[Mesa-dev] [PATCH 01/13] nir: Add a couple quick-and-dirty out-of-SSA helpers
Timothy Arceri
timothy.arceri at collabora.com
Thu Dec 22 02:26:04 UTC 2016
From: Jason Ekstrand <jason at jlekstrand.net>
These are designed for use within an optimization pass when SSA becomes
more pain than it's worth. They're very naive and don't generate
anything close to optimal register-based NIR. Also, they may result in
shaders which do not validate because of, for instance, registers in phi
sources. However, the register-based into-SSA pass should be pretty
efficient at cleaning up the mess.
---
src/compiler/nir/nir.h | 3 +
src/compiler/nir/nir_from_ssa.c | 189 +++++++++++++++++++++++++++++++++++++---
2 files changed, 180 insertions(+), 12 deletions(-)
diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
index ba88a24..2c7237b 100644
--- a/src/compiler/nir/nir.h
+++ b/src/compiler/nir/nir.h
@@ -2493,6 +2493,9 @@ bool nir_repair_ssa(nir_shader *shader);
*/
void nir_convert_from_ssa(nir_shader *shader, bool phi_webs_only);
+bool nir_lower_phis_to_regs_block(nir_block *block);
+bool nir_lower_ssa_defs_to_regs_block(nir_block *block);
+
bool nir_opt_algebraic(nir_shader *shader);
bool nir_opt_algebraic_late(nir_shader *shader);
bool nir_opt_constant_folding(nir_shader *shader);
diff --git a/src/compiler/nir/nir_from_ssa.c b/src/compiler/nir/nir_from_ssa.c
index 15e89a3..603d1a5 100644
--- a/src/compiler/nir/nir_from_ssa.c
+++ b/src/compiler/nir/nir_from_ssa.c
@@ -447,6 +447,19 @@ aggressive_coalesce_block(nir_block *block, struct from_ssa_state *state)
return true;
}
+static nir_register *
+create_reg_for_ssa_def(nir_ssa_def *def, nir_function_impl *impl)
+{
+ nir_register *reg = nir_local_reg_create(impl);
+
+ reg->name = def->name;
+ reg->num_components = def->num_components;
+ reg->bit_size = def->bit_size;
+ reg->num_array_elems = 0;
+
+ return reg;
+}
+
static bool
rewrite_ssa_def(nir_ssa_def *def, void *void_state)
{
@@ -463,13 +476,8 @@ rewrite_ssa_def(nir_ssa_def *def, void *void_state)
* the things in the merge set should be the same so it doesn't
* matter which node's definition we use.
*/
- if (node->set->reg == NULL) {
- node->set->reg = nir_local_reg_create(state->impl);
- node->set->reg->name = def->name;
- node->set->reg->num_components = def->num_components;
- node->set->reg->bit_size = def->bit_size;
- node->set->reg->num_array_elems = 0;
- }
+ if (node->set->reg == NULL)
+ node->set->reg = create_reg_for_ssa_def(def, state->impl);
reg = node->set->reg;
} else {
@@ -482,11 +490,7 @@ rewrite_ssa_def(nir_ssa_def *def, void *void_state)
if (def->parent_instr->type == nir_instr_type_load_const)
return true;
- reg = nir_local_reg_create(state->impl);
- reg->name = def->name;
- reg->num_components = def->num_components;
- reg->bit_size = def->bit_size;
- reg->num_array_elems = 0;
+ reg = create_reg_for_ssa_def(def, state->impl);
}
nir_ssa_def_rewrite_uses(def, nir_src_for_reg(reg));
@@ -810,3 +814,164 @@ nir_convert_from_ssa(nir_shader *shader, bool phi_webs_only)
nir_convert_from_ssa_impl(function->impl, phi_webs_only);
}
}
+
+
+static void
+place_phi_read(nir_shader *shader, nir_register *reg,
+ nir_ssa_def *def, nir_block *block)
+{
+ if (block != def->parent_instr->block) {
+ /* Try to go up the single-successor tree */
+ bool all_single_successors = true;
+ struct set_entry *entry;
+ set_foreach(block->predecessors, entry) {
+ nir_block *pred = (nir_block *)entry->key;
+ if (pred->successors[0] && pred->successors[1]) {
+ all_single_successors = false;
+ break;
+ }
+ }
+
+ if (all_single_successors) {
+ /* All predecessors of this block have exactly one successor and it
+ * is this block so they must eventually lead here without
+ * intersecting each other. Place the reads in the predecessors
+ * instead of this block.
+ */
+ set_foreach(block->predecessors, entry)
+ place_phi_read(shader, reg, def, (nir_block *)entry->key);
+ return;
+ }
+ }
+
+ nir_alu_instr *mov = nir_alu_instr_create(shader, nir_op_imov);
+ mov->src[0].src = nir_src_for_ssa(def);
+ mov->dest.dest = nir_dest_for_reg(reg);
+ mov->dest.write_mask = (1 << reg->num_components) - 1;
+ nir_instr_insert(nir_after_block_before_jump(block), &mov->instr);
+}
+
+/** Lower all of the phi nodes in a block to imov's to and from a register
+ *
+ * This provides a very quick-and-dirty out-of-SSA pass that you can run on a
+ * single block to convert all of it's phis to a register and some imov's.
