[Mesa-dev] [PATCH 04/12] nir: Add a LCSAA-pass
Timothy Arceri
timothy.arceri at collabora.com
Wed Dec 21 04:03:24 UTC 2016
On Tue, 2016-12-20 at 15:14 -0800, Jason Ekstrand wrote:
> I did have a couple of "real" comments on this one that I'd like to
> at least see a reply to. Does look pretty good though.
>
> On Sun, Dec 18, 2016 at 9:47 PM, Timothy Arceri <timothy.arceri at colla
> bora.com> wrote:
> > From: Thomas Helland <thomashelland90 at gmail.com>
> >
> > V2: Do a "depth first search" to convert to LCSSA
> >
> > V3: Small comment fixup
> >
> > V4: Rebase, adapt to removal of function overloads
> >
> > V5: Rebase, adapt to relocation of nir to compiler/nir
> > Still need to adapt to potential if-uses
> > Work around nir_validate issue
> >
> > V6 (Timothy):
> > - tidy lcssa and stop leaking memory
> > - dont rewrite the src for the lcssa phi node
> > - validate lcssa phi srcs to avoid postvalidate assert
> > - don't add new phi if one already exists
> > - more lcssa phi validation fixes
> > - Rather than marking ssa defs inside a loop just mark blocks
> > inside
> > a loop. This is simpler and fixes lcssa for intrinsics which do
> > not have a destination.
> > - don't create LCSSA phis for loops we won't unroll
> > - require loop metadata for lcssa pass
> > - handle case were the ssa defs use outside the loop is already a
> > phi
> >
> > V7: (Timothy)
> > - pass indirect mask to metadata call
> >
> > v8: (Timothy)
> > - make convert to lcssa a helper function rather than a nir pass
> > - replace inside loop bitset with on the fly block index logic.
> > - remove lcssa phi validation special cases
> > - inline code from useless helpers, suggested by Jason.
> > - always do lcssa on loops, suggested by Jason.
> > - stop making lcssa phis special. Add as many source as the block
> > has predecessors, suggested by Jason.
> >
> > V9: (Timothy)
> > - fix regression with the is_lcssa_phi field not being initialised
> > to false now that ralloc() doesn't zero out memory.
> >
> > V10: (Timothy)
> > - remove extra braces in SSA example, pointed out by Topi
> >
> > V11: (Timothy)
> > - add missing support for LCSSA phis in if conditions.
> > ---
> > src/compiler/Makefile.sources | 1 +
> > src/compiler/nir/nir.c | 1 +
> > src/compiler/nir/nir.h | 4 +
> > src/compiler/nir/nir_to_lcssa.c | 215
> > ++++++++++++++++++++++++++++++++++++++++
> > 4 files changed, 221 insertions(+)
> > create mode 100644 src/compiler/nir/nir_to_lcssa.c
> >
> > diff --git a/src/compiler/Makefile.sources
> > b/src/compiler/Makefile.sources
> > index ca8a056..e8f7b02 100644
> > --- a/src/compiler/Makefile.sources
> > +++ b/src/compiler/Makefile.sources
> > @@ -254,6 +254,7 @@ NIR_FILES = \
> > nir/nir_split_var_copies.c \
> > nir/nir_sweep.c \
> > nir/nir_to_ssa.c \
> > + nir/nir_to_lcssa.c \
> > nir/nir_validate.c \
> > nir/nir_vla.h \
> > nir/nir_worklist.c \
> > diff --git a/src/compiler/nir/nir.c b/src/compiler/nir/nir.c
> > index 2c3531c..e522a67 100644
> > --- a/src/compiler/nir/nir.c
> > +++ b/src/compiler/nir/nir.c
> > @@ -561,6 +561,7 @@ nir_phi_instr_create(nir_shader *shader)
> > {
> > nir_phi_instr *instr = ralloc(shader, nir_phi_instr);
> > instr_init(&instr->instr, nir_instr_type_phi);
> > + instr->is_lcssa_phi = false;
> >
> > dest_init(&instr->dest);
> > exec_list_make_empty(&instr->srcs);
> > diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> > index 28010aa..75a91ea 100644
> > --- a/src/compiler/nir/nir.h
> > +++ b/src/compiler/nir/nir.h
> > @@ -1360,6 +1360,8 @@ typedef struct {
> > struct exec_list srcs; /** < list of nir_phi_src */
> >
> > nir_dest dest;
> > +
> > + bool is_lcssa_phi;
> > } nir_phi_instr;
> >
> > typedef struct {
> > @@ -2526,6 +2528,8 @@ void nir_convert_to_ssa(nir_shader *shader);
> > bool nir_repair_ssa_impl(nir_function_impl *impl);
> > bool nir_repair_ssa(nir_shader *shader);
> >
> > +void nir_convert_loop_to_lcssa(nir_loop *loop);
> > +
> > /* If phi_webs_only is true, only convert SSA values involved in
> > phi nodes to
> > * registers. If false, convert all values (even those not
> > involved in a phi
> > * node) to registers.
