[Mesa-dev] [PATCH 141/133] nir/lower_variables: Add a bunch of comments and
Jason Ekstrand
jason at jlekstrand.net
Thu Dec 18 13:51:43 PST 2014
This commit seeks to make the lower_variables pass much more clear by
adding a pile of comments and re-arranging a few things. There are no
functional or algorithmic changes.
---
src/glsl/nir/nir_lower_variables.c | 227 +++++++++++++++++++++++++++----------
1 file changed, 170 insertions(+), 57 deletions(-)
diff --git a/src/glsl/nir/nir_lower_variables.c b/src/glsl/nir/nir_lower_variables.c
index 0d940d0..e08df09 100644
--- a/src/glsl/nir/nir_lower_variables.c
+++ b/src/glsl/nir/nir_lower_variables.c
@@ -52,7 +52,12 @@ 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 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.
+ */
struct hash_table *deref_leaves;
/* A hash table mapping phi nodes to deref_state data */
@@ -63,6 +68,9 @@ struct lower_variables_state {
* variable dreferences. When the hash or equality function encounters an
* array, all indirects are treated as equal and are never equal to a
* direct dereference or a wildcard.
+ *
+ * Some of the magic numbers here were taken from _mesa_hash_data and one
+ * was just a big prime I found on the internet.
*/
static uint32_t
hash_deref(const void *void_deref)
@@ -170,6 +178,10 @@ deref_node_create(struct deref_node *parent,
return node;
}
+/* 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.
+ */
static struct deref_node *
get_deref_node(nir_deref_var *deref, bool add_to_leaves,
struct lower_variables_state *state)
@@ -272,56 +284,7 @@ get_deref_node(nir_deref_var *deref, bool add_to_leaves,
return parent;
}
-static void
-register_load_instr(nir_intrinsic_instr *load_instr, bool create_node,
- struct lower_variables_state *state)
-{
- struct deref_node *node = get_deref_node(load_instr->variables[0],
- create_node, state);
- if (node == NULL)
- return;
-
- if (node->loads == NULL)
- node->loads = _mesa_set_create(state->dead_ctx,
- _mesa_key_pointer_equal);
-
- _mesa_set_add(node->loads, _mesa_hash_pointer(load_instr), load_instr);
-}
-
-static void
-register_store_instr(nir_intrinsic_instr *store_instr, bool create_node,
- struct lower_variables_state *state)
-{
- struct deref_node *node = get_deref_node(store_instr->variables[0],
- create_node, state);
- if (node == NULL)
- return;
-
- if (node->stores == NULL)
- node->stores = _mesa_set_create(state->dead_ctx,
- _mesa_key_pointer_equal);
-
- _mesa_set_add(node->stores, _mesa_hash_pointer(store_instr), store_instr);
-}
-
-static void
-register_copy_instr(nir_intrinsic_instr *copy_instr, bool create_node,
- struct lower_variables_state *state)
-{
- for (unsigned idx = 0; idx < 2; idx++) {
- struct deref_node *node = get_deref_node(copy_instr->variables[idx],
- create_node, state);
- if (node == NULL)
- continue;
-
- if (node->copies == NULL)
- node->copies = _mesa_set_create(state->dead_ctx,
- _mesa_key_pointer_equal);
-
- _mesa_set_add(node->copies, _mesa_hash_pointer(copy_instr), copy_instr);
- }
-}
-
+/* \sa foreach_deref_node_match */
static bool
foreach_deref_node_worker(struct deref_node *node, nir_deref *deref,
bool (* cb)(struct deref_node *node,
@@ -360,6 +323,18 @@ foreach_deref_node_worker(struct deref_node *node, nir_deref *deref,
}
}
+/* Walks over every "matching" deref_node and calls the callback. A node
+ * is considered to "match" if either refers to that deref or matches up t
+ * a wildcard. In other words, the following would match a[6].foo[3].bar:
+ *
+ * a[6].foo[3].bar
+ * a[*].foo[3].bar
+ * a[6].foo[*].bar
+ * a[*].foo[*].bar
+ *
+ * The given deref must be a full-length and fully qualified (no wildcards
+ * or indirexcts) deref chain.
+ */
static bool
foreach_deref_node_match(nir_deref_var *deref,
bool (* cb)(struct deref_node *node,
@@ -376,9 +351,7 @@ foreach_deref_node_match(nir_deref_var *deref,
return foreach_deref_node_worker(node, &deref->deref, cb, state);
}
-/* This question can only be asked about leaves. Searching down the tree
- * is much harder than searching up.
- */
+/* \sa deref_may_be_aliased */
static bool
deref_may_be_aliased_node(struct deref_node *node, nir_deref *deref,
struct lower_variables_state *state)
@@ -422,6 +395,15 @@ 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.
+ * 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.
