Notes - Some newer optimisation ideas/notes

1 files changed, 29 insertions, 7 deletions

diff --git a/Notes/MIR-Optimisations.md b/Notes/MIR-Optimisations.md
index d20828ed..baa737eb 100644
--- a/Notes/MIR-Optimisations.md
+++ b/Notes/MIR-Optimisations.md
@@ -206,11 +206,33 @@ Purpose: Remove borrows generated by method calls that have been inlined
 
 Overview
 --------
-- Detect potential replacements:
-  - Find borrows of locals
-  - If destination of borrow is used directly (not via a deref), skip
-  - If the destination is re-assigned, skip
-    - Note, this should be able to tell if the re-assignment would be able to
-      invalidate the previous statements
-- Replace all dereference usages of the destination with the original.
+- Find locals that are only assigned once, and only ever used via a deref
+  operation.
+  - Allow drops to use the destination by value
+- If the assignment was a Borrow RValue, continue
+- Iterate forward, replacing usage of the value with the original source while:
+  - The source isn't invalidated
+  - The number of replacements is less than the number of known usage sites
+- If the original borrow itself contained a deref of a local, update that
+  local's usage count.
+- If all usage sites were updated, erase the original borrow
+
+
+Borrow Elimination 2
+====================
+Purpose: Remove borrows of borrows that are never used again
 
+Overview
+--------
+- Find assign-once/use-once locals (ignoring drop)
+- If the source is a borrow of a deref of a local/argument, continue
+- If the source inner value is of the same type as the local, continue
+  - I.e. we're not re-borrowing a `&mut` as `&`
+- If the variable's type is not `&` (i.e. it's &mut or &uniq)
+  - Check the usage count for the source value, and only continue is this is
+    the only usage site of the source
+  - Ideally: This would only check forwards, but that's hard
+- If the variable's type is `&`, check the path between assignment and usage
+  for invalidation.
+- If no invalidation found, replace usage with inner of assignment and erase
+  assignment.