = intrinsic load_uniform () () (232)
>                 vec1 ssa_588 = ige ssa_2195, ssa_8139
>                 /* succs: block_2 block_3 */
>                 if ssa_588 {
>                         block block_2:
>                         /* preds: block_1 */
>                         break
>                         /* succs: block_5 */
>                 } else {
>                         block block_3:
>                         /* preds: block_1 */
>                         /* succs: block_4 */
>                 }
>                 block block_4:
>                 /* preds: block_3 */
>                 vec1 ssa_994 = iadd ssa_2195, ssa_2150
>                 /* succs: block_1 */
>         }
>
> where after removing the second, third, and fourth phi nodes, the loop becomes
> entirely dead, and this patch will cause the loop to be deleted entirely.
>
> No piglit regressions.
>
> Shader-db results on bdw:
>
> instructions in affected programs:     5824 -> 5664 (-2.75%)
> helped:                                32
> HURT:                                  0

I added loop-count support to report.py recently.

instructions in affected programs:     5824 -> 5664 (-2.75%)
total loops in shared programs:        2234 -> 2202 (-1.43%)
helped:                                0

(report.py doesn't count programs in which the number of loops changed
as helped/hurt because sometimes a loop is unrolled and the
instruction count triples)

I'd feel fine about changing helped: 0 to helped: 32 like you have.

>
> Signed-off-by: Connor Abbott <cwabbott0 at gmail.com>
> ---
> Seems like this one fell through the cracks...
>
>  src/glsl/nir/nir_opt_remove_phis.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/src/glsl/nir/nir_opt_remove_phis.c b/src/glsl/nir/nir_opt_remove_phis.c
> index bf4a67e..0a5793a 100644
> --- a/src/glsl/nir/nir_opt_remove_phis.c
> +++ b/src/glsl/nir/nir_opt_remove_phis.c
> @@ -58,6 +58,21 @@ remove_phis_block(nir_block *block, void *state)
>
>        nir_foreach_phi_src(phi, src) {
>           assert(src->src.is_ssa);
> +
> +         /* For phi nodes at the beginning of loops, we may encounter some
> +          * sources from backedges that point back to the destination of the
> +          * same phi, i.e. something like:
> +          *
> +          * a = phi(a, b, ...)
> +          *
> +          * We can safely ignore these sources, since if all of the normal
> +          * sources point to the same definition, then that definition must
> +          * still dominate the phi node, and the phi will still always take
> +          * the value of that definition.
> +          */
> +

I'd remove the newline here.

> +         if (src->src.ssa == &phi->dest.ssa)
> +            continue;
>
>           if (def == NULL) {
>              def  = src->src.ssa;
> @@ -72,6 +87,11 @@ remove_phis_block(nir_block *block, void *state)
>        if (!srcs_same)
>           continue;
>
> +      /* We must have found at least one definition, since there must be at
> +       * least one forward edge.
> +       */
> +      assert(def != NULL);
> +
>        assert(phi->dest.is_ssa);
>        nir_ssa_def_rewrite_uses(&phi->dest.ssa, nir_src_for_ssa(def));
>        nir_instr_remove(instr);
> --
> 2.1.0

Look good to me. Thanks for remembering this.

Reviewed-by: Matt Turner <mattst88 at gmail.com>

One strange thing I noticed... this doesn't affect any programs on
Haswell. All the shaders are vertex shaders, so they're vec4 on
Haswell but scalar on Broadwell, so it's probably somehow related to
that.

Want to take a look at what's going on with Haswell? Seems like if the
loop is dead... it should be dead.