[Mesa-dev] [PATCH v2 1/3] nir: Add a pass to lower vector phi nodes to scalar phi nodes
Jason Ekstrand
jason at jlekstrand.net
Tue Jan 27 14:07:52 PST 2015
oops, this should have been a v3
On Tue, Jan 27, 2015 at 1:32 PM, Jason Ekstrand <jason at jlekstrand.net>
wrote:
> v2 Jason Ekstrand <jason.ekstrand at intel.com>:
> - Add better comments
> - Use nir_ssa_dest_init and nir_src_for_ssa more places
> - Fix some void * casts
> ---
> src/glsl/Makefile.sources | 1 +
> src/glsl/nir/nir.h | 2 +
> src/glsl/nir/nir_lower_phis_to_scalar.c | 251
> ++++++++++++++++++++++++++++++++
> 3 files changed, 254 insertions(+)
> create mode 100644 src/glsl/nir/nir_lower_phis_to_scalar.c
>
> diff --git a/src/glsl/Makefile.sources b/src/glsl/Makefile.sources
> index 96c4ec5..02d0780 100644
> --- a/src/glsl/Makefile.sources
> +++ b/src/glsl/Makefile.sources
> @@ -28,6 +28,7 @@ NIR_FILES = \
> nir/nir_lower_global_vars_to_local.c \
> nir/nir_lower_locals_to_regs.c \
> nir/nir_lower_io.c \
> + nir/nir_lower_phis_to_scalar.c \
> nir/nir_lower_samplers.cpp \
> nir/nir_lower_system_values.c \
> nir/nir_lower_to_source_mods.c \
> diff --git a/src/glsl/nir/nir.h b/src/glsl/nir/nir.h
> index 119ca01..cda14aa 100644
> --- a/src/glsl/nir/nir.h
> +++ b/src/glsl/nir/nir.h
> @@ -1523,6 +1523,8 @@ void nir_remove_dead_variables(nir_shader *shader);
> void nir_lower_vec_to_movs(nir_shader *shader);
> void nir_lower_alu_to_scalar(nir_shader *shader);
>
> +void nir_lower_phis_to_scalar(nir_shader *shader);
> +
> void nir_lower_samplers(nir_shader *shader,
> struct gl_shader_program *shader_program,
> struct gl_program *prog);
> diff --git a/src/glsl/nir/nir_lower_phis_to_scalar.c
> b/src/glsl/nir/nir_lower_phis_to_scalar.c
> new file mode 100644
> index 0000000..d8c3f57
> --- /dev/null
> +++ b/src/glsl/nir/nir_lower_phis_to_scalar.c
> @@ -0,0 +1,251 @@
> +/*
> + * Copyright © 2015 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 a pass that lowers vector phi nodes to scalar phi nodes when
> + * we don't think it will hurt anything.
> + */
> +
> +struct lower_phis_to_scalar_state {
> + void *mem_ctx;
> + void *dead_ctx;
> +
> + /* Hash table marking which phi nodes are scalarizable. The key is
> + * pointers to phi instructions and the entry is either NULL for not
> + * scalarizable or non-null for scalarizable.
> + */
> + struct hash_table *phi_table;
> +};
> +
> +/* Determines if the given phi node should be lowered. The only phi nodes
> + * we will scalarize at the moment are those where all of the sources are
> + * scalarizable.
> + *
> + * The reason for this comes down to coalescing. Since phi sources can't
> + * swizzle, swizzles on phis have to be resolved by inserting a mov right
> + * before the phi. The choice then becomes between movs to pick off
> + * components for a scalar phi or potentially movs to recombine components
> + * for a vector phi. The problem is that the movs generated to pick off
> + * the components are almost uncoalescable. We can't coalesce them in NIR
> + * because we need them to pick off components and we can't coalesce them
> + * in the backend because the source register is a vector and the
> + * destination is a scalar that may be used at other places in the
> program.
