[Mesa-dev] [PATCH v2 11/12] nir: Add a global code motion (GCM) pass
Connor Abbott
cwabbott0 at gmail.com
Mon Feb 9 12:01:35 PST 2015
So last time you posted this, I had some suggestions but I realized
that in light of the fact that we want to do value numbering, it
didn't seem like my suggestions were any good. But now that I've
thought about it a little bit, it seems to me like a better plan might
be to pull instructions out of their basic blocks and into the global
list of instructions earlier. That is, instead of marking instructions
as pinned in gcm_pin_instructions_block(), put the ones that aren't
pinned in state->instrs and then instead of walking over all the basic
blocks and bailing on pinned instructions when scheduling early and
late, just walk over state->instrs. This should be faster, and when we
do value-numbering there's a natural place to do it, i.e. after
pulling instructions but before scheduling -- this means we only
value-number the non-pinned instructions, which is what we want since
we're not coalescing the pinned instructions in this pass. This is
also more similar to how it's presented in the paper (although our
scheduling algorithm will still be different I guess).
On Mon, Feb 9, 2015 at 3:19 AM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> v2 Jason Ekstrand <jason.ekstrand at intel.com>:
> - Use nir_dominance_lca for computing least common anscestors
> - Use the block index for comparing dominance tree depths
> - Pin things that do partial derivatives
> ---
> src/glsl/Makefile.sources | 1 +
> src/glsl/nir/nir.h | 2 +
> src/glsl/nir/nir_opt_gcm.c | 501 +++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 504 insertions(+)
> create mode 100644 src/glsl/nir/nir_opt_gcm.c
>
> diff --git a/src/glsl/Makefile.sources b/src/glsl/Makefile.sources
> index a580b6e..69cb2e6 100644
> --- a/src/glsl/Makefile.sources
> +++ b/src/glsl/Makefile.sources
> @@ -43,6 +43,7 @@ NIR_FILES = \
> nir/nir_opt_copy_propagate.c \
> nir/nir_opt_cse.c \
> nir/nir_opt_dce.c \
> + nir/nir_opt_gcm.c \
> nir/nir_opt_global_to_local.c \
> nir/nir_opt_peephole_select.c \
> nir/nir_opt_remove_phis.c \
> diff --git a/src/glsl/nir/nir.h b/src/glsl/nir/nir.h
> index 90a7001..55fb43d 100644
> --- a/src/glsl/nir/nir.h
> +++ b/src/glsl/nir/nir.h
> @@ -1589,6 +1589,8 @@ bool nir_opt_cse(nir_shader *shader);
> bool nir_opt_dce_impl(nir_function_impl *impl);
> bool nir_opt_dce(nir_shader *shader);
>
> +void nir_opt_gcm(nir_shader *shader);
> +
> bool nir_opt_peephole_select(nir_shader *shader);
> bool nir_opt_peephole_ffma(nir_shader *shader);
>
> diff --git a/src/glsl/nir/nir_opt_gcm.c b/src/glsl/nir/nir_opt_gcm.c
> new file mode 100644
> index 0000000..d48518b
> --- /dev/null
> +++ b/src/glsl/nir/nir_opt_gcm.c
> @@ -0,0 +1,501 @@
> +/*
> + * 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"
> +
> +/*
> + * Implements Global Code Motion. A description of GCM can be found in
> + * "Global Code Motion; Global Value Numbering" by Cliff Click.
> + * Unfortunately, the algorithm presented in the paper is broken in a
> + * number of ways. The algorithm used here differs substantially from the
> + * one in the paper but it is, in my opinion, much easier to read and
> + * verify correcness.
> + */
> +
> +struct gcm_block_info {
> + /* Number of loops this block is inside */
> + unsigned loop_depth;
> +
> + /* The last instruction inserted into this block. This is used as we
> + * traverse the instructions and insert them back into the program to
> + * put them in the right order.
> + */
> + nir_instr *last_instr;
> +};
> +
> +struct gcm_state {
> + nir_function_impl *impl;
> + nir_instr *instr;
> +
> + /* Marks all instructions that have been visited by the curren pass */
> + BITSET_WORD *visited;
> +
> + /* Marks instructions that are "pinned", i.e. cannot be moved from their
> + * basic block by code motion */
> + BITSET_WORD *pinned;
Note that there's already a boolean field embedded in each instruction
called "live" since it's used by DCE. Maybe we can replace it with a
uint8_t or so with a more generic name like "data" or "bitfield", and
then we can use it for this pass as well to replace both visited and
pinned. We'd only need 4 bits (pinned, scheduled early, scheduled
late, and placed). This would also mean we wouldn't need the
per-instruction index any more.
