SILOptimizer: a new optimization to hoist destroys of memory locations

DestroyHoisting moves destroys of memory locations up the control flow as far as possible.
Beside destroy_addr, also "store [assign]" is considered a destroy, because is is equivalent to an destroy_addr + a "store [init]".
The main purpose of this optimization is to minimize copy-on-write operations for arrays, etc. Especially if such COW containers  are used as enum payloads and modified in-place. E.g.

switch e {
  case .A(var arr):
    arr.append(x)
    self = .A(arr)
...

In such a case DestroyHoisting can move the destroy of the self-assignment up before the switch and thus let the array buffer only be single-referenced at the time of the append.

When we have ownership SIL throughout the pass pipeline this optimization will replace the current destroy hoisting optimization in CopyForwarding.
For now, this optimization only runs in the mandatory pipeline (but not for -Onone) where we already have ownership SIL.

SR-10605
rdar://problem/50463362

Author: eeckstein

include/swift/SIL/SILInstruction.h

@@ -3240,6 +3240,9 @@ class LoadInst
     return LoadOwnershipQualifier(
       SILInstruction::Bits.LoadInst.OwnershipQualifier);
   }
+  void setOwnershipQualifier(LoadOwnershipQualifier qualifier) {
+    SILInstruction::Bits.LoadInst.OwnershipQualifier = unsigned(qualifier);
+  }
 };
 
 // *NOTE* When serializing, we can only represent up to 4 values here. If more
@@ -3279,6 +3282,9 @@ class StoreInst
     return StoreOwnershipQualifier(
       SILInstruction::Bits.StoreInst.OwnershipQualifier);
   }
+  void setOwnershipQualifier(StoreOwnershipQualifier qualifier) {
+    SILInstruction::Bits.StoreInst.OwnershipQualifier = unsigned(qualifier);
+  }
 };
 
 class EndBorrowInst;

include/swift/SILOptimizer/PassManager/Passes.def

@@ -132,6 +132,8 @@ PASS(DeadObjectElimination, "deadobject-elim",
      "Dead Object Elimination for Classes with Trivial Destruction")
 PASS(DefiniteInitialization, "definite-init",
      "Definite Initialization for Diagnostics")
+PASS(DestroyHoisting, "destroy-hoisting",
+     "Hoisting of value destroys")
 PASS(Devirtualizer, "devirtualizer",
      "Indirect Call Devirtualization")
 PASS(DiagnoseInfiniteRecursion, "diagnose-infinite-recursion",

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -104,6 +104,7 @@ static void addMandatoryOptPipeline(SILPassPipelinePlan &P) {
   P.addClosureLifetimeFixup();
   if (Options.shouldOptimize()) {
     P.addSemanticARCOpts();
+    P.addDestroyHoisting();
   }
   if (!Options.StripOwnershipAfterSerialization)
     P.addOwnershipModelEliminator();

lib/SILOptimizer/Transforms/CMakeLists.txt

@@ -15,6 +15,7 @@ silopt_register_sources(
   DeadCodeElimination.cpp
   DeadObjectElimination.cpp
   DeadStoreElimination.cpp
+  DestroyHoisting.cpp
   Devirtualizer.cpp
   GenericSpecializer.cpp
   MergeCondFail.cpp