[gcc/devel/loop-unswitch-support-switches] Add top-level comment.

[gcc/devel/loop-unswitch-support-switches] Add top-level comment.

[Martin Liska]

https://gcc.gnu.org/g:149d1e4ec97c60610b592b945bbb6e0f19aa4355

commit 149d1e4ec97c60610b592b945bbb6e0f19aa4355
Author: Martin Liska <mliska@suse.cz>
Date:   Thu Jan 13 17:02:38 2022 +0100

    Add top-level comment.

Diff:
---
 gcc/tree-ssa-loop-unswitch.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/gcc/tree-ssa-loop-unswitch.c b/gcc/tree-ssa-loop-unswitch.c
index 1ed18154339..32c1e38c1fb 100644
--- a/gcc/tree-ssa-loop-unswitch.c
+++ b/gcc/tree-ssa-loop-unswitch.c
@@ -79,6 +79,30 @@ along with GCC; see the file COPYING3.  If not see
    tree-ssa-loop-im.c ensures that all the suitable conditions are in this
    shape.  */
 
+/* Loop unswitching algorithm for innermost loops works in the following steps:
+
+   1) Number of instructions is estimated for each BB that belongs to a loop.
+   2) Unswitching candidates are found for gcond and gswitch statements
+      (note that unswitching precicate for a gswitch actually corresponds
+       to a non-default edge - can contain multiple cases).
+   3) The so called unswitinch predicates are stored in a cache where gimple_uid
+      is an index to the cache.
+   4) We consider one by one the unswitching candidate and calculate BBs that
+      will be reachable in the unswitch version.
+   5) A selected predicate is chosen and we simplify CFG (dead edges) in both
+      versions of the loop.  We utilize both Ranger for condition simplification
+      and also symbol equivalence.  The folded if conditions are replaced with
+      true/false values, while for gswitch we mark the corresponding edge
+      with a pass-defined unreachable flag.
+   6) Every time we unswitch a loop, we save unswitch_predicate to a vector
+      together with information if true or false edge was taken.  Doing that
+      we have a so called PREDICATE_PATH that is utilized for simplification
+      of the cloned loop.
+   7) The process is repeated until we reach a growth threshold or all
+      unswitching opportunities are taken.
+
+*/
+
 /* A tuple that holds GIMPLE condition and value range for an unswitching
    predicate.  */