/*
* 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@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;
};
static bool
should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state);
static bool
is_phi_src_scalarizable(nir_phi_src *src,
struct lower_phis_to_scalar_state *state)
{
/* 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:
/* A phi is scalarizable if we're going to lower it */
return should_lower_phi(nir_instr_as_phi(src_instr), state);
case nir_instr_type_load_const:
/* These are trivially scalarizable */
return true;
case nir_instr_type_intrinsic: {
nir_intrinsic_instr *src_intrin = nir_instr_as_intrinsic(src_instr);
switch (src_intrin->intrinsic) {
case nir_intrinsic_load_var:
return src_intrin->variables[0]->var->data.mode == nir_var_shader_in ||
src_intrin->variables[0]->var->data.mode == nir_var_uniform;
case nir_intrinsic_interp_var_at_centroid:
case nir_intrinsic_interp_var_at_sample:
case nir_intrinsic_interp_var_at_offset:
case nir_intrinsic_load_uniform:
case nir_intrinsic_load_uniform_indirect:
case nir_intrinsic_load_ubo:
case nir_intrinsic_load_ubo_indirect:
case nir_intrinsic_load_input:
case nir_intrinsic_load_input_indirect:
return true;
default:
break;
}
}
default:
/* We can't scalarize this type of instruction */
return false;
}
}
/**
* 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;
/* Insert an entry and mark it as scalarizable for now. That way
* we don't recurse forever and a cycle in the dependence graph
* won't automatically make us fail to scalarize.
*/
entry = _mesa_hash_table_insert(state->phi_table, phi, (void *)(intptr_t)1);
bool scalarizable = true;
nir_foreach_phi_src(phi, src) {
scalarizable = is_phi_src_scalarizable(src, state);
if (!scalarizable)
break;
}
entry->data = (void *)(intptr_t)scalarizable;
return scalarizable;
}
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;
nir_src_copy(&mov->src[0].src, &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(new_phi, 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);
}
}