> +
> + /* The list of non-pinned instructions. As we do the late scheduling,
> + * we pull non-pinned instructions out of their blocks and place them in
> + * this list. This saves us from having linked-list problems when we go
> + * to put instructions back in their blocks.
> + */
> + struct exec_list instrs;
> +
> + struct gcm_block_info *blocks;
> +};
> +
> +/* Recursively walks the CFG and builds the block_info structure */
> +static void
> +gcm_build_block_info(struct exec_list *cf_list, struct gcm_state *state,
> + unsigned loop_depth)
> +{
> + foreach_list_typed(nir_cf_node, node, node, cf_list) {
> + switch (node->type) {
> + case nir_cf_node_block: {
> + nir_block *block = nir_cf_node_as_block(node);
> + state->blocks[block->index].loop_depth = loop_depth;
> + break;
> + }
> + case nir_cf_node_if: {
> + nir_if *if_stmt = nir_cf_node_as_if(node);
> + gcm_build_block_info(&if_stmt->then_list, state, loop_depth);
> + gcm_build_block_info(&if_stmt->else_list, state, loop_depth);
> + break;
> + }
> + case nir_cf_node_loop: {
> + nir_loop *loop = nir_cf_node_as_loop(node);
> + gcm_build_block_info(&loop->body, state, loop_depth + 1);
> + break;
> + }
> + default:
> + unreachable("Invalid CF node type");
> + }
> + }
> +}
> +
> +/* Walks the instruction list and marks immovable instructions as pinned */
> +static bool
> +gcm_pin_instructions_block(nir_block *block, void *void_state)
> +{
> + struct gcm_state *state = void_state;
> +
> + nir_foreach_instr_safe(block, instr) {
> + bool pinned;
> + switch (instr->type) {
> + case nir_instr_type_alu:
> + switch (nir_instr_as_alu(instr)->op) {
> + case nir_op_fddx:
> + case nir_op_fddy:
> + case nir_op_fddx_fine:
> + case nir_op_fddy_fine:
> + case nir_op_fddx_coarse:
> + case nir_op_fddy_coarse:
> + /* These can only go in uniform control flow; pin them for now */
> + pinned = true;
> +
> + default:
> + pinned = false;
> + }
> + break;
> +
> + case nir_instr_type_tex:
> + /* We need to pin texture ops that do partial derivatives */
> + pinned = nir_instr_as_tex(instr)->op == nir_texop_txd;
I don't think this is correct -- txd is when we supply the derivatives
ourselves, so there are no implicit partial derivatives. I *think* the
ones where the HW takes partial derivatives are tex, txb, lod, and tg4
(gather). I'd double-check that with someone else, though, since it's
been a while since I had to do texture stuff.
> + break;
> +
> + case nir_instr_type_load_const:
> + pinned = false;
> + break;
> +
> + case nir_instr_type_intrinsic: {
> + const nir_intrinsic_info *info =
> + &nir_intrinsic_infos[nir_instr_as_intrinsic(instr)->intrinsic];
> + pinned = !(info->flags & NIR_INTRINSIC_CAN_ELIMINATE) ||
> + !(info->flags & NIR_INTRINSIC_CAN_REORDER);
> + break;
> + }
> +
> + case nir_instr_type_jump:
> + case nir_instr_type_ssa_undef:
> + case nir_instr_type_phi:
> + pinned = true;
> + break;
> +
> + default:
> + unreachable("Invalid instruction type in GCM");
> + }
> +
> + if (pinned)
> + BITSET_SET(state->pinned, instr->index);
> + }
> +
> + return true;
> +}
> +
> +static void
> +gcm_schedule_early_instr(nir_instr *instr, struct gcm_state *state);
> +
> +/** Update an instructions schedule for the given source
> + *
> + * This function is called iteratively as we walk the sources of an
> + * instruction. It ensures that the given source instruction has been
> + * scheduled and then update this instruction's block if the source
> + * instruction is lower down the tree.
