[Mesa-dev] [PATCH 051/133] nir: Add an SSA-based liveness analysis pass.
Connor Abbott
cwabbott0 at gmail.com
Wed Dec 17 13:52:00 PST 2014
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.
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.
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.
> +};
> +
> +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
>
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