[OSSACanOwned] Copy for guaranteed final consume.

Because discovery of defs walks into reborrows and borrowed-from
instructions, copies may be processed whose underlying value is a
guaranteed value (namely, a reborrow or a borrowed-from instruction).
The last consume of such a copy is a final consume, but even so, a copy
is still necessary: the original value is guaranteed and can't serve as
an operand to an instruction which requries an owned value.

rdar://139842132

Author: nate-chandler

lib/SILOptimizer/Utils/CanonicalizeOSSALifetime.cpp

@@ -1170,7 +1170,7 @@ void CanonicalizeOSSALifetime::rewriteCopies(
   //
   // Return true if the operand can use the current definition. Return false if
   // it requires a copy.
-  auto visitUse = [&](Operand *use) {
+  auto visitUse = [&](Operand *use, Def def) {
     auto *user = use->getUser();
     // The current def can always be used for a copy_value.
     if (auto *copy = dyn_cast<CopyValueInst>(user)) {
@@ -1205,6 +1205,11 @@ void CanonicalizeOSSALifetime::rewriteCopies(
       return false;
     }
 
+    // This is a final consume, but a copy will still be necessary if the
+    // original value is guaranteed.
+    if (def.getOriginalValue()->getOwnershipKind() == OwnershipKind::Guaranteed)
+      return false;
+
     return true;
   };
 
@@ -1223,7 +1228,7 @@ void CanonicalizeOSSALifetime::rewriteCopies(
       for (auto useIter = value->use_begin(), endIter = value->use_end();
            useIter != endIter;) {
         Operand *use = *useIter++;
-        if (!visitUse(use)) {
+        if (!visitUse(use, def)) {
           copyLiveUse(use, getCallbacks());
         }
       }
@@ -1237,7 +1242,7 @@ void CanonicalizeOSSALifetime::rewriteCopies(
       for (auto useIter = srcCopy->use_begin();
            useIter != srcCopy->use_end();) {
         Operand *use = *useIter++;
-        if (!visitUse(use)) {
+        if (!visitUse(use, def)) {
           if (!reusedCopyOp && srcCopy->getParent() == use->getParentBlock()) {
             reusedCopyOp = use;
           } else {

test/SILOptimizer/canonicalize_ossa_lifetime_unit.sil

@@ -789,3 +789,32 @@ entry(%c : @owned $C):
   specify_test "canonicalize_ossa_lifetime true false true %c"
   unreachable
 }
+
+// CHECK-LABEL: begin running test {{.*}} on consume_copy_of_borrowed_from
+// CHECK-LABEL: sil [ossa] @consume_copy_of_borrowed_from : {{.*}} {
+// The copy can't be eliminated because its root is a guaranteed value.
+// CHECK:         copy_value
+// CHECK-LABEL: } // end sil function 'consume_copy_of_borrowed_from'
+// CHECK-LABEL: end running test {{.*}} on consume_copy_of_borrowed_from
+sil [ossa] @consume_copy_of_borrowed_from : $@convention(thin) (@owned C) -> () {
+bb0(%instance : @owned $C):
+  %borrow = begin_borrow %instance : $C
+  br bb1(%instance : $C, %borrow : $C)
+
+bb1(%instance2 : @owned $C, %reborrowed : @reborrow $C):
+  specify_test "canonicalize_ossa_lifetime true false true %instance2"
+  %reborrow = borrowed %reborrowed : $C from (%instance2 : $C)
+  cond_br undef, bb2, bb3
+
+bb2:
+  end_borrow %reborrow : $C
+  destroy_value %instance2 : $C
+  %retval = tuple ()
+  return %retval : $()
+
+bb3:
+  %copy = copy_value %reborrow : $C
+  %takeC = function_ref @takeC : $@convention(thin) (@owned C) -> ()
+  apply %takeC(%copy) : $@convention(thin) (@owned C) -> ()
+  unreachable
+}