[Mesa-dev] [PATCH 051/133] nir: Add an SSA-based liveness analysis pass.
Jason Ekstrand
jason at jlekstrand.net
Mon Dec 15 22:05:01 PST 2014
---
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);
/* 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;
+};
+
+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];
+ 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;
+ 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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