On Tue, Mar 31, 2015 at 3:57 PM, Rob Clark <robdcl...@gmail.com> wrote:
> From: Rob Clark <robcl...@freedesktop.org>
>
> Freedreno and vc4 want this behavior for the time being (until we have
> real flow control). Even after that, we probably want to turn this into
> some sort of driver tunable threshold, since for at least some hardware,
> reasonably large if/else is best flattend rather than having divergent
> flow control.
>
> NOTE: wasn't sure about some of the additional restrictions in
> block_check_for_allowed_instrs().. are there some other cases where
> I might need to fix things up in order to be guaranteed to be able to
> flatten?
>
> NOTE: drop vc4 hunk if this is merged first, ofc
>
> Signed-off-by: Rob Clark <robcl...@freedesktop.org>
> ---
> src/gallium/drivers/vc4/vc4_program.c | 2 +-
> src/glsl/nir/nir.h | 2 +-
> src/glsl/nir/nir_opt_peephole_select.c | 21 +++++++++++++++------
> src/mesa/drivers/dri/i965/brw_fs_nir.cpp | 2 +-
> 4 files changed, 18 insertions(+), 9 deletions(-)
>
> diff --git a/src/gallium/drivers/vc4/vc4_program.c
> b/src/gallium/drivers/vc4/vc4_program.c
> index db51665..09896ce 100644
> --- a/src/gallium/drivers/vc4/vc4_program.c
> +++ b/src/gallium/drivers/vc4/vc4_program.c
> @@ -1673,7 +1673,7 @@ vc4_optimize_nir(struct nir_shader *s)
> progress = nir_copy_prop(s) || progress;
> progress = nir_opt_dce(s) || progress;
> progress = nir_opt_cse(s) || progress;
> - progress = nir_opt_peephole_select(s) || progress;
> + progress = nir_opt_peephole_select(s, true) || progress;
> progress = nir_opt_algebraic(s) || progress;
> progress = nir_opt_constant_folding(s) || progress;
> } while (progress);
> diff --git a/src/glsl/nir/nir.h b/src/glsl/nir/nir.h
> index 74927e5..cd03d6b 100644
> --- a/src/glsl/nir/nir.h
> +++ b/src/glsl/nir/nir.h
> @@ -1631,7 +1631,7 @@ bool nir_opt_dce(nir_shader *shader);
>
> void nir_opt_gcm(nir_shader *shader);
>
> -bool nir_opt_peephole_select(nir_shader *shader);
> +bool nir_opt_peephole_select(nir_shader *shader, bool flatten_all);
> bool nir_opt_peephole_ffma(nir_shader *shader);
>
> bool nir_opt_remove_phis(nir_shader *shader);
> diff --git a/src/glsl/nir/nir_opt_peephole_select.c
> b/src/glsl/nir/nir_opt_peephole_select.c
> index b89451b..e48fc7a 100644
> --- a/src/glsl/nir/nir_opt_peephole_select.c
> +++ b/src/glsl/nir/nir_opt_peephole_select.c
> @@ -44,10 +44,16 @@
> * whose only use is one of the following phi nodes. This happens all the
> * time when the SSA form comes from a conditional assignment with a
> * swizzle.
> + *
> + * If 'flatten_all' is true, then flatten *all* if/else into one block
> + * and use select to pick the winners. This is useful for drivers that
> + * do not (yet) have proper flow control. Eventually we probably want
> + * to make this a more clever driver tunable threshold.
> */
>
> struct peephole_select_state {
> void *mem_ctx;
> + bool flatten_all;
> bool progress;
> };
>
> @@ -150,9 +156,10 @@ nir_opt_peephole_select_block(nir_block *block, void
> *void_state)
> nir_block *else_block = nir_cf_node_as_block(else_node);
>
> /* ... and those blocks must only contain "allowed" instructions. */
> - if (!block_check_for_allowed_instrs(then_block) ||
> - !block_check_for_allowed_instrs(else_block))
> - return true;
> + if (!state->flatten_all)
> + if (!block_check_for_allowed_instrs(then_block) ||
> + !block_check_for_allowed_instrs(else_block))
> + return true;
This isn't valid. We can't blindly flatten if either list of blocks
contains an instruction that has side-effects. In order to do that,
we would have to add predication back into NIR.
> /* At this point, we know that the previous CFG node is an if-then
> * statement containing only moves to phi nodes in this block. We can
> @@ -217,11 +224,12 @@ nir_opt_peephole_select_block(nir_block *block, void
> *void_state)
> }
>
> static bool
> -nir_opt_peephole_select_impl(nir_function_impl *impl)
> +nir_opt_peephole_select_impl(nir_function_impl *impl, bool flatten_all)
> {
> struct peephole_select_state state;
>
> state.mem_ctx = ralloc_parent(impl);
> + state.flatten_all = flatten_all;
> state.progress = false;
>
> nir_foreach_block(impl, nir_opt_peephole_select_block, &state);
> @@ -233,13 +241,14 @@ nir_opt_peephole_select_impl(nir_function_impl *impl)
> }
>
> bool
> -nir_opt_peephole_select(nir_shader *shader)
> +nir_opt_peephole_select(nir_shader *shader, bool flatten_all)
As matt said, boolean arguments aren't great. I also don't like
arbitrary bitfields. We do, however, have a shader params struct.
You could add a "flatten" flag to it I guess. However, doing so kind
of implies that we actually flatten everything which (as stated above)
we can't here.
--Jason
> {
> bool progress = false;
>
> nir_foreach_overload(shader, overload) {
> if (overload->impl)
> - progress |= nir_opt_peephole_select_impl(overload->impl);
> + progress |= nir_opt_peephole_select_impl(overload->impl,
> + flatten_all);
> }
>
> return progress;
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> index 0b8ed1a..7bd7cd3 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> @@ -47,7 +47,7 @@ nir_optimize(nir_shader *nir)
> nir_validate_shader(nir);
> progress |= nir_opt_cse(nir);
> nir_validate_shader(nir);
> - progress |= nir_opt_peephole_select(nir);
> + progress |= nir_opt_peephole_select(nir, false);
> nir_validate_shader(nir);
> progress |= nir_opt_algebraic(nir);
> nir_validate_shader(nir);
> --
> 2.1.0
>
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