> + */
> +static bool
> +gcm_schedule_early_src(nir_src *src, void *void_state)
> +{
> + struct gcm_state *state = void_state;
> + nir_instr *instr = state->instr;
> +
> + assert(src->is_ssa);
> +
> + gcm_schedule_early_instr(src->ssa->parent_instr, void_state);
> +
> + /* While the index isn't a proper dominance depth, it does have the
> + * property that if A dominates B then A->index <= B->index. Since we
> + * know that this instruction must have been dominated by all of its
> + * sources at some point (even if it's gone through value-numbering),
> + * all of the sources must lie on the same branch of the dominance tree.
> + * Therefore, we can just go ahead and just compare indices.
> + */
> + if (instr->block->index < src->ssa->parent_instr->block->index)
> + instr->block = src->ssa->parent_instr->block;
> +
> + /* We need to restore the state instruction because it may have been
> + * changed through the gcm_schedule_early_instr call above. Since we
> + * may still be iterating through sources and future calls to
> + * gcm_schedule_early_src for the same instruction will still need it.
> + */
> + state->instr = instr;
> +
> + return true;
> +}
> +
> +/** Schedules an instruction early
> + *
> + * This function performs a recursive depth-first search starting at the
> + * given instruction and proceeding through the sources to schedule
> + * instructions as early as they can possibly go in the dominance tree.
> + * The instructions are "scheduled" by updating their instr->block field.
> + */
> +static void
> +gcm_schedule_early_instr(nir_instr *instr, struct gcm_state *state)
> +{
> + if (BITSET_TEST(state->visited, instr->index))
> + return;
> +
> + BITSET_SET(state->visited, instr->index);
> +
> + /* Pinned instructions are already scheduled so we don't need to do
> + * anything. Also, bailing here keeps us from ever following the
> + * sources of phi nodes which can be back-edges.
> + */
> + if (BITSET_TEST(state->pinned, instr->index))
> + return;
> +
> + /* Start with the instruction at the top. As we iterate over the
> + * sources, it will get moved down as needed.
> + */
> + instr->block = state->impl->start_block;
> + state->instr = instr;
> +
> + nir_foreach_src(instr, gcm_schedule_early_src, state);
> +}
> +
> +static bool
> +gcm_schedule_early_block(nir_block *block, void *state)
> +{
> + nir_foreach_instr(block, instr)
> + gcm_schedule_early_instr(instr, state);
> +
> + return true;
> +}
> +
> +static void
> +gcm_schedule_late_instr(nir_instr *instr, struct gcm_state *state);
> +
> +/** Schedules the instruction associated with the given SSA def late
> + *
> + * This function works by first walking all of the uses of the given SSA
> + * definition, ensuring that they are scheduled, and then computing the LCA
> + * (least common ancestor) of its uses. It then schedules this instruction
> + * as close to the LCA as possible while trying to stay out of loops.
> + */
> +static bool
> +gcm_schedule_late_def(nir_ssa_def *def, void *void_state)
> +{
> + struct gcm_state *state = void_state;
> +
> + nir_block *lca = NULL;
> +
> + struct set_entry *entry;
> + set_foreach(def->uses, entry) {
> + nir_instr *use_instr = (nir_instr *)entry->key;
> +
> + gcm_schedule_late_instr(use_instr, state);
> +
> + /* Phi instructions are a bit special. SSA definitions don't have to
> + * dominate the sources of the phi nodes that use them; instead, they
> + * have to dominate the predecessor block corresponding to the phi
> + * source. We handle this by looking through the sources, finding
> + * any that are usingg this SSA def, and using those blocks instead
> + * of the one the phi lives in.
> + */
> + if (use_instr->type == nir_instr_type_phi) {
> + nir_phi_instr *phi = nir_instr_as_phi(use_instr);
> +
> + nir_foreach_phi_src(phi, phi_src) {
> + if (phi_src->src.ssa == def)
> + lca = nir_dominance_lca(lca, phi_src->pred);
> + }
> + } else {
> + lca = nir_dominance_lca(lca, use_instr->block);
> + }
> + }
> +
> + set_foreach(def->if_uses, entry) {
> + nir_if *if_stmt = (nir_if *)entry->key;
> +
> + /* For if statements, we consider the block to be the one immediately
> + * preceding the if CF node.
