On Wed, Dec 17, 2014 at 2:23 PM, Connor Abbott <cwabbo...@gmail.com> wrote:
>
> On Wed, Dec 17, 2014 at 5:11 PM, Jason Ekstrand <ja...@jlekstrand.net>
> wrote:
> > On Wed, Dec 17, 2014 at 1:52 PM, Connor Abbott <cwabbo...@gmail.com>
> wrote:
> >>
> >> I'm sure you're already aware, but there are two things we could do to
> >> speed this up:
> >>
> >> 1. Pre-compute def/kill sets for each block similar to what i965 does.
> >
> >
> > Sure, but we walk the instructions at most deepest block depth + 1 and
> the
> > depest we've ever seen in the wild is 2.
>
> Well, there's always orbital explorer ;)
>
> >
> >> 2. Use a worklist + an array of flags for "this block is in the
> >> worklist" rather than walking all the basic blocks in reverse to find
> >> the few we need to update.
> >
> >
> > Sure, we could, but I don't see how pushing the blocks onto a stack and
> then
> > popping them back off really gains us anything over just walking them.
> If
> > there's something I'm missing, please let me know because it's not
> jumping
> > out at me. I'll freely admit that I'm not terribly familiar with
> worklists.
>
> I didn't mean a stack, I meant a FIFO queue. The idea is that rather
> than walking through every instruction, even when there are only
> changes to be made, we pop a pointer to a block off the worklist, see
> if it's live-in set changed, and if it did, we push every predecessor
> not already in the worklist onto the worklist and then mark the block
> as not being in the worklist. We start by pushing all the blocks in
> the worklist in reverse order, since that's the most efficient
> ordering. Then, we're only processing blocks that we know might change
> rather than every possible one, and for control flow without loops we
> only walk the instructions once instead of twice. I think this is
> going to be a pretty common thing for optimization passes to do, so it
> would be nice to have a common implementation of the worklist.
>
Reading through that a second time and it makes more sense now. Sure, we
could do it that way and it is more efficient. However, that's also a
pretty solid rewrite so I think it'll have to wait until more pressing
things get done.
--Jason
>
> >
> >>
> >>
> >> Wrt #2, we already use a worklist in the DCE pass, but it's kinda lame
> >> because it's using a linked list when we could just allocate an array
> >> of pointers up-front based on the maximum size (the number of blocks
> >> in this case, the number of SSA definitions in that case) and use it
> >> as a ringbuffer. It would be a nice cleanup to implement such a
> >> bounded worklist and share it between these two passes, since we'll
> >> probably want to use it for lots of other passes too.
> >>
> >> I don't either thing should block merging this, though.
> >>
> >> A few other comments below.
> >>
> >> On Tue, Dec 16, 2014 at 1:05 AM, Jason Ekstrand <ja...@jlekstrand.net>
> >> wrote:
> >> > ---
> >> > src/glsl/Makefile.sources | 1 +
> >> > src/glsl/nir/nir.h | 13 ++
> >> > src/glsl/nir/nir_live_variables.c | 282
> >> > ++++++++++++++++++++++++++++++++++++++
> >> > src/glsl/nir/nir_metadata.c | 2 +
> >> > src/mesa/main/bitset.h | 1 +
> >> > 5 files changed, 299 insertions(+)
> >> > create mode 100644 src/glsl/nir/nir_live_variables.c
> >> >
> >> > diff --git a/src/glsl/Makefile.sources b/src/glsl/Makefile.sources
> >> > index 4eb6320..433224e 100644
> >> > --- a/src/glsl/Makefile.sources
> >> > +++ b/src/glsl/Makefile.sources
> >> > @@ -20,6 +20,7 @@ NIR_FILES = \
> >> > $(GLSL_SRCDIR)/nir/nir_from_ssa.c \
> >> > $(GLSL_SRCDIR)/nir/nir_intrinsics.c \
> >> > $(GLSL_SRCDIR)/nir/nir_intrinsics.h \
> >> > + $(GLSL_SRCDIR)/nir/nir_live_variables.c \
> >> > $(GLSL_SRCDIR)/nir/nir_lower_atomics.c \
> >> > $(GLSL_SRCDIR)/nir/nir_lower_samplers.cpp \
> >> > $(GLSL_SRCDIR)/nir/nir_lower_system_values.c \
> >> > diff --git a/src/glsl/nir/nir.h b/src/glsl/nir/nir.h
> >> > index f744736..2f2edb6 100644
> >> > --- a/src/glsl/nir/nir.h
> >> > +++ b/src/glsl/nir/nir.h
> >> > @@ -420,6 +420,9 @@ typedef struct {
> >> > /** generic SSA definition index. */
> >> > unsigned index;
> >> >
> >> > + /** Index into the live_in and live_out bitfields */
> >> > + unsigned live_index;
> >> > +
> >> > nir_instr *parent_instr;
> >> >
> >> > struct set *uses;
> >> > @@ -999,6 +1002,10 @@ typedef struct nir_block {
> >> > struct nir_block **dom_children;
> >> >
> >> > struct set *dom_frontier;
> >> > +
> >> > + /* live in and out for this block; used for liveness analysis */
> >> > + BITSET_WORD *live_in;
> >> > + BITSET_WORD *live_out;
> >> > } nir_block;
> >> >
> >> > #define nir_block_first_instr(block) \
> >> > @@ -1047,6 +1054,7 @@ typedef enum {
> >> > nir_metadata_none = 0x0,
> >> > nir_metadata_block_index = 0x1,
> >> > nir_metadata_dominance = 0x2,
> >> > + nir_metadata_live_variables = 0x4,
> >> > } nir_metadata;
> >> >
> >> > typedef struct {
> >> > @@ -1274,6 +1282,8 @@ bool nir_foreach_src(nir_instr *instr,
> >> > nir_foreach_src_cb cb, void *state);
> >> > typedef bool (*nir_foreach_block_cb)(nir_block *block, void *state);
> >> > bool nir_foreach_block(nir_function_impl *impl, nir_foreach_block_cb
> >> > cb,
> >> > void *state);
> >> > +bool nir_foreach_block_reverse(nir_function_impl *impl,
> >> > nir_foreach_block_cb cb,
> >> > + void *state);
> >>
> >> This hunk should go in the previous patch that defined this function.
> >>
> >> >
> >> > /* If the following CF node is an if, this function returns that if.
> >> > * Otherwise, it returns NULL.
> >> > @@ -1318,6 +1328,9 @@ void nir_lower_system_values(nir_shader
> *shader);
> >> >
> >> > void nir_lower_atomics(nir_shader *shader);
> >> >
> >> > +void nir_live_variables_impl(nir_function_impl *impl);
> >> > +bool nir_ssa_defs_interfere(nir_ssa_def *a, nir_ssa_def *b);
> >> > +
> >> > void nir_convert_to_ssa_impl(nir_function_impl *impl);
> >> > void nir_convert_to_ssa(nir_shader *shader);
> >> > void nir_convert_from_ssa(nir_shader *shader);
> >> > diff --git a/src/glsl/nir/nir_live_variables.c
> >> > b/src/glsl/nir/nir_live_variables.c
> >> > new file mode 100644
> >> > index 0000000..7d99a06
> >> > --- /dev/null
> >> > +++ b/src/glsl/nir/nir_live_variables.c
> >> > @@ -0,0 +1,282 @@
> >> > +/*
> >> > + * Copyright © 2014 Intel Corporation
> >> > + *
> >> > + * 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.
> >> > + *
> >> > + * Authors:
> >> > + * Jason Ekstrand (ja...@jlekstrand.net)
> >> > + */
> >> > +
> >> > +#include "nir.h"
> >> > +
> >> > +/*
> >> > + * Basic liveness analysis. This works only in SSA form.
> >> > + *
> >> > + * This liveness pass treats phi nodes as being melded to the space
> >> > between
> >> > + * blocks so that the destinations of a phi are in the livein of the
> >> > block
> >> > + * in which it resides and the sources are in the liveout of the
> >> > + * corresponding block. By formulating the liveness information in
> >> > this
> >> > + * way, we ensure that the definition of any variable dominates its
> >> > entire
> >> > + * live range. This is true because the only way that the definition
> >> > of an
> >> > + * SSA value may not dominate a use is if the use is in a phi node
> and
> >> > the
> >> > + * uses in phi no are in the live-out of the corresponding
> predecessor
> >> > + * block but not in the live-in of the block containing the phi node.
> >> > + */
> >> > +
> >> > +struct live_variables_state {
> >> > + unsigned num_ssa_defs;
> >> > + unsigned bitset_words;
> >> > + BITSET_WORD progress;
> >>
> >> I'd rather have this be
> >>
> >> bool progress;
> >>
> >> since it's effectively being used as a boolean, no need to confuse
> >> people with this and it's just unnecessary micro-optimization.
> >
> >
> > Sure. That can be done.
> > --Jason
> >
> >>
> >>
> >> > +};
> >> > +
> >> > +static bool
> >> > +index_dest(nir_dest *dest, void *void_state)
> >> > +{
> >> > + struct live_variables_state *state = void_state;
> >> > +
> >> > + if (dest->is_ssa)
> >> > + dest->ssa.live_index = state->num_ssa_defs++;
> >> > +
> >> > + return true;
> >> > +}
> >> > +
> >> > +static bool
> >> > +index_ssa_definitions_block(nir_block *block, void *void_state)
> >> > +{
> >> > + struct live_variables_state *state = void_state;
> >> > +
> >> > + nir_foreach_instr(block, instr) {
> >> > + if (instr->type == nir_instr_type_ssa_undef) {
> >> > + nir_ssa_undef_instr *undef = nir_instr_as_ssa_undef(instr);
> >> > + undef->def.live_index = 0;
> >> > + } else {
> >> > + nir_foreach_dest(instr, index_dest, state);
> >> > + }
> >> > + }
> >> > +
> >> > + return true;
> >> > +}
> >> > +
> >> > +static bool
> >> > +init_liveness_block(nir_block *block, void *void_state)
> >> > +{
> >> > + struct live_variables_state *state = void_state;
> >> > +
> >> > + block->live_in = reralloc(block, block->live_in, BITSET_WORD,
> >> > + state->bitset_words);
> >> > + memset(block->live_in, 0, state->bitset_words *
> >> > sizeof(BITSET_WORD));
> >> > +
> >> > + block->live_out = reralloc(block, block->live_out, BITSET_WORD,
> >> > + state->bitset_words);
> >> > + memset(block->live_out, 0, state->bitset_words *
> >> > sizeof(BITSET_WORD));
> >> > +
> >> > + return true;
> >> > +}
> >> > +
> >> > +static bool
> >> > +set_src_live(nir_src *src, void *void_live)
> >> > +{
> >> > + BITSET_WORD *live = void_live;
> >> > +
> >> > + if (!src->is_ssa)
> >> > + return true;
> >> > +
> >> > + if (src->ssa->live_index == 0)
> >> > + return true; /* undefined variables are never live */
> >> > +
> >> > + BITSET_SET(live, src->ssa->live_index);
> >> > +
> >> > + return true;
> >> > +}
> >> > +
> >> > +static bool
> >> > +set_dest_dead(nir_dest *dest, void *void_live)
> >> > +{
> >> > + BITSET_WORD *live = void_live;
> >> > +
> >> > + if (dest->is_ssa)
> >> > + BITSET_CLEAR(live, dest->ssa.live_index);
> >> > +
> >> > + return true;
> >> > +}
> >> > +
> >> > +/* Phi nodes exist "between" blocks and all the phi nodes at the
> start
> >> > of a
> >> > + * block act "in parallel". When we propagate from the live_in of
> one
> >> > + * block to the live out of the other, we have to kill any writes
> from
> >> > phis
> >> > + * and make live any sources.
