[Mesa-dev] [PATCH 20/21] nir/cf: add new control modification API's

[Connor Abbott]

These will help us do a number of things, including:

- Early return elimination.
- Dead control flow elimination.
- Various optimizations, such as replacing:

if (foo) {
    ...
}
if (!foo) {
    ...
}

with:

if (foo) {
    ...
} else {
    ...
}

Signed-off-by: Connor Abbott <connor.w.abbott at intel.com>
---
 src/glsl/nir/nir_control_flow.c | 62 ++++++++++++++++++++++++++++++++++
 src/glsl/nir/nir_control_flow.h | 75 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 137 insertions(+)

diff --git a/src/glsl/nir/nir_control_flow.c b/src/glsl/nir/nir_control_flow.c
index ba39205..7864138 100644
--- a/src/glsl/nir/nir_control_flow.c
+++ b/src/glsl/nir/nir_control_flow.c
@@ -739,3 +739,65 @@ nir_cf_node_remove(nir_cf_node *node)
    cleanup_cf_node(node, impl);
 }
 
+void
+nir_cf_extract(nir_cf_list *extracted, nir_cursor begin, nir_cursor end)
+{
+   nir_block *block_begin, *block_end, *block_before, *block_after;
+
+   /* In the case where begin points to an instruction in some basic block and
+    * end points to the end of the same basic block, we rely on the fact that
+    * splitting on an instruction moves earlier instructions into a new basic
+    * block. If the later instructions were moved instead, then the end cursor
+    * would be pointing to the same place that begin used to point to, which
+    * is obviously not what we want.
+    */
+   split_block_cursor(begin, &block_before, &block_begin);
+   split_block_cursor(end, &block_end, &block_after);
+
+   extracted->impl = nir_cf_node_get_function(&block_begin->cf_node);
+   exec_list_make_empty(&extracted->list);
+
+   nir_cf_node *cf_node = &block_begin->cf_node;
+   nir_cf_node *cf_node_end = &block_end->cf_node;
+   while (true) {
+      nir_cf_node *next = nir_cf_node_next(cf_node);
+
+      exec_node_remove(&cf_node->node);
+      cf_node->parent = NULL;
+      exec_list_push_tail(&extracted->list, &cf_node->node);
+
+      if (cf_node == cf_node_end)
+         break;
+
+      cf_node = next;
+   }
+
+   stitch_blocks(block_before, block_after);
+}
+
+void
+nir_cf_reinsert(nir_cf_list *cf_list, nir_cursor cursor)
+{
+   nir_block *before, *after;
+
+   split_block_cursor(cursor, &before, &after);
+
+   foreach_list_typed_safe(nir_cf_node, node, node, &cf_list->list) {
+      exec_node_remove(&node->node);
+      node->parent = before->cf_node.parent;
+      exec_node_insert_node_before(&after->cf_node.node, &node->node);
+   }
+
+   stitch_blocks(before,
+                 nir_cf_node_as_block(nir_cf_node_next(&before->cf_node)));
+   stitch_blocks(nir_cf_node_as_block(nir_cf_node_prev(&after->cf_node)),
+                 after);
+}
+
+void
+nir_cf_delete(nir_cf_list *cf_list)
+{
+   foreach_list_typed(nir_cf_node, node, node, &cf_list->list) {
+      cleanup_cf_node(node, cf_list->impl);
+   }
+}
diff --git a/src/glsl/nir/nir_control_flow.h b/src/glsl/nir/nir_control_flow.h
index 5a3be26..45aff3e 100644
--- a/src/glsl/nir/nir_control_flow.h
+++ b/src/glsl/nir/nir_control_flow.h
@@ -37,6 +37,12 @@ extern "C" {
  *
  * This file contains various API's that make modifying control flow in NIR,
  * while maintaining the invariants checked by the validator, much easier.
+ * There are two parts to this:
+ *
+ * 1. Inserting control flow (if's and loops) in various places, for creating
+ *    IR either from scratch or as part of some lowering pass.
+ * 2. Taking existing pieces of the IR and either moving them around or
+ *    deleting them.
  */
 
 /* Helper struct for representing a point to extract/insert. Helps reduce the
@@ -164,6 +170,75 @@ nir_cf_node_insert_end(struct exec_list *list, nir_cf_node *node)
 /** removes a control flow node, doing any cleanup necessary */
 void nir_cf_node_remove(nir_cf_node *node);
 
+/** Control flow motion.
+ *
+ * These functions let you take a part of a control flow list (basically
+ * equivalent to a series of statement in GLSL) and "extract" it from the IR,
+ * so that it's a free-floating piece of IR that can be either re-inserted
+ * somewhere else or deleted entirely. A few notes on using it:
+ *
+ * 1. Phi nodes are considered attached to the piece of control flow that
+ *    their sources come from. There are three places where phi nodes can
+ *    occur, which are the three places where a block can have multiple
+ *    predecessors:
+ *
+ *    1) After an if statement, if neither branch ends in a jump.
+ *    2) After a loop, if there are multiple break's.
+ *    3) At the beginning of a loop.
+ *
+ *    For #1, the phi node is considered to be part of the if, and for #2 and
+ *    #3 the phi node is considered to be part of the loop. This allows us to
+ *    keep phi's intact, but it means that phi nodes cannot be separated from
+ *    the control flow they come from. For example, extracting an if without
+ *    extracting all the phi nodes after it is not allowed, and neither is
+ *    extracting only some of the phi nodes at the beginning of a block. It
+ *    also means that extracting from the beginning of a basic block actually
+ *    means extracting from the first non-phi instruction, since there's no
+ *    situation where extracting phi nodes without extracting what comes
+ *    before them makes any sense.
+ *
+ * 2. Phi node sources are guaranteed to remain valid, meaning that they still
+ *    correspond one-to-one with the predecessors of the basic block they're
+ *    part of. In addition, the original sources will be preserved unless they
+ *    correspond to a break or continue that was deleted. However, no attempt
+ *    is made to ensure that SSA form is maintained. In particular, it is
+ *    *not* guaranteed that definitions of SSA values will dominate all their
+ *    uses after all is said and done. Either the caller must ensure that this
+ *    is the case, or it must insert extra phi nodes to restore SSA.
+ *
+ * 3. It is invalid to move a piece of IR with a break/continue outside of the
+ *    loop it references. Doing this will result in invalid
+ *    successors/predecessors and phi node sources.
+ *
+ * 4. It is invalid to move a piece of IR from one function implementation to
+ *    another.
+ *
+ * 5. Extracting a control flow list will leave lots of dangling references to
+ *    and from other pieces of the IR. It also leaves things in a not 100%
+ *    consistent state. This means that some things (e.g. inserting
+ *    instructions) might not work reliably on the extracted control flow. It
+ *    also means that extracting control flow without re-inserting it or
+ *    deleting it is a Bad Thing (tm).
+ */
+
+typedef struct {
+   struct exec_list list;
+   nir_function_impl *impl; /* for cleaning up if the list is deleted */
+} nir_cf_list;
+
+void nir_cf_extract(nir_cf_list *extracted, nir_cursor begin, nir_cursor end);
+
+void nir_cf_reinsert(nir_cf_list *cf_list, nir_cursor cursor);
+
+void nir_cf_delete(nir_cf_list *cf_list);
+
+static inline void
+nir_cf_list_extract(nir_cf_list *extracted, struct exec_list *cf_list)
+{
+   nir_cf_extract(extracted, nir_before_cf_list(cf_list),
+                  nir_after_cf_list(cf_list));
+}
+
 #ifdef __cplusplus
 }
 #endif
-- 
2.4.3