> + * On the other hand, if we have a bunch of scalars going into a vector
> + * phi, the situation is much better. In this case, if the SSA def is
> + * generated in the predecessor block to the corresponding phi source, the
> + * backend code will be an ALU op into a temporary and then a mov into the
> + * given vector component; this move can almost certainly be coalesced
> + * away.
> + */
> +static bool
> +should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state
> *state)
> +{
> + /* Already scalar */
> + if (phi->dest.ssa.num_components == 1)
> + return false;
> +
> + struct hash_entry *entry = _mesa_hash_table_search(state->phi_table,
> phi);
> + if (entry)
> + return entry->data != NULL;
> +
> + nir_foreach_phi_src(phi, src) {
> + /* Don't know what to do with non-ssa sources */
> + if (!src->src.is_ssa)
> + return false;
> +
> + nir_instr *src_instr = src->src.ssa->parent_instr;
> + switch (src_instr->type) {
> + case nir_instr_type_alu: {
> + nir_alu_instr *src_alu = nir_instr_as_alu(src_instr);
> +
> + /* ALU operations with output_size == 0 should be scalarized. We
> + * will also see a bunch of vecN operations from scalarizing ALU
> + * operations and, since they can easily be copy-propagated, they
> + * are ok too.
> + */
> + return nir_op_infos[src_alu->op].output_size == 0 ||
> + src_alu->op != nir_op_vec2 ||
> + src_alu->op != nir_op_vec3 ||
> + src_alu->op != nir_op_vec4;
> + }
> +
> + case nir_instr_type_phi: {
> + nir_phi_instr *src_phi = nir_instr_as_phi(src_instr);
> +
> + /* Insert an entry and mark it as scalarizable for now. That way
> + * we don't recurse forever and a cycle in the depencence graph
> + * won't automatically make us fail to scalarize.
> + */
> + entry = _mesa_hash_table_insert(state->phi_table,
> + src_phi, (void *)(intptr_t)1);
> + bool scalarizable = should_lower_phi(src_phi, state);
> + entry->data = (void *)(intptr_t)scalarizable;
> +
> + return scalarizable;
> + }
> +
> + default:
> + /* We can't scalarize this type of instruction */
> + return false;
> + }
> + }
> +
> + return true;
> +}
> +
> +static bool
> +lower_phis_to_scalar_block(nir_block *block, void *void_state)
> +{
> + struct lower_phis_to_scalar_state *state = void_state;
> +
> + /* Find the last phi node in the block */
> + nir_phi_instr *last_phi = NULL;
> + nir_foreach_instr(block, instr) {
> + if (instr->type != nir_instr_type_phi)
> + break;
> +
> + last_phi = nir_instr_as_phi(instr);
> + }
> +
> + /* We have to handle the phi nodes in their own pass due to the way
> + * we're modifying the linked list of instructions.
> + */
> + nir_foreach_instr_safe(block, instr) {
> + if (instr->type != nir_instr_type_phi)
> + break;
> +
> + nir_phi_instr *phi = nir_instr_as_phi(instr);
> +
> + if (!should_lower_phi(phi, state))
> + continue;
> +
> + /* Create a vecN operation to combine the results. Most of these
> + * will be redundant, but copy propagation should clean them up for
> + * us. No need to add the complexity here.