+ */
static bool
deref_may_be_aliased(nir_deref_var *deref,
struct lower_variables_state *state)
@@ -437,8 +419,59 @@ deref_may_be_aliased(nir_deref_var *deref,
return deref_may_be_aliased_node(node, &deref->deref, state);
}
+static void
+register_load_instr(nir_intrinsic_instr *load_instr, bool create_node,
+ struct lower_variables_state *state)
+{
+ struct deref_node *node = get_deref_node(load_instr->variables[0],
+ create_node, state);
+ if (node == NULL)
+ return;
+
+ if (node->loads == NULL)
+ node->loads = _mesa_set_create(state->dead_ctx,
+ _mesa_key_pointer_equal);
+
+ _mesa_set_add(node->loads, _mesa_hash_pointer(load_instr), load_instr);
+}
+
+static void
+register_store_instr(nir_intrinsic_instr *store_instr, bool create_node,
+ struct lower_variables_state *state)
+{
+ struct deref_node *node = get_deref_node(store_instr->variables[0],
+ create_node, state);
+ if (node == NULL)
+ return;
+
+ if (node->stores == NULL)
+ node->stores = _mesa_set_create(state->dead_ctx,
+ _mesa_key_pointer_equal);
+
+ _mesa_set_add(node->stores, _mesa_hash_pointer(store_instr), store_instr);
+}
+
+static void
+register_copy_instr(nir_intrinsic_instr *copy_instr, bool create_node,
+ struct lower_variables_state *state)
+{
+ for (unsigned idx = 0; idx < 2; idx++) {
+ struct deref_node *node = get_deref_node(copy_instr->variables[idx],
+ create_node, state);
+ if (node == NULL)
+ continue;
+
+ if (node->copies == NULL)
+ node->copies = _mesa_set_create(state->dead_ctx,
+ _mesa_key_pointer_equal);
+
+ _mesa_set_add(node->copies, _mesa_hash_pointer(copy_instr), copy_instr);
+ }
+}
+
+/* Registers all variable uses in the given block. */
static bool
-fill_deref_tables_block(nir_block *block, void *void_state)
+register_variable_uses_block(nir_block *block, void *void_state)
{
struct lower_variables_state *state = void_state;
@@ -469,6 +502,11 @@ fill_deref_tables_block(nir_block *block, void *void_state)
return true;
}
+/* Walks down the deref chain and returns the next deref in the chain whose
+ * child is a wildcard. In other words, given the chain a[1].foo[*].bar,
+ * this function will return the deref to foo. Calling it a second time
+ * with the [*].bar, it will return NULL.
+ */
static nir_deref *
deref_next_wildcard_parent(nir_deref *deref)
{
@@ -485,6 +523,8 @@ deref_next_wildcard_parent(nir_deref *deref)
return NULL;
}
+/* Returns the last deref in the chain.
+ */
static nir_deref *
get_deref_tail(nir_deref *deref)
{
@@ -494,25 +534,51 @@ get_deref_tail(nir_deref *deref)
return deref;
}
+/* This function recursively walks the given deref chain and replaces the
+ * given copy instruction with an equivalent sequence load/store
+ * operations.
+ *
+ * @copy_instr The copy instruction to replace; new instructions will be
+ * inserted before this one
+ *
+ * @dest_head The head of the destination variable deref chain
+ *
+ * @src_head The head of the source variable deref chain
+ *
+ * @dest_tail The current tail of the destination variable deref chain;
+ * this is used for recursion and external callers of this
+ * function should call it with tail == head
+ *
+ * @src_tail The current tail of the source variable deref chain;
+ * this is used for recursion and external callers of this
+ * function should call it with tail == head
+ *
+ * @state The current variable lowering state
+ */
static void
emit_copy_load_store(nir_intrinsic_instr *copy_instr,
nir_deref_var *dest_head, nir_deref_var *src_head,
nir_deref *dest_tail, nir_deref *src_tail,
struct lower_variables_state *state)
{
+ /* Find the next pair of wildcards */
nir_deref *src_arr_parent = deref_next_wildcard_parent(src_tail);
nir_deref *dest_arr_parent = deref_next_wildcard_parent(dest_tail);
if (src_arr_parent || dest_arr_parent) {
+ /* Wildcards had better come in matched pairs */
assert(dest_arr_parent && dest_arr_parent);
nir_deref_array *src_arr = nir_deref_as_array(src_arr_parent->child);
nir_deref_array *dest_arr = nir_deref_as_array(dest_arr_parent->child);
unsigned length = type_get_length(src_arr_parent->type);
+ /* The wildcards should represent the same number of elements */
assert(length == type_get_length(dest_arr_parent->type));
assert(length > 0);
+ /* Walk over all of the elements that this wildcard refers to and
+ * call emit_copy_load_store on each one of them */
src_arr->deref_array_type = nir_deref_array_type_direct;
dest_arr->deref_array_type = nir_deref_array_type_direct;
for (unsigned i = 0; i < length; i++) {
@@ -524,7 +590,8 @@ emit_copy_load_store(nir_intrinsic_instr *copy_instr,
src_arr->deref_array_type = nir_deref_array_type_wildcard;
dest_arr->deref_array_type = nir_deref_array_type_wildcard;
} else {
- /* Base case. Actually do the copy */
+ /* In this case, we have no wildcards anymore, so all we have to do
+ * is just emit the load and store operations. */
src_tail = get_deref_tail(src_tail);
dest_tail = get_deref_tail(dest_tail);
@@ -557,6 +624,9 @@ emit_copy_load_store(nir_intrinsic_instr *copy_instr,
}
}
+/* Walks over all of the copy instructions to or from the given deref_node
+ * and lowers them to load/store intrinsics.