> + */
> + nir_block *pred_block =
> + nir_cf_node_as_block(nir_cf_node_prev(&if_stmt->cf_node));
> +
> + lca = nir_dominance_lca(lca, pred_block);
> + }
> +
> + /* Some instructions may never be used. We'll just leave them scheduled
> + * early and let dead code clean them up.
> + */
> + if (lca == NULL)
> + return true;
> +
> + /* We know have the LCA of all of the uses. If our invariants hold,
> + * this is dominated by the block that we chose when scheduling early.
> + * We now walk up the dominance tree and pick the lowest block that is
> + * as far outside loops as we can get.
> + */
> + nir_block *best = lca;
> + while (lca != def->parent_instr->block) {
> + assert(lca);
> + if (state->blocks[lca->index].loop_depth <
> + state->blocks[best->index].loop_depth)
> + best = lca;
> + lca = lca->imm_dom;
> + }
> + def->parent_instr->block = best;
I think this loop will exclude def->parent_instr->block from
consideration unless lca == def->parent_instr->block. On the last
iteration, we'll check lca to see if it's the best, then move lca up
the tree, then discover that lca is equal to def->parent_instr->block,
then bail out without checking it. I think you can fix this by
swapping the if-statement and the last statement in the loop, which
will also have the benefit of avoiding the unnecessary test on the
first iteration of the loop. Of course, you'd want to move the assert
as well, so it would look like:
nir_block *best = lca;
while (lca != def->parent_instr->block) {
lca = lca->imm_dom;
assert(lca);
if (state->blocks[lca->index].loop_depth <
state->blocks[best->index].loop_depth)
best = lca;
}
> +
> + return true;
> +}
> +
> +/** Schedules an instruction late
> + *
> + * This function performs a depth-first search starting at the given
> + * instruction and proceeding through its uses to schedule instructions as
> + * late as they can reasonably go in the dominance tree. The instructions
> + * are "scheduled" by updating their instr->block field.
> + *
> + * The name of this function is actually a bit of a misnomer as it doesn't
> + * schedule them "as late as possible" as the paper implies. Instead, it
> + * first finds the lates possible place it can schedule the instruction and
> + * then possibly schedules it earlier than that. The actual location is as
> + * far down the tree as we can go while trying to stay out of loops.
> + */
> +static void
> +gcm_schedule_late_instr(nir_instr *instr, struct gcm_state *state)
> +{
> + if (BITSET_TEST(state->visited, instr->index))
> + return;
> +
> + BITSET_SET(state->visited, instr->index);
> +
> + /* Pinned instructions are already scheduled so we don't need to do
> + * anything. Also, bailing here keeps us from ever following phi nodes
> + * which can be back-edges.
> + */
> + if (BITSET_TEST(state->pinned, instr->index))
> + return;
> +
> + nir_foreach_ssa_def(instr, gcm_schedule_late_def, state);
> +}
> +
> +static bool
> +gcm_schedule_late_block(nir_block *block, void *void_state)
> +{
> + struct gcm_state *state = void_state;
> +
> + nir_foreach_instr_safe(block, instr) {
> + gcm_schedule_late_instr(instr, state);
> +
> + if (!BITSET_TEST(state->pinned, instr->index)) {
> + /* If this is an instruction we can move, go ahead and pull it out
> + * of the program and put it on the instrs list. This keeps us
> + * from causing linked list confusion when we're trying to put
> + * everything in its proper place.
> + *
> + * Note that we don't use nir_instr_remove here because that also
> + * cleans up uses and defs and we want to keep that information.
> + */
> + exec_node_remove(&instr->node);
> + exec_list_push_tail(&state->instrs, &instr->node);
> + }
> + }
> +
> + return true;
> +}
> +
> +static void
> +gcm_place_instr(nir_instr *instr, struct gcm_state *state);
> +
> +static bool
> +gcm_place_instr_def(nir_ssa_def *def, void *state)
> +{
> + struct set_entry *entry;
> + set_foreach(def->uses, entry)
> + gcm_place_instr((nir_instr *)entry->key, state);
> +
> + return false;
> +}
> +
> +/** Places an instrution back into the program
> + *
> + * The earlier passes of GCM simply choose blocks for each instruction and
> + * otherwise leave them alone. This pass actually places the instructions
> + * into their chosen blocks.
