CopyPropagation: Avoid regenerating destroys.

CanonicalizeOSSA is now used iteratively in SILCombine. To avoid
endless worklist iteration based on whether InstructionDeleter's
callbacks fired, ensure that destroys are only deleted and recreated
when necessary.

Author: atrick

include/swift/SILOptimizer/Utils/CanonicalOSSALifetime.h

@@ -357,8 +357,8 @@ class CanonicalizeOSSALifetime {
 
   void findOrInsertDestroys();
 
-  void insertDestroyOnCFGEdge(SILBasicBlock *predBB, SILBasicBlock *succBB,
-                              bool needsPoison);
+  void findOrInsertDestroyOnCFGEdge(SILBasicBlock *predBB,
+                                    SILBasicBlock *succBB, bool needsPoison);
 
   void rewriteCopies();
 

lib/SILOptimizer/Utils/CanonicalOSSALifetime.cpp

@@ -343,25 +343,48 @@ void CanonicalizeOSSALifetime::extendLivenessThroughOverlappingAccess() {
 // liveness computed in Step 1.
 //===----------------------------------------------------------------------===//
 
+// Look past destroys and incidental uses to find a destroy on \p edgeBB that
+// destroys \p def.
+static DestroyValueInst *findDestroyOnCFGEdge(SILBasicBlock *edgeBB,
+                                              SILValue def) {
+  for (auto &inst : *edgeBB) {
+    if (isIncidentalUse(&inst))
+      continue;
+    if (auto *destroy = dyn_cast<DestroyValueInst>(&inst)) {
+      if (destroy->getOperand() == def)
+        return destroy;
+      continue;
+    }
+    break;
+  }
+  return nullptr;
+}
+
 /// The liveness boundary is at a CFG edge `predBB` -> `succBB`, meaning that
 /// `currentDef` is live out of at least one other `predBB` successor.
 ///
 /// Create and record a final destroy_value at the beginning of `succBB`
 /// (assuming no critical edges).
-void CanonicalizeOSSALifetime::insertDestroyOnCFGEdge(
-  SILBasicBlock *predBB, SILBasicBlock *succBB, bool needsPoison) {
+///
+/// Avoid deleting and recreating a destroy that was already placed on this
+/// edge. Ignore any intervening destroys that may have been placed while
+/// canonicalizing other values.  This is especially important when
+/// canonicalization is called within an iterative worklist such as SILCombine.
+void CanonicalizeOSSALifetime::findOrInsertDestroyOnCFGEdge(
+    SILBasicBlock *predBB, SILBasicBlock *succBB, bool needsPoison) {
 
   assert(succBB->getSinglePredecessorBlock() == predBB
          && "value is live-out on another predBB successor: critical edge?");
-
-  auto pos = succBB->begin();
-  // TODO: to improve debugability, consider advancing the poison position ahead
-  // of operations that are known not to access weak references.
-  SILBuilderWithScope builder(pos);
-  auto loc = RegularLocation::getAutoGeneratedLocation(pos->getLoc());
-  auto *di = builder.createDestroyValue(loc, currentDef, needsPoison);
-  getCallbacks().createdNewInst(di);
-
+  auto *di = findDestroyOnCFGEdge(succBB, currentDef);
+  if (!di) {
+    auto pos = succBB->begin();
+    // TODO: to improve debugability, consider advancing the poison position
+    // ahead of operations that are known not to access weak references.
+    SILBuilderWithScope builder(pos);
+    auto loc = RegularLocation::getAutoGeneratedLocation(pos->getLoc());
+    di = builder.createDestroyValue(loc, currentDef, needsPoison);
+    getCallbacks().createdNewInst(di);
+  }
   consumes.recordFinalConsume(di);
 
   ++NumDestroysGenerated;
@@ -431,7 +454,7 @@ void CanonicalizeOSSALifetime::findOrInsertDestroyInBlock(SILBasicBlock *bb) {
       // Insert a destroy after this non-consuming use.
       if (isa<TermInst>(inst)) {
         for (auto &succ : bb->getSuccessors()) {
-          insertDestroyOnCFGEdge(bb, succ, poisonRefsMode);
+          findOrInsertDestroyOnCFGEdge(bb, succ, poisonRefsMode);
         }
       } else {
         if (existingDestroy) {
@@ -540,7 +563,7 @@ void CanonicalizeOSSALifetime::findOrInsertDestroys() {
       // Continue searching upward to find the pruned liveness boundary.
       for (auto *predBB : bb->getPredecessorBlocks()) {
         if (liveness.getBlockLiveness(predBB) == PrunedLiveBlocks::LiveOut) {
-          insertDestroyOnCFGEdge(predBB, bb, poisonRefsMode);
+          findOrInsertDestroyOnCFGEdge(predBB, bb, poisonRefsMode);
         } else {
           if (poisonRefsMode) {
             remnantLiveOutBlocks.insert(predBB);