Don't duplicate builtin "once"

lib/SIL/IR/SILInstruction.cpp

@@ -1426,6 +1426,12 @@ bool SILInstruction::isTriviallyDuplