[Mesa-dev] [PATCH 151/133] nir/lower_variables: Improve documentation
Jason Ekstrand
jason at jlekstrand.net
Wed Jan 14 12:36:24 PST 2015
On Fri, Jan 9, 2015 at 11:31 AM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> On Tue, Jan 6, 2015 at 7:34 PM, Jason Ekstrand <jason at jlekstrand.net>
> wrote:
> > 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.
> > ---
> > src/glsl/nir/nir_lower_variables.c | 103
> +++++++++++++++++++++++++++----------
> > 1 file changed, 76 insertions(+), 27 deletions(-)
> >
> > diff --git a/src/glsl/nir/nir_lower_variables.c
> b/src/glsl/nir/nir_lower_variables.c
> > index 6c3e6cc..085c6fe 100644
> > --- a/src/glsl/nir/nir_lower_variables.c
> > +++ b/src/glsl/nir/nir_lower_variables.c
> > @@ -53,12 +53,18 @@ 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 to
> > + * deref_node data.
>
> Since this is the first time you mention "fully-qualified direct
> dereferences" in the comments, I'd add a little explanation here e.g.
> "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 don't do anything interesting with variable copy
> > + * instructions other than splitting up wildcards and lowering them
> to
> > + * load/store instructions. We only really care about lowering a
> > + * particular deref to SSA values if it is used in a load or store
> > + * operation. Since wildcards can only occur in load/store
> operations,
>
> Copy operations?
>
> > + * the leaves are precicly the derefs we can actually consider
> lowering
>
> leaves?
>
> > + * to SSA values.
> > */
>
> Here's what I would say:
>
> At the moment, we don't try and lower most variable copy instructions.
> 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.
>
I fixed the minor things and more-or-less copied the above. Anything else
that needs to be done?
>
> > - 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;
> > @@ -184,14 +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;
> >
> > @@ -247,7 +254,7 @@ 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:
> > @@ -256,7 +263,7 @@ 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:
> > @@ -271,8 +278,8 @@ 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,
> > + if (is_direct && add_to_direct_deref_nodes)
> > + _mesa_hash_table_insert(state->direct_deref_nodes,
> > hash_deref(deref), deref, node);
> >
> > return node;
> > @@ -327,7 +334,7 @@ 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,
> > @@ -390,13 +397,18 @@ 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,
> > @@ -948,7 +960,7 @@ 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;
> > @@ -1021,6 +1033,15 @@ 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);
> > @@ -1030,7 +1051,7 @@ 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)
> > @@ -1088,6 +1109,34 @@ 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 derefss we plan to lower to SSA values and
>
> derefs
>
> > + * 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)
> > {
> > @@ -1099,8 +1148,8 @@ nir_lower_variables_impl(nir_function_impl *impl)
> >
> > state.deref_var_nodes = _mesa_hash_table_create(state.dead_ctx,
> >
> _mesa_key_pointer_equal);
> > - state.deref_leaves = _mesa_hash_table_create(state.dead_ctx,
> > - derefs_equal);
> > + state.direct_deref_nodes = _mesa_hash_table_create(state.dead_ctx,
> > + derefs_equal);
> > state.phi_table = _mesa_hash_table_create(state.dead_ctx,
> > _mesa_key_pointer_equal);
> >
> > @@ -1114,17 +1163,17 @@ 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;
> > }
> >
> > --
> > 2.2.0
> >
> > _______________________________________________
> > mesa-dev mailing list
> > mesa-dev at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
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