Commit e2763339 authored by Jason Ekstrand's avatar Jason Ekstrand
Browse files

nir/lower_variables: Add a bunch of comments and re-arrange a few things



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.
Reviewed-by: default avatarConnor Abbott <cwabbott0@gmail.com>
parent 40ca129e
......@@ -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)
......@@ -268,56 +280,7 @@ 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,
......@@ -356,6 +319,18 @@ 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,
......@@ -372,9 +347,7 @@ 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)
......@@ -418,6 +391,15 @@ 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)
......@@ -433,8 +415,59 @@ 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;
......@@ -465,6 +498,11 @@ 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)
{
......@@ -481,6 +519,8 @@ 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)
{
......@@ -490,25 +530,51 @@ 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++) {
......@@ -520,7 +586,8 @@ 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);
......@@ -553,6 +620,9 @@ 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)
......@@ -588,6 +658,10 @@ 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)
......@@ -645,6 +719,15 @@ 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)
......@@ -668,6 +751,16 @@ 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)
......@@ -696,6 +789,10 @@ 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)
......@@ -724,6 +821,16 @@ 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)
{
......@@ -859,6 +966,12 @@ 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)
{
......@@ -948,7 +1061,7 @@ nir_lower_variables_impl(nir_function_impl *impl)
_mesa_hash_pointer,
_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);
......
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