> >> > + */
> >> > +static void
> >> > +propagate_across_edge(nir_block *pred, nir_block *succ,
> >> > + struct live_variables_state *state)
> >> > +{
> >> > + BITSET_WORD live[state->bitset_words];
> >> > + memcpy(live, succ->live_in, sizeof live);
> >> > +
> >> > + nir_foreach_instr(succ, instr) {
> >> > + if (instr->type != nir_instr_type_phi)
> >> > + break;
> >> > + nir_phi_instr *phi = nir_instr_as_phi(instr);
> >> > +
> >> > + set_dest_dead(&phi->dest, live);
> >> > + }
> >> > +
> >> > + nir_foreach_instr(succ, instr) {
> >> > + if (instr->type != nir_instr_type_phi)
> >> > + break;
> >> > + nir_phi_instr *phi = nir_instr_as_phi(instr);
> >> > +
> >> > + foreach_list_typed(nir_phi_src, src, node, &phi->srcs) {
> >> > + if (src->pred == pred) {
> >> > + set_src_live(&src->src, live);
> >> > + break;
> >> > + }
> >> > + }
> >> > + }
> >> > +
> >> > + for (unsigned i = 0; i < state->bitset_words; ++i) {
> >> > + state->progress |= live[i] & ~pred->live_out[i];
> >>
> >> state->progress = state->progress || (live[i] & ~pred->live_out[i]) !=
> 0;
> >>
> >> > + pred->live_out[i] |= live[i];
> >> > + }
> >> > +}
> >> > +
> >> > +static bool
> >> > +walk_instructions_block(nir_block *block, void *void_state)
> >> > +{
> >> > + struct live_variables_state *state = void_state;
> >> > +
> >> > + /* The live out is the union (modulo phi nodes) of the live ins of
> >> > its
> >> > + * successors */
> >> > + if (block->successors[0])
> >> > + propagate_across_edge(block, block->successors[0], state);
> >> > + if (block->successors[1])
> >> > + propagate_across_edge(block, block->successors[1], state);
> >> > +
> >> > + memcpy(block->live_in, block->live_out,
> >> > + state->bitset_words * sizeof(BITSET_WORD));
> >> > +
> >> > + nir_if *following_if = nir_block_following_if(block);
> >> > + if (following_if)
> >> > + set_src_live(&following_if->condition, block->live_in);
> >> > +
> >> > + nir_foreach_instr_reverse(block, instr) {
> >> > + /* Phi nodes are handled seperately so we want to skip them.
> >> > Since
> >> > + * we are going backwards and they are at the beginning, we can
> >> > just
> >> > + * break as soon as we see one.
> >> > + */
> >> > + if (instr->type == nir_instr_type_phi)
> >> > + break;
> >> > +
> >> > + nir_foreach_dest(instr, set_dest_dead, block->live_in);
> >> > + nir_foreach_src(instr, set_src_live, block->live_in);
> >> > + }
> >> > +
> >> > + return true;
> >> > +}
> >> > +
> >> > +static bool
> >> > +src_does_not_use_def(nir_src *src, void *def)
> >> > +{
> >> > + return !src->is_ssa || src->ssa != (nir_ssa_def *)def;
> >> > +}
> >> > +
> >> > +static bool
> >> > +search_for_use_after_instr(nir_instr *start, nir_ssa_def *def)
> >> > +{
> >> > + /* Only look for a use strictly after the given instruction */
> >> > + struct exec_node *node = start->node.next;
> >> > + while (!exec_node_is_tail_sentinel(node)) {
> >> > + nir_instr *instr = exec_node_data(nir_instr, node, node);
> >> > + if (!nir_foreach_src(instr, src_does_not_use_def, def))
> >> > + return true;
> >> > + node = node->next;
> >> > + }
> >> > + return false;
> >> > +}
> >> > +
> >> > +/* Returns true if def is live at instr assuming that def comes
> before
> >> > + * instr in a pre DFS search of the dominance tree.
> >> > + */
> >> > +static bool
> >> > +nir_ssa_def_is_live_at(nir_ssa_def *def, nir_instr *instr)
> >> > +{
> >> > + if (BITSET_TEST(instr->block->live_out, def->live_index)) {
> >> > + /* Since def dominates instr, if def is in the liveout of the
> >> > block,
> >> > + * it's live at instr
> >> > + */
> >> > + return true;
> >> > + } else {
> >> > + if (BITSET_TEST(instr->block->live_in, def->live_index) ||
> >> > + def->parent_instr->block == instr->block) {
> >> > + /* In this case it is either live coming into instr's block
> or
> >> > it
> >> > + * is defined in the same block. In this case, we simply
> need
> >> > to
> >> > + * see if it is used after instr.