+ * The code that is generated, while not optimal for actual codegen in a
+ * back-end, is easy to generate, correct, and will turn into the same set of
+ * phis after you call regs_to_ssa and do some copy propagation.
+ *
+ * The one intelligent thing this pass does is that it places the moves from
+ * the phi sources as high up the predecessor tree as possible instead of in
+ * the exact predecessor. This means that, in particular, it will crawl into
+ * the deepest nesting of any if-ladders. In order to ensure that doing so is
+ * safe, it stops as soon as one of the predecessors has multiple successors.
+ */
+bool
+nir_lower_phis_to_regs_block(nir_block *block)
+{
+ nir_function_impl *impl = nir_cf_node_get_function(&block->cf_node);
+ nir_shader *shader = impl->function->shader;
+
+ bool progress = false;
+ nir_foreach_instr_safe(instr, block) {
+ if (instr->type != nir_instr_type_phi)
+ break;
+
+ nir_phi_instr *phi = nir_instr_as_phi(instr);
+ assert(phi->dest.is_ssa);
+
+ nir_register *reg = create_reg_for_ssa_def(&phi->dest.ssa, impl);
+
+ nir_alu_instr *mov = nir_alu_instr_create(shader, nir_op_imov);
+ mov->src[0].src = nir_src_for_reg(reg);
+ mov->dest.write_mask = (1 << phi->dest.ssa.num_components) - 1;
+ nir_ssa_dest_init(&mov->instr, &mov->dest.dest,
+ phi->dest.ssa.num_components, phi->dest.ssa.bit_size,
+ phi->dest.ssa.name);
+ nir_instr_insert(nir_after_instr(&phi->instr), &mov->instr);
+
+ nir_ssa_def_rewrite_uses(&phi->dest.ssa,
+ nir_src_for_ssa(&mov->dest.dest.ssa));
+
+ nir_foreach_phi_src(src, phi) {
+ assert(src->src.is_ssa);
+ place_phi_read(shader, reg, src->src.ssa, src->pred);
+ }
+
+ nir_instr_remove(&phi->instr);
+
+ progress = true;
+ }
+
+ return progress;
+}
+
+struct ssa_def_to_reg_state {
+ nir_function_impl *impl;
+ bool progress;
+};
+
+static bool
+dest_replace_ssa_with_reg(nir_dest *dest, void *void_state)
+{
+ struct ssa_def_to_reg_state *state = void_state;
+
+ if (!dest->is_ssa)
+ return true;
+
+ nir_register *reg = create_reg_for_ssa_def(&dest->ssa, state->impl);
+
+ nir_ssa_def_rewrite_uses(&dest->ssa, nir_src_for_reg(reg));
+
+ nir_instr *instr = dest->ssa.parent_instr;
+ *dest = nir_dest_for_reg(reg);
+ dest->reg.parent_instr = instr;
+ list_addtail(&dest->reg.def_link, ®->defs);
+
+ state->progress = true;
+
+ return true;
+}
+
+/** Lower all of the SSA defs in a block to registers
+ *
+ * This performs the very simple operation of blindly replacing all of the SSA
+ * defs in the given block with registers. If not used carefully, this may
+ * result in phi nodes with register sources which is technically invalid.
+ * Fortunately, the register-based into-SSA pass handles them anyway.
+ */
+bool
+nir_lower_ssa_defs_to_regs_block(nir_block *block)
+{
+ nir_function_impl *impl = nir_cf_node_get_function(&block->cf_node);
+ nir_shader *shader = impl->function->shader;
+
+ struct ssa_def_to_reg_state state = {
+ .impl = impl,
+ .progress = false,
+ };
+
+ nir_foreach_instr(instr, block) {
+ if (instr->type == nir_instr_type_ssa_undef) {
+ /* Undefs are just a read of something never written. */
+ nir_ssa_undef_instr *undef = nir_instr_as_ssa_undef(instr);
+ nir_register *reg = create_reg_for_ssa_def(&undef->def, state.impl);
+ nir_ssa_def_rewrite_uses(&undef->def, nir_src_for_reg(reg));
+ } else if (instr->type == nir_instr_type_load_const) {
+ /* Constant loads are SSA-only, we need to insert a move */
+ nir_load_const_instr *load = nir_instr_as_load_const(instr);
+ nir_register *reg = create_reg_for_ssa_def(&load->def, state.impl);
+ nir_ssa_def_rewrite_uses(&load->def, nir_src_for_reg(reg));
+
+ nir_alu_instr *mov = nir_alu_instr_create(shader, nir_op_imov);
+ mov->src[0].src = nir_src_for_ssa(&load->def);
+ mov->dest.dest = nir_dest_for_reg(reg);
+ mov->dest.write_mask = (1 << reg->num_components) - 1;
+ nir_instr_insert(nir_after_instr(&load->instr), &mov->instr);
+ } else {
+ nir_foreach_dest(instr, dest_replace_ssa_with_reg, &state);
+ }
+ nir_foreach_dest(instr, dest_replace_ssa_with_reg, &state);
+ }
+
+ return state.progress;
+}
--
2.9.3
More information about the mesa-dev
mailing list