[CanonOSSALifetime] Fixed term boundary extension.

If multiple terminators which branch to the same merge block are added
to the boundary, depending on whether a destroy_value can be found
within the block either (a) every terminator must be added to the
boundary or (b) the destroy_value must be added to the boundary exactly
once.

Author: nate-chandler

lib/SILOptimizer/Utils/CanonicalizeOSSALifetime.cpp

@@ -741,6 +741,17 @@ class ExtendBoundaryToDestroys final {
         // before means it has multiple predecessors, so this must be \p block's
         // unique successor.
         assert(block->getSingleSuccessorBlock() == successor);
+        // When this merge point was encountered the first time, a
+        // destroy_value was sought from its top.  If one was found, it was
+        // added to the boundary. If no destroy_value was found, _that_ user
+        // (i.e. the one on behalf of which extendBoundaryFromTerminator was
+        // called which inserted successor into seenMergePoints) was added to
+        // the boundary.
+        //
+        // This time, if a destroy was found, it's already in the boundary.  If
+        // no destroy was found, though, _this_ user must be added to the
+        // boundary.
+        foundDestroy = findDestroyFromBlockBegin(successor, currentDef);
         continue;
       }
       if (auto *dvi = findDestroyFromBlockBegin(successor, currentDef)) {

test/SILOptimizer/canonicalize_ossa_lifetime_unit.sil

@@ -1,6 +1,7 @@
 // RUN: %target-sil-opt -unit-test-runner %s -o /dev/null 2>&1 | %FileCheck %s
 
 class C {}
+sil @getOwned : $@convention(thin) () -> @owned C
 
 // When access scopes are respected, the lifetime which previously extended
 // beyond the access scope still extends beyond it.
@@ -45,3 +46,31 @@ bb0(%addr : $*C):
   %retval = tuple ()
   return %retval : $()
 }
+
+
+// CHECK-LABEL: begin running test 1 of 1 on reuse_destroy_after_barrier_phi: canonicalize-ossa-lifetime with: true, false, true, @trace
+// CHECK-LABEL: sil [ossa] @reuse_destroy_after_barrier_phi : {{.*}} {
+// CHECK:       {{bb[0-9]+}}([[INSTANCE:%[^,]+]] :
+// CHECK:       {{bb[0-9]+}}({{%[^,]+}}
+// CHECK:         destroy_value [[INSTANCE]]
+// CHECK-LABEL: } // end sil function 'reuse_destroy_after_barrier_phi'
+// CHECK-LABEL: end running test 1 of 1 on reuse_destroy_after_barrier_phi: canonicalize-ossa-lifetime with: true, false, true, @trace
+sil [ossa] @reuse_destroy_after_barrier_phi : $@convention(thin) (@owned C) -> @owned C {
+entry(%instance : @owned $C):
+  debug_value [trace] %instance : $C
+  test_specification "canonicalize-ossa-lifetime true false true @trace"
+  %get = function_ref @getOwned : $@convention(thin) () -> @owned C
+  cond_br undef, through, loop
+
+through:
+  %4 = copy_value %instance : $C
+  br exit(%4 : $C)
+
+loop:
+  %other = apply %get() : $@convention(thin) () -> @owned C
+  br exit(%other : $C)
+
+exit(%out : @owned $C):
+  destroy_value %instance : $C
+  return %out : $C
+}