> >> > + */
> >> > + return search_for_use_after_instr(instr, def);
> >> > + } else {
> >> > + return false;
> >> > + }
> >> > + }
> >> > +}
> >> > +
> >> > +bool
> >> > +nir_ssa_defs_interfere(nir_ssa_def *a, nir_ssa_def *b)
> >> > +{
> >> > + if (a->parent_instr == b->parent_instr) {
> >> > + /* Two variables defined at the same time interfere assuming at
> >> > + * least one isn't dead.
> >> > + */
> >> > + return true;
> >> > + } else if (a->live_index == 0 || b->live_index == 0) {
> >> > + /* If either variable is an ssa_undef, then there's no
> >> > interference */
> >> > + return false;
> >> > + } else if (a->live_index < b->live_index) {
> >> > + return nir_ssa_def_is_live_at(a, b->parent_instr);
> >> > + } else {
> >> > + return nir_ssa_def_is_live_at(b, a->parent_instr);
> >> > + }
> >> > +}
> >> > +
> >> > +void
> >> > +nir_live_variables_impl(nir_function_impl *impl)
> >> > +{
> >> > + struct live_variables_state state;
> >> > +
> >> > + /* We start at 1 because we reserve the index value of 0 for
> >> > ssa_undef
> >> > + * instructions. Those are never live, so their liveness
> >> > information
> >> > + * can be compacted into a single bit.
> >> > + */
> >> > + state.num_ssa_defs = 1;
> >> > + nir_foreach_block(impl, index_ssa_definitions_block, &state);
> >> > +
> >> > + /* We now know how many unique ssa definitions we have and we can
> go
> >> > + * ahead and allocate live_in and live_out sets
> >> > + */
> >> > + state.bitset_words = BITSET_WORDS(state.num_ssa_defs);
> >> > + nir_foreach_block(impl, init_liveness_block, &state);
> >> > +
> >> > + /* We need to propagate the liveness back through the CFG. Thanks
> >> > to
> >> > + * the wonders of SSA, this will run no more times than the depth
> of
> >> > the
> >> > + * deepest loop + 1.
> >> > + */
> >> > + do {
> >> > + state.progress = 0;
> >>
> >> state.progress = false;
> >>
> >> > + nir_foreach_block_reverse(impl, walk_instructions_block,
> &state);
> >> > + } while (state.progress);
> >> > +}
> >> > diff --git a/src/glsl/nir/nir_metadata.c b/src/glsl/nir/nir_metadata.c
> >> > index 070cb74..a4d618c 100644
> >> > --- a/src/glsl/nir/nir_metadata.c
> >> > +++ b/src/glsl/nir/nir_metadata.c
> >> > @@ -39,6 +39,8 @@ nir_metadata_require(nir_function_impl *impl,
> >> > nir_metadata required)
> >> > nir_index_blocks(impl);
> >> > if (NEEDS_UPDATE(nir_metadata_dominance))
> >> > nir_calc_dominance_impl(impl);
> >> > + if (NEEDS_UPDATE(nir_metadata_live_variables))
> >> > + nir_live_variables_impl(impl);
> >> >
> >> > #undef NEEDS_UPDATE
> >> >
> >> > diff --git a/src/mesa/main/bitset.h b/src/mesa/main/bitset.h
> >> > index 601fd0e..2558da4 100644
> >> > --- a/src/mesa/main/bitset.h
> >> > +++ b/src/mesa/main/bitset.h
> >> > @@ -32,6 +32,7 @@
> >> > #define BITSET_H
> >> >
> >> > #include "imports.h"
> >> > +#include "macros.h"
> >> >
> >> >
> >> >
> /****************************************************************************
> >> > * generic bitset implementation
> >> > --
> >> > 2.2.0
> >> >
> >> > _______________________________________________
> >> > mesa-dev mailing list
> >> > mesa-dev@lists.freedesktop.org
> >> > http://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
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