[Mesa-dev] [PATCH 051/133] nir: Add an SSA-based liveness analysis pass.
Connor Abbott
cwabbott0 at gmail.com
Wed Dec 17 18:03:54 PST 2014
On Wed, Dec 17, 2014 at 8:48 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>
>
> On Wed, Dec 17, 2014 at 2:23 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
>>
>> On Wed, Dec 17, 2014 at 5:11 PM, Jason Ekstrand <jason at jlekstrand.net>
>> wrote:
>> > On Wed, Dec 17, 2014 at 1:52 PM, Connor Abbott <cwabbott0 at 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
Ok.
>
>>
>>
>> >
>> >>
>> >>
>> >> 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 <jason at 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 (jason at 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 at lists.freedesktop.org
>> >> > http://lists.freedesktop.org/mailman/listinfo/mesa-dev
More information about the mesa-dev
mailing list