diff options
Diffstat (limited to 'src/compiler/nir/nir_phi_builder.c')
| -rw-r--r-- | src/compiler/nir/nir_phi_builder.c | 65 |
1 files changed, 53 insertions, 12 deletions
diff --git a/src/compiler/nir/nir_phi_builder.c b/src/compiler/nir/nir_phi_builder.c index 5429083e5c8..a39e3606fd5 100644 --- a/src/compiler/nir/nir_phi_builder.c +++ b/src/compiler/nir/nir_phi_builder.c @@ -52,6 +52,7 @@ struct nir_phi_builder_value { /* Needed so we can create phis and undefs */ unsigned num_components; + unsigned bit_size; /* The list of phi nodes associated with this value. Phi nodes are not * added directly. Instead, they are created, the instr->block pointer @@ -61,8 +62,18 @@ struct nir_phi_builder_value { */ struct exec_list phis; - /* Array of SSA defs, indexed by block. If a phi needs to be inserted - * in a given block, it will have the magic value NEEDS_PHI. + /* Array of SSA defs, indexed by block. For each block, this array has has + * one of three types of values: + * + * - NULL. Indicates that there is no known definition in this block. If + * you need to find one, look at the block's immediate dominator. + * + * - NEEDS_PHI. Indicates that the block may need a phi node but none has + * been created yet. If a def is requested for a block, a phi will need + * to be created. + * + * - A regular SSA def. This will be either the result of a phi node or + * one of the defs provided by nir_phi_builder_value_set_blocK_def(). */ nir_ssa_def *defs[0]; }; @@ -101,7 +112,7 @@ nir_phi_builder_create(nir_function_impl *impl) struct nir_phi_builder_value * nir_phi_builder_add_value(struct nir_phi_builder *pb, unsigned num_components, - const BITSET_WORD *defs) + unsigned bit_size, const BITSET_WORD *defs) { struct nir_phi_builder_value *val; unsigned i, w_start = 0, w_end = 0; @@ -109,6 +120,7 @@ nir_phi_builder_add_value(struct nir_phi_builder *pb, unsigned num_components, val = rzalloc_size(pb, sizeof(*val) + sizeof(val->defs[0]) * pb->num_blocks); val->builder = pb; val->num_components = num_components; + val->bit_size = bit_size; exec_list_make_empty(&val->phis); exec_list_push_tail(&pb->values, &val->node); @@ -127,8 +139,7 @@ nir_phi_builder_add_value(struct nir_phi_builder *pb, unsigned num_components, set_foreach(cur->dom_frontier, dom_entry) { nir_block *next = (nir_block *) dom_entry->key; - /* - * If there's more than one return statement, then the end block + /* If there's more than one return statement, then the end block * can be a join point for some definitions. However, there are * no instructions in the end block, so nothing would use those * phi nodes. Of course, we couldn't place those phi nodes @@ -139,6 +150,10 @@ nir_phi_builder_add_value(struct nir_phi_builder *pb, unsigned num_components, continue; if (val->defs[next->index] == NULL) { + /* Instead of creating a phi node immediately, we simply set the + * value to the magic value NEEDS_PHI. Later, we create phi nodes + * on demand in nir_phi_builder_value_get_block_def(). + */ val->defs[next->index] = NEEDS_PHI; if (pb->work[next->index] < pb->iter_count) { @@ -163,7 +178,9 @@ nir_ssa_def * nir_phi_builder_value_get_block_def(struct nir_phi_builder_value *val, nir_block *block) { + /* For each block, we have one of three types of values */ if (val->defs[block->index] == NULL) { + /* NULL indicates that we have no SSA def for this block. */ if (block->imm_dom) { /* Grab it from our immediate dominator. We'll stash it here for * easy access later. @@ -185,17 +202,36 @@ nir_phi_builder_value_get_block_def(struct nir_phi_builder_value *val, return &undef->def; } } else if (val->defs[block->index] == NEEDS_PHI) { - /* If we need a phi instruction, go ahead and create one but don't - * add it to the program yet. Later, we'll go through and set up phi - * sources and add the instructions will be added at that time. + /* The magic value NEEDS_PHI indicates that the block needs a phi node + * but none has been created. We need to create one now so we can + * return it to the caller. + * + * Because a phi node may use SSA defs that it does not dominate (this + * happens in loops), we do not yet have enough information to fully + * fill out the phi node. Instead, the phi nodes we create here will be + * empty (have no sources) and won't actually be placed in the block's + * instruction list yet. Later, in nir_phi_builder_finish(), we walk + * over all of the phi instructions, fill out the sources lists, and + * place them at the top of their respective block's instruction list. + * + * Creating phi nodes on-demand allows us to avoid creating dead phi + * nodes that will just get deleted later. While this probably isn't a + * big win for a full into-SSA pass, other users may use the phi builder + * to make small SSA form repairs where most of the phi nodes will never + * be used. */ nir_phi_instr *phi = nir_phi_instr_create(val->builder->shader); - nir_ssa_dest_init(&phi->instr, &phi->dest, val->num_components, NULL); + nir_ssa_dest_init(&phi->instr, &phi->dest, val->num_components, + val->bit_size, NULL); phi->instr.block = block; exec_list_push_tail(&val->phis, &phi->instr.node); val->defs[block->index] = &phi->dest.ssa; return &phi->dest.ssa; } else { + /* In this case, we have an actual SSA def. It's either the result of a + * phi node created by the case above or one passed to us through + * nir_phi_builder_value_set_block_def(). + */ return val->defs[block->index]; } } @@ -216,9 +252,14 @@ nir_phi_builder_finish(struct nir_phi_builder *pb) NIR_VLA(nir_block *, preds, num_blocks); foreach_list_typed(struct nir_phi_builder_value, val, node, &pb->values) { - /* We can't iterate over the list of phis normally because we are - * removing them as we go and, in some cases, adding new phis as we - * build the source lists of others. + /* We treat the linked list of phi nodes like a worklist. The list is + * pre-populated by calls to nir_phi_builder_value_get_block_def() that + * create phi nodes. As we fill in the sources of phi nodes, more may + * be created and are added to the end of the list. + * + * Because we are adding and removing phi nodes from the list as we go, + * we can't iterate over it normally. Instead, we just iterate until + * the list is empty. */ while (!exec_list_is_empty(&val->phis)) { struct exec_node *head = exec_list_get_head(&val->phis); |