nir/cf: add new control modification API's

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 <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

1 files changed, 75 insertions, 0 deletions

diff --git a/src/glsl/nir/nir_control_flow.h b/src/glsl/nir/nir_control_flow.h
index 5a3be260f63..45aff3e61be 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