Commit 347ab2bf authored by Jason Ekstrand's avatar Jason Ekstrand
Browse files

nir/lower_variables: Improve documentation



Additional description was added to a variety of places.  Also, we no
longer use the term "leaf" to describe fully-qualified direct derefs.
Instead, we simply use the term "direct" or spell it out completely.
Reviewed-by: default avatarConnor Abbott <cwabbott0@gmail.com>
parent 8016fa39
......@@ -53,12 +53,21 @@ struct lower_variables_state {
/* A hash table mapping variables to deref_node data */
struct hash_table *deref_var_nodes;
/* A hash table mapping dereference leaves to deref_node data. A deref
* is considered a leaf if it is fully-qualified (no wildcards) and
* direct. In short, these are the derefs we can actually consider
* lowering to SSA values.
/* A hash table mapping fully-qualified direct dereferences, i.e.
* dereferences with no indirect or wildcard array dereferences, to
* deref_node data.
*
* At the moment, we only lower loads, stores, and copies that can be
* trivially lowered to loads and stores, i.e. copies with no indirects
* and no wildcards. If a part of a variable that is being loaded from
* and/or stored into is also involved in a copy operation with
* wildcards, then we lower that copy operation to loads and stores, but
* otherwise we leave copies with wildcards alone. Since the only derefs
* used in these loads, stores, and trivial copies are ones with no
* wildcards and no indirects, these are precisely the derefs that we
* can actually consider lowering.
*/
struct hash_table *deref_leaves;
struct hash_table *direct_deref_nodes;
/* A hash table mapping phi nodes to deref_state data */
struct hash_table *phi_table;
......@@ -181,14 +190,15 @@ deref_node_create(struct deref_node *parent,
}
/* Gets the deref_node for the given deref chain and creates it if it
* doesn't yet exist. If the deref is a leaf (fully-qualified and direct)
* and add_to_leaves is true, it will be added to the hash table of leaves.
* doesn't yet exist. If the deref is fully-qualified and direct and
* add_to_direct_deref_nodes is true, it will be added to the hash table of
* of fully-qualified direct derefs.
*/
static struct deref_node *
get_deref_node(nir_deref_var *deref, bool add_to_leaves,
get_deref_node(nir_deref_var *deref, bool add_to_direct_deref_nodes,
struct lower_variables_state *state)
{
bool is_leaf = true;
bool is_direct = true;
struct deref_node *node;
......@@ -242,7 +252,7 @@ get_deref_node(nir_deref_var *deref, bool add_to_leaves,
state->dead_ctx);
node = node->indirect;
is_leaf = false;
is_direct = false;
break;
case nir_deref_array_type_wildcard:
......@@ -251,7 +261,7 @@ get_deref_node(nir_deref_var *deref, bool add_to_leaves,
state->dead_ctx);
node = node->wildcard;
is_leaf = false;
is_direct = false;
break;
default:
......@@ -266,8 +276,8 @@ get_deref_node(nir_deref_var *deref, bool add_to_leaves,
assert(node);
if (is_leaf && add_to_leaves)
_mesa_hash_table_insert(state->deref_leaves, deref, node);
if (is_direct && add_to_direct_deref_nodes)
_mesa_hash_table_insert(state->direct_deref_nodes, deref, node);
return node;
}
......@@ -321,7 +331,7 @@ foreach_deref_node_worker(struct deref_node *node, nir_deref *deref,
* a[*].foo[*].bar
*
* The given deref must be a full-length and fully qualified (no wildcards
* or indirexcts) deref chain.
* or indirects) deref chain.
*/
static bool
foreach_deref_node_match(nir_deref_var *deref,
......@@ -384,13 +394,18 @@ deref_may_be_aliased_node(struct deref_node *node, nir_deref *deref,
}
/* Returns true if there are no indirects that can ever touch this deref.
* This question can only be asked about fully-qualified derefs.
*
* For example, if the given deref is a[6].foo, then any uses of a[i].foo
* would cause this to return false, but a[i].bar would not affect it
* because it's a different structure member. A var_copy involving of
* a[*].bar also doesn't affect it because that can be lowered to entirely
* direct load/stores.
*
* We only support asking this question about fully-qualified derefs.
* Obviously, it's pointless to ask this about indirects, but we also
* rule-out wildcards. For example, if the given deref is a[6].foo, then
* any uses of a[i].foo would case this to return false, but a[i].bar would
* not affect it because it's a different structure member. A var_copy
* involving of a[*].bar also doesn't affect it because that can be lowered
* to entirely direct load/stores.