> + */
> + nir_op vec_op;
> + switch (phi->dest.ssa.num_components) {
> + case 2: vec_op = nir_op_vec2; break;
> + case 3: vec_op = nir_op_vec3; break;
> + case 4: vec_op = nir_op_vec4; break;
> + default: unreachable("Invalid number of components");
> + }
> +
> + nir_alu_instr *vec = nir_alu_instr_create(state->mem_ctx, vec_op);
> + nir_ssa_dest_init(&vec->instr, &vec->dest.dest,
> + phi->dest.ssa.num_components, NULL);
> + vec->dest.write_mask = (1 << phi->dest.ssa.num_components) - 1;
> +
> + for (unsigned i = 0; i < phi->dest.ssa.num_components; i++) {
> + nir_phi_instr *new_phi = nir_phi_instr_create(state->mem_ctx);
> + nir_ssa_dest_init(&new_phi->instr, &new_phi->dest, 1, NULL);
> +
> + vec->src[i].src = nir_src_for_ssa(&new_phi->dest.ssa);
> +
> + nir_foreach_phi_src(phi, src) {
> + /* We need to insert a mov to grab the i'th component of src
> */
> + nir_alu_instr *mov = nir_alu_instr_create(state->mem_ctx,
> + nir_op_imov);
> + nir_ssa_dest_init(&mov->instr, &mov->dest.dest, 1, NULL);
> + mov->dest.write_mask = 1;
> + mov->src[0].src = nir_src_copy(src->src, state->mem_ctx);
> + mov->src[0].swizzle[0] = i;
> +
> + /* Insert at the end of the predecessor but before the jump */
> + nir_instr *pred_last_instr = nir_block_last_instr(src->pred);
> + if (pred_last_instr && pred_last_instr->type ==
> nir_instr_type_jump)
> + nir_instr_insert_before(pred_last_instr, &mov->instr);
> + else
> + nir_instr_insert_after_block(src->pred, &mov->instr);
> +
> + nir_phi_src *new_src = ralloc(state->mem_ctx, nir_phi_src);
> + new_src->pred = src->pred;
> + new_src->src = nir_src_for_ssa(&mov->dest.dest.ssa);
> +
> + exec_list_push_tail(&new_phi->srcs, &new_src->node);
> + }
> +
> + nir_instr_insert_before(&phi->instr, &new_phi->instr);
> + }
> +
> + nir_instr_insert_after(&last_phi->instr, &vec->instr);
> +
> + nir_ssa_def_rewrite_uses(&phi->dest.ssa,
> + nir_src_for_ssa(&vec->dest.dest.ssa),
> + state->mem_ctx);
> +
> + ralloc_steal(state->dead_ctx, phi);
> + nir_instr_remove(&phi->instr);
> +
> + /* We're using the safe iterator and inserting all the newly
> + * scalarized phi nodes before their non-scalarized version so
> that's
> + * ok. However, we are also inserting vec operations after all of
> + * the last phi node so once we get here, we can't trust even the
> + * safe iterator to stop properly. We have to break manually.
> + */
> + if (instr == &last_phi->instr)
> + break;
> + }
> +
> + return true;
> +}
> +
> +static void
> +lower_phis_to_scalar_impl(nir_function_impl *impl)
> +{
> + struct lower_phis_to_scalar_state state;
> +
> + state.mem_ctx = ralloc_parent(impl);
> + state.dead_ctx = ralloc_context(NULL);
> + state.phi_table = _mesa_hash_table_create(state.dead_ctx,
> _mesa_hash_pointer,
> + _mesa_key_pointer_equal);
> +
> + nir_foreach_block(impl, lower_phis_to_scalar_block, &state);
> +
> + nir_metadata_preserve(impl, nir_metadata_block_index |
> + nir_metadata_dominance);
> +
> + ralloc_free(state.dead_ctx);
> +}
> +
> +/** A pass that lowers vector phi nodes to scalar
> + *
> + * This pass loops through the blocks and lowers looks for vector phi
> nodes
> + * it can lower to scalar phi nodes. Not all phi nodes are lowered. For
> + * instance, if one of the sources is a non-scalarizable vector, then we
> + * don't bother lowering because that would generate hard-to-coalesce
> movs.
> + */
> +void
> +nir_lower_phis_to_scalar(nir_shader *shader)
> +{
> + nir_foreach_overload(shader, overload) {
> + if (overload->impl)
> + lower_phis_to_scalar_impl(overload->impl);
> + }
> +}
> --
> 2.2.2
>
>
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