[Mem2Reg] Canonicalize new phis.

Mem2Reg introduces new OSSA values in the form of phis.  Take
responsibility for canonicalizing their lifetimes.

Author: nate-chandler

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -1948,7 +1948,8 @@ class MemoryToRegisters {
 
   /// Canonicalize the lifetimes of the specified owned and guaranteed values.
   void canonicalizeValueLifetimes(StackList<SILValue> &owned,
-                                  StackList<SILValue> &guaranteed);
+                                  StackList<SILValue> &guaranteed,
+                                  BasicBlockSetVector &livePhiBlocks);
 
 public:
   /// C'tor
@@ -2163,10 +2164,29 @@ void MemoryToRegisters::collectStoredValues(AllocStackInst *asi,
 }
 
 void MemoryToRegisters::canonicalizeValueLifetimes(
-    StackList<SILValue> &owned, StackList<SILValue> &guaranteed) {
+    StackList<SILValue> &owned, StackList<SILValue> &guaranteed,
+    BasicBlockSetVector &livePhiBlocks) {
   if (Mem2RegDisableLifetimeCanonicalization)
     return;
 
+  for (auto *block : livePhiBlocks) {
+    // When a single alloc_stack is promoted, any block gains at most a single
+    // new phi, which appears at the end of its argument list.  The collection
+    // \p livePhiBlocks consists of exactly those blocks which gained such a
+    // new phi.
+    SILPhiArgument *argument =
+        cast<SILPhiArgument>(block->getArgument(block->getNumArguments() - 1));
+    switch (argument->getOwnershipKind()) {
+    case OwnershipKind::Owned:
+      owned.push_back(argument);
+      break;
+    case OwnershipKind::Guaranteed:
+      guaranteed.push_back(argument);
+      break;
+    default:
+      break;
+    }
+  }
   CanonicalizeOSSALifetime canonicalizer(
       /*pruneDebug=*/true, /*maximizeLifetime=*/!f.shouldOptimize(), &f,
       accessBlockAnalysis, domInfo, calleeAnalysis, deleter);
@@ -2278,7 +2298,8 @@ bool MemoryToRegisters::run() {
         }
         instructionsToDelete.clear();
         ++NumInstRemoved;
-        canonicalizeValueLifetimes(ownedValues, guaranteedValues);
+        canonicalizeValueLifetimes(ownedValues, guaranteedValues,
+                                   livePhiBlocks);
         madeChange = true;
       }
     }

test/SILOptimizer/mem2reg_lifetime.sil

@@ -311,8 +311,9 @@ bb2:
 // CHECK: bb3([[ARG:%.*]] : @owned $@callee_owned () -> Int):
 bb3:
   %13 = load [copy] %1 : $*@callee_owned () -> Int
-// CHECK: [[COPY:%.*]] = copy_value [[ARG]]
-// CHECK: [[RESULT:%.*]] = apply [[COPY]]
+//      : [[COPY:%.*]] = copy_value [[ARG]]
+// The above copy is eliminated by owned value canonicalization.
+// CHECK: [[RESULT:%.*]] = apply [[ARG]]
   %15 = apply %13() : $@callee_owned () -> Int
   br bb4
 
@@ -321,7 +322,8 @@ bb3:
   //       block.
 // CHECK: bb4
 bb4:
-// CHECK: destroy_value [[ARG]]
+//      : destroy_value [[ARG]]
+// The above destroy is eliminated by owned value canonicalization.
   destroy_addr %1 : $*@callee_owned () -> Int
   dealloc_stack %1 : $*@callee_owned () -> Int
   return %15 : $Int

test/SILOptimizer/mem2reg_ossa.sil

@@ -325,8 +325,9 @@ bb2:
 bb3:
 // CHECK-NOT: load
   %13 = load [copy] %1 : $*@callee_owned () -> Int
-// CHECK: [[COPY:%.*]] = copy_value [[ARG]]
-// CHECK: [[RESULT:%.*]] = apply [[COPY]]
+//      : [[COPY:%.*]] = copy_value [[ARG]]
+// The above copy is eliminated by owned value canonicalization.
+// CHECK: [[RESULT:%.*]] = apply [[ARG]]
   %15 = apply %13() : $@callee_owned () -> Int
   br bb4