* rule-out wildcards. Handling Wildcard dereferences would involve
* checking each array index to make sure that there aren't any indirect
* references.
*/
static bool
deref_may_be_aliased(nir_deref_var *deref,
......@@ -940,7 +955,7 @@ rename_variables_block(nir_block *block, struct lower_variables_state *state)
def_stack_push(node, &mov->dest.dest.ssa, state);
/* We'll wait to remove the unstruction until the next pass
/* We'll wait to remove the instruction until the next pass
* where we pop the node we just pushed back off the stack.
*/
break;
......@@ -1012,6 +1027,15 @@ insert_phi_nodes(struct lower_variables_state *state)
{
unsigned work[state->impl->num_blocks];
unsigned has_already[state->impl->num_blocks];
/*
* Since the work flags already prevent us from inserting a node that has
* ever been inserted into W, we don't need to use a set to represent W.
* Also, since no block can ever be inserted into W more than once, we know
* that the maximum size of W is the number of basic blocks in the
* function. So all we need to handle W is an array and a pointer to the
* next element to be inserted and the next element to be removed.
*/
nir_block *W[state->impl->num_blocks];
memset(work, 0, sizeof work);
......@@ -1021,7 +1045,7 @@ insert_phi_nodes(struct lower_variables_state *state)
unsigned iter_count = 0;
struct hash_entry *deref_entry;
hash_table_foreach(state->deref_leaves, deref_entry) {
hash_table_foreach(state->direct_deref_nodes, deref_entry) {
struct deref_node *node = deref_entry->data;
if (node->stores == NULL)
......@@ -1078,6 +1102,34 @@ insert_phi_nodes(struct lower_variables_state *state)
}
}
/** Implements a pass to lower variable uses to SSA values
*
* This path walks the list of instructions and tries to lower as many
* local variable load/store operations to SSA defs and uses as it can.
* The process involves four passes:
*
* 1) Iterate over all of the instructions and mark where each local
* variable deref is used in a load, store, or copy. While we're at
* it, we keep track of all of the fully-qualified (no wildcards) and
* fully-direct references we see and store them in the
* direct_deref_nodes hash table.
*
* 2) Walk over the the list of fully-qualified direct derefs generated in
* the previous pass. For each deref, we determine if it can ever be
* aliased, i.e. if there is an indirect reference anywhere that may
* refer to it. If it cannot be aliased, we mark it for lowering to an
* SSA value. At this point, we lower any var_copy instructions that
* use the given deref to load/store operations and, if the deref has a
* constant initializer, we go ahead and add a load_const value at the
* beginning of the function with the initialized value.
*
* 3) Walk over the list of derefs we plan to lower to SSA values and
* insert phi nodes as needed.
*
* 4) Perform "variable renaming" by replacing the load/store instructions
* with SSA definitions and SSA uses.
*/
static bool
nir_lower_variables_impl(nir_function_impl *impl)
{
......@@ -1090,8 +1142,8 @@ nir_lower_variables_impl(nir_function_impl *impl)
state.deref_var_nodes = _mesa_hash_table_create(state.dead_ctx,
_mesa_hash_pointer,
_mesa_key_pointer_equal);
state.deref_leaves = _mesa_hash_table_create(state.dead_ctx,
hash_deref, derefs_equal);
state.direct_deref_nodes = _mesa_hash_table_create(state.dead_ctx,
hash_deref, derefs_equal);
state.phi_table = _mesa_hash_table_create(state.dead_ctx,
_mesa_hash_pointer,
_mesa_key_pointer_equal);
......@@ -1106,17 +1158,17 @@ nir_lower_variables_impl(nir_function_impl *impl)
nir_metadata_require(impl, nir_metadata_block_index);
struct hash_entry *entry;
hash_table_foreach(state.deref_leaves, entry) {
hash_table_foreach(state.direct_deref_nodes, entry) {
nir_deref_var *deref = (void *)entry->key;
struct deref_node *node = entry->data;
if (deref->var->data.mode != nir_var_local) {
_mesa_hash_table_remove(state.deref_leaves, entry);
_mesa_hash_table_remove(state.direct_deref_nodes, entry);
continue;
}
if (deref_may_be_aliased(deref, &state)) {
_mesa_hash_table_remove(state.deref_leaves, entry);
_mesa_hash_table_remove(state.direct_deref_nodes, entry);
continue;
}
......
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