+ */
static bool
lower_copies_to_load_store(struct deref_node *node,
struct lower_variables_state *state)
@@ -592,6 +662,10 @@ lower_copies_to_load_store(struct deref_node *node,
return true;
}
+/* Returns a load_const instruction that represents the constant
+ * initializer for the given deref chain. The caller is responsible for
+ * ensuring that there actually is a constant initializer.
+ */
static nir_load_const_instr *
get_const_initializer_load(const nir_deref_var *deref,
struct lower_variables_state *state)
@@ -649,6 +723,15 @@ get_const_initializer_load(const nir_deref_var *deref,
return load;
}
+/** Pushes an SSA def onto the def stack for the given node
+ *
+ * Each node is potentially associated with a stack of SSA definitions.
+ * This stack is used for determining what SSA definition reaches a given
+ * point in the program for variable renaming. The stack is always kept in
+ * dominance-order with at most one SSA def per block. If the SSA
+ * definition on the top of the stack is in the same block as the one being
+ * pushed, the top element is replaced.
+ */
static void
def_stack_push(struct deref_node *node, nir_ssa_def *def,
struct lower_variables_state *state)
@@ -672,6 +755,16 @@ def_stack_push(struct deref_node *node, nir_ssa_def *def,
*(++node->def_stack_tail) = def;
}
+/** Retrieves the SSA definition associated with the given node that
+ * reaches the current point in the program
+ *
+ * If the SSA def on the top of the stack is in the given block or some
+ * other block that dominates the given block, then the top of the stack is
+ * returned. Otherwise, the stack is popped until we get to an SSA
+ * definition that dominates the given block and that is returned. If we
+ * pop the stack all the way to empty, then we return the constant
+ * initializer (if it exists) or an SSA undef.
+ */
static nir_ssa_def *
get_ssa_def_for_block(struct deref_node *node, nir_block *block,
struct lower_variables_state *state)
@@ -700,6 +793,10 @@ get_ssa_def_for_block(struct deref_node *node, nir_block *block,
return &undef->def;
}
+/* Given a block and one of its predecessors, this function fills in the
+ * souces of the phi nodes to take SSA defs from the given predecessor.
+ * This function must be called exactly once per block/predecessor pair.
+ */
static void
add_phi_sources(nir_block *block, nir_block *pred,
struct lower_variables_state *state)
@@ -729,6 +826,16 @@ add_phi_sources(nir_block *block, nir_block *pred,
}
}
+/* Performs variable renaming by doing a DFS of the dominance tree
+ *
+ * This algorithm is very similar to the one outlined in "Efficiently
+ * Computing Static Single Assignment Form and the Control Dependence
+ * Graph" by Cytron et. al. The primary difference is in how the stacks of
+ * SSA definitions are handled. In the Cytron paper, they explicitly pop
+ * the old elements off the stack after visiting the dominance children.
+ * In our algorithm, popping old elements off the stack is implicitly
+ * handled by get_ssa_def_for_block.
+ */
static bool
rename_variables_block(nir_block *block, struct lower_variables_state *state)
{
@@ -865,6 +972,12 @@ rename_variables_block(nir_block *block, struct lower_variables_state *state)
return true;
}
+/* Inserts phi nodes for all variables marked lower_to_ssa
+ *
+ * This is the same algorithm as presented in "Efficiently Computing Static
+ * Single Assignment Form and the Control Dependence Graph" by Cytron et.
+ * al.
+ */
static void
insert_phi_nodes(struct lower_variables_state *state)
{
@@ -953,7 +1066,7 @@ nir_lower_variables_impl(nir_function_impl *impl)
state.phi_table = _mesa_hash_table_create(state.dead_ctx,
_mesa_key_pointer_equal);
- nir_foreach_block(impl, fill_deref_tables_block, &state);
+ nir_foreach_block(impl, register_variable_uses_block, &state);
struct set *outputs = _mesa_set_create(state.dead_ctx,
_mesa_key_pointer_equal);
--
2.2.0
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