> + *
> + * To do so, we use a standard post-order depth-first search linearization
> + * algorithm. We walk over the uses of the given instruction and ensure
> + * that they are placed and then place this instruction. Because we are
> + * working on multiple blocks at a time, we keep track of the last inserted
> + * instruction per-block in the state structure's block_info array. When
> + * we insert an instruction in a block we insert it before the last
> + * instruction inserted in that block rather than the last instruction
> + * inserted globally.
> + */
> +static void
> +gcm_place_instr(nir_instr *instr, struct gcm_state *state)
> +{
> + if (BITSET_TEST(state->visited, instr->index))
> + return;
> +
> + BITSET_SET(state->visited, instr->index);
> +
> + /* Phi nodes are our once source of back-edges. Since right now we are
> + * only doing scheduling within blocks, we don't need to worry about
> + * them since they are always at the top. Just skip them completely.
> + */
> + if (instr->type == nir_instr_type_phi) {
> + assert(BITSET_TEST(state->pinned, instr->index));
> + return;
> + }
> +
> + nir_foreach_ssa_def(instr, gcm_place_instr_def, state);
> +
> + if (BITSET_TEST(state->pinned, instr->index)) {
> + /* Pinned instructions have an implicit dependence on the pinned
> + * instructions that come after them in the block. Since the pinned
> + * instructions will naturally "chain" together, we only need to
> + * explicitly visit one of them.
> + */
> + for (nir_instr *after = nir_instr_next(instr);
> + after;
> + after = nir_instr_next(after)) {
> + if (BITSET_TEST(state->pinned, after->index)) {
> + gcm_place_instr(after, state);
> + break;
> + }
> + }
> + }
> +
> + struct gcm_block_info *block_info = &state->blocks[instr->block->index];
> + if (!BITSET_TEST(state->pinned, instr->index)) {
> + exec_node_remove(&instr->node);
> +
> + if (block_info->last_instr) {
> + exec_node_insert_node_before(&block_info->last_instr->node,
> + &instr->node);
> + } else {
> + /* Schedule it at the end of the block */
> + nir_instr *jump_instr = nir_block_last_instr(instr->block);
> + if (jump_instr && jump_instr->type == nir_instr_type_jump) {
> + exec_node_insert_node_before(&jump_instr->node, &instr->node);
> + } else {
> + exec_list_push_tail(&instr->block->instr_list, &instr->node);
> + }
> + }
> + }
> +
> + block_info->last_instr = instr;
> +}
> +
> +static void
> +opt_gcm_impl(nir_function_impl *impl)
> +{
> + struct gcm_state state;
> +
> + unsigned num_instrs = nir_index_instrs(impl);
> + unsigned instr_words = BITSET_WORDS(num_instrs);
> +
> + state.impl = impl;
> + state.instr = NULL;
> + state.visited = rzalloc_array(NULL, BITSET_WORD, instr_words);
> + state.pinned = rzalloc_array(NULL, BITSET_WORD, instr_words);
> + exec_list_make_empty(&state.instrs);
> + state.blocks = rzalloc_array(NULL, struct gcm_block_info, impl->num_blocks);
> +
> + nir_metadata_require(impl, nir_metadata_block_index |
> + nir_metadata_dominance);
> +
> + gcm_build_block_info(&impl->body, &state, 0);
> + nir_foreach_block(impl, gcm_pin_instructions_block, &state);
> +
> + nir_foreach_block(impl, gcm_schedule_early_block, &state);
> +
> + memset(state.visited, 0, instr_words * sizeof(*state.visited));
> + nir_foreach_block(impl, gcm_schedule_late_block, &state);
> +
> + memset(state.visited, 0, instr_words * sizeof(*state.visited));
> + while (!exec_list_is_empty(&state.instrs)) {
> + nir_instr *instr = exec_node_data(nir_instr,
> + state.instrs.tail_pred, node);
> + gcm_place_instr(instr, &state);
> + }
> +
> + ralloc_free(state.visited);
> + ralloc_free(state.blocks);
> +}
> +
> +void
> +nir_opt_gcm(nir_shader *shader)
> +{
> + nir_foreach_overload(shader, overload) {
> + if (overload->impl)
> + opt_gcm_impl(overload->impl);
> + }
> +}
> --
> 2.2.2
>
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