> > diff --git a/src/compiler/nir/nir_to_lcssa.c
> > b/src/compiler/nir/nir_to_lcssa.c
> > new file mode 100644
> > index 0000000..8afdc54
> > --- /dev/null
> > +++ b/src/compiler/nir/nir_to_lcssa.c
> > @@ -0,0 +1,215 @@
> > +/*
> > + * Copyright © 2015 Thomas Helland
> > + *
> > + * Permission is hereby granted, free of charge, to any person
> > obtaining a
> > + * copy of this software and associated documentation files (the
> > "Software"),
> > + * to deal in the Software without restriction, including without
> > limitation
> > + * the rights to use, copy, modify, merge, publish, distribute,
> > sublicense,
> > + * and/or sell copies of the Software, and to permit persons to
> > whom the
> > + * Software is furnished to do so, subject to the following
> > conditions:
> > + *
> > + * The above copyright notice and this permission notice
> > (including the next
> > + * paragraph) shall be included in all copies or substantial
> > portions of the
> > + * Software.
> > + *
> > + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
> > EXPRESS OR
> > + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
> > MERCHANTABILITY,
> > + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO
> > EVENT SHALL
> > + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
> > DAMAGES OR OTHER
> > + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
> > ARISING
> > + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
> > OTHER DEALINGS
> > + * IN THE SOFTWARE.
> > + */
> > +
> > +/*
> > + * This pass converts the ssa-graph into "Loop Closed SSA form".
> > This is
> > + * done by placing phi nodes at the exits of the loop for all
> > values
> > + * that are used outside the loop. The result is it transforms:
> > + *
> > + * loop { -> loop {
> > + * ssa2 = .... -> ssa2 = ...
> > + * if (cond) -> if (cond)
> > + * break; -> break;
> > + * ssa3 = ssa2 * ssa4 -> ssa3 = ssa2 * ssa4
> > + * } -> }
> > + * ssa6 = ssa2 + 4 -> ssa5 = lcssa_phi(ssa2)
> > + * ssa6 = ssa5 + 4
> > + */
> > +
> > +#include "nir.h"
> > +
> > +typedef struct {
> > + /* The nir_shader we are transforming */
> > + nir_shader *shader;
> > +
> > + /* The loop we store information for */
> > + nir_loop *loop;
> > +
> > +} lcssa_state;
> > +
> > +static bool
> > +is_if_use_inside_loop(nir_src *use, nir_loop* loop)
> > +{
> > + nir_block *block_before_loop =
> > + nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
> > + nir_block *block_after_loop =
> > + nir_cf_node_as_block(nir_cf_node_next(&loop->cf_node));
> > +
> > + nir_block *prev_block =
> > + nir_cf_node_as_block(nir_cf_node_prev(&use->parent_if-
> > >cf_node));
> > + if (prev_block->index <= block_before_loop->index ||
> > + prev_block->index >= block_after_loop->index) {
>
> I'm not sure you want "<=" and ">=" here. It seems like you either
> want <= and nir_loop_first_block above or keep what you have above
> and use <. As currently written, this will return true if it's used
> in the block before or after the loop not just inside the loop.
>
> > + return false;
> > + }
> > +
> > + return true;
> > +}
> > +
> > +static bool
> > +is_use_inside_loop(nir_src *use, nir_loop* loop)
> > +{
> > + nir_block *block_before_loop =
> > + nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
> > + nir_block *block_after_loop =
> > + nir_cf_node_as_block(nir_cf_node_next(&loop->cf_node));
> > +
> > + if (use->parent_instr->block->index <= block_before_loop->index
> > ||
> > + use->parent_instr->block->index >= block_after_loop->index)
> > {
>
> Same here
>
> > + return false;
> > + }
> > +
> > + return true;
> > +}
> > +
> > +static bool
> > +convert_loop_exit_for_ssa(nir_ssa_def *def, void *void_state)
> > +{
> > + lcssa_state *state = void_state;
> > + bool all_uses_inside_loop = true;
> > +
> > + nir_block *block_after_loop =
> > + nir_cf_node_as_block(nir_cf_node_next(&state->loop-
> > >cf_node));
> > +
> > + /* If a LCSSA-phi from a nested loop is use outside the parent
> > loop there
> > + * is no reason to create another LCSSA-phi for the parent
> > loop.
> > + */
>
> I don't think this is true. The whole point of LCSSA is that no SSA
> def declared inside a loop is able to escape the loop except through
> a phi in the block after the loop. If it's an LCSSA phi from an
> inner loop, I think you very much want it to have another phi for the
> outer loop.
>
> > + if (def->parent_instr->type == nir_instr_type_phi &&
> > + nir_instr_as_phi(def->parent_instr)->is_lcssa_phi) {
> > + return true;
> > + }
> > +
> > + nir_foreach_use(use, def) {
> > + if (use->parent_instr->type == nir_instr_type_phi &&
> > + block_after_loop == use->parent_instr->block) {
>
> I'm silly but would you mind flipping this equality around? That way
> the LHS is use->parent_instr->something for both clauses.
