[CanonicalizeOSSALifetime] Separated steps.

Rather than having finding the boundary be a single combined step,
separate finding the original boundary from extending that boundary.
This enables inserting an optional step between those steps, namely to
extend unconsumed liveness to its original extent at Onone.

Author: nate-chandler

include/swift/SILOptimizer/Utils/CanonicalizeOSSALifetime.h

@@ -347,7 +347,10 @@ class CanonicalizeOSSALifetime final {
 
   void extendLivenessThroughOverlappingAccess();
 
-  void findExtendedBoundary(PrunedLivenessBoundary &boundary);
+  void findOriginalBoundary(PrunedLivenessBoundary &boundary);
+
+  void findExtendedBoundary(PrunedLivenessBoundary const &originalBoundary,
+                            PrunedLivenessBoundary &boundary);
 
   void insertDestroysOnBoundary(PrunedLivenessBoundary const &boundary);
 

lib/SILOptimizer/Utils/CanonicalizeOSSALifetime.cpp

@@ -20,13 +20,16 @@
 /// 1. Compute "pruned" liveness of def and its copies, ignoring original
 ///    destroys. Initializes `liveness`.
 ///
-/// 2. Find the "extended" boundary of liveness by walking out from the boundary
+/// 2. Find the "original" boundary of liveness using
+///    PrunedLiveness::computeBoundary.
+///
+/// 3. Find the "extended" boundary of liveness by walking out from the boundary
 ///    computed by PrunedLiveness out to destroys which aren't separated from
 ///    the original destory by "interesting" instructions.
 ///
-/// 3. Initializes `consumes` and inserts new destroy_value instructions.
+/// 4. Initializes `consumes` and inserts new destroy_value instructions.
 ///
-/// 4. Rewrite `def`s original copies and destroys, inserting new copies where
+/// 5. Rewrite `def`s original copies and destroys, inserting new copies where
 ///    needed. Deletes original copies and destroys and inserts new copies.
 ///
 /// See CanonicalizeOSSALifetime.h for examples.
@@ -410,8 +413,20 @@ void CanonicalizeOSSALifetime::extendLivenessThroughOverlappingAccess() {
 }
 
 //===----------------------------------------------------------------------===//
-// MARK: Step 2. Find the destroy points of the current def based on the pruned
-// liveness computed in Step 1.
+// MARK: Step 2. Find the "original" (unextended) boundary determined by the
+//               liveness built up in step 1.
+//===----------------------------------------------------------------------===//
+
+void CanonicalizeOSSALifetime::findOriginalBoundary(
+    PrunedLivenessBoundary &boundary) {
+  assert(boundary.lastUsers.size() == 0 && boundary.boundaryEdges.size() == 0 &&
+         boundary.deadDefs.size() == 0);
+  liveness.computeBoundary(boundary, consumingBlocks.getArrayRef());
+}
+
+//===----------------------------------------------------------------------===//
+// MARK: Step 3. Extend the "original" boundary from step 2 up to destroys that
+//               aren't separated from it by "interesting" instructions.
 //===----------------------------------------------------------------------===//
 
 namespace {
@@ -614,17 +629,17 @@ class ExtendBoundaryToDestroys final {
 } // anonymous namespace
 
 void CanonicalizeOSSALifetime::findExtendedBoundary(
+    PrunedLivenessBoundary const &originalBoundary,
     PrunedLivenessBoundary &boundary) {
-  PrunedLivenessBoundary originalBoundary;
-  liveness.computeBoundary(originalBoundary, consumingBlocks.getArrayRef());
-
+  assert(boundary.lastUsers.size() == 0 && boundary.boundaryEdges.size() == 0 &&
+         boundary.deadDefs.size() == 0);
   ExtendBoundaryToDestroys extender(liveness, originalBoundary,
                                     getCurrentDef());
   extender.extend(boundary);
 }
 
 //===----------------------------------------------------------------------===//
-// MARK: Step 3. Insert destroys onto the boundary found in step 2 where needed.
+// MARK: Step 4. Insert destroys onto the boundary found in step 3 where needed.
 //===----------------------------------------------------------------------===//
 
 /// Create a new destroy_value instruction before the specified instruction and
@@ -714,7 +729,7 @@ void CanonicalizeOSSALifetime::insertDestroysOnBoundary(
 }
 
 //===----------------------------------------------------------------------===//
-// MARK: Step 4. Rewrite copies and destroys
+// MARK: Step 5. Rewrite copies and destroys
 //===----------------------------------------------------------------------===//
 
 /// The lifetime extends beyond given consuming use. Copy the value.
@@ -880,12 +895,15 @@ bool CanonicalizeOSSALifetime::canonicalizeValueLifetime(SILValue def) {
     return false;
   }
   extendLivenessThroughOverlappingAccess();
-  // Step 2: compute boundary
+  // Step 2: compute original boundary
+  PrunedLivenessBoundary originalBoundary;
+  findOriginalBoundary(originalBoundary);
+  // Step 3: extend boundary to destroys
   PrunedLivenessBoundary boundary;
-  findExtendedBoundary(boundary);
-  // Step 3: insert destroys and record consumes
+  findExtendedBoundary(originalBoundary, boundary);
+  // Step 4: insert destroys and record consumes
   insertDestroysOnBoundary(boundary);
-  // Step 4: rewrite copies and delete extra destroys
+  // Step 5: rewrite copies and delete extra destroys
   rewriteCopies();
 
   clearLiveness();