>
> > + continue;
> > + }
> > +
> > + if (!is_use_inside_loop(use, state->loop)) {
> > + all_uses_inside_loop = false;
> > + }
> > + }
> > +
> > + nir_foreach_if_use(use, def) {
> > + if (!is_if_use_inside_loop(use, state->loop)) {
> > + all_uses_inside_loop = false;
> > + }
> > + }
> > +
> > + /* There where no sources that had defs outside the loop */
> > + if (all_uses_inside_loop)
> > + return true;
> > +
> > + /* Initialize a phi-instruction */
> > + nir_phi_instr *phi = nir_phi_instr_create(state->shader);
> > + phi->instr.block = block_after_loop;
>
> You don't need this. nir_instr_insert will set it for you.
>
> > + nir_ssa_dest_init(&phi->instr, &phi->dest,
> > + def->num_components, def->bit_size, "LCSSA-
> > phi");
> > + phi->is_lcssa_phi = true;
> > +
> > + /* Create a phi node with as many sources pointing to the same
> > ssa_def as
> > + * the block has predecessors.
> > + */
> > + struct set_entry *entry;
> > + set_foreach(block_after_loop->predecessors, entry) {
> > + nir_phi_src *phi_src = ralloc(phi, nir_phi_src);
> > + phi_src->src = nir_src_for_ssa(def);
> > +
> > + /* This is a lie to pass validation */
> > + phi_src->pred = (nir_block *) entry->key;
>
> This is not a lie. It really is the value we get coming in from that
> predecessor.
>
> > +
> > + exec_list_push_tail(&phi->srcs, &phi_src->node);
> > + }
> > +
> > + nir_instr_insert_before_block(phi->instr.block, &phi->instr);
> > +
> > + /* Run through all uses and rewrite those outside the loop to
> > point to
> > + * the phi instead of pointing to the ssa-def.
> > + */
> > + nir_foreach_use_safe(use, def) {
> > + if (use->parent_instr->type == nir_instr_type_phi &&
> > + block_after_loop == use->parent_instr->block) {
> > + continue;
> > + }
> > +
> > + if (!is_use_inside_loop(use, state->loop)) {
> > + nir_instr_rewrite_src(use->parent_instr, use,
> > + nir_src_for_ssa(&phi->dest.ssa));
> > + }
> > + }
> > +
> > + nir_foreach_if_use_safe(use, def) {
> > + if (!is_if_use_inside_loop(use, state->loop)) {
> > + nir_if_rewrite_condition(use->parent_if,
> > + nir_src_for_ssa(&phi-
> > >dest.ssa));
> > + }
> > + }
> > +
> > + return true;
> > +}
> > +
> > +static void
> > +convert_to_lcssa(nir_cf_node *cf_node, lcssa_state *state)
> > +{
> > + switch (cf_node->type) {
> > + case nir_cf_node_block:
> > + nir_foreach_instr(instr, nir_cf_node_as_block(cf_node))
> > + nir_foreach_ssa_def(instr, convert_loop_exit_for_ssa,
> > state);
> > + return;
> > + case nir_cf_node_if: {
> > + nir_if *if_stmt = nir_cf_node_as_if(cf_node);
> > + foreach_list_typed(nir_cf_node, nested_node, node, &if_stmt-
> > >then_list)
> > + convert_to_lcssa(nested_node, state);
> > + foreach_list_typed(nir_cf_node, nested_node, node, &if_stmt-
> > >else_list)
> > + convert_to_lcssa(nested_node, state);
> > + return;
> > + }
> > + case nir_cf_node_loop: {
> > + nir_loop *parent_loop = state->loop;
> > + state->loop = nir_cf_node_as_loop(cf_node);
> > +
> > + foreach_list_typed(nir_cf_node, nested_node, node, &state-
> > >loop->body)
> > + convert_to_lcssa(nested_node, state);
> > +
> > + state->loop = parent_loop;
> > + return;
> > + }
> > + default:
> > + unreachable("unknown cf node type");
> > + }
> > +}
> > +
> > +void
> > +nir_convert_loop_to_lcssa(nir_loop *loop) {
> > + nir_function_impl *impl = nir_cf_node_get_function(&loop-
> > >cf_node);
> > +
> > + nir_metadata_require(impl, nir_metadata_block_index);
> > +
> > + lcssa_state *state = rzalloc(NULL, lcssa_state);
> > + state->loop = loop;
> > + state->shader = impl->function->shader;
> > +
> > + foreach_list_typed(nir_cf_node, node, node, &state->loop->body)
> > + convert_to_lcssa(node, state);
>
> We have better iterators now! This and the whole convert_to_lcssa
> function can be replaced with
>
> nir_foreach_block_in_cf_node(block, &loop->cf_node) {
> nir_foreach_instr(instr, block)
> nir_foreach_ssa_def(instr, convert_loop_exit_for_ssa, state);
> }
>
> Much simpler!
I don't think I can use that or we won't be setting state->loop
>
> > +
> > + ralloc_free(state);
> > +}
> > --
> > 2.9.3
> >
> > _______________________________________________
> > mesa-dev mailing list
> > mesa-dev at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/mesa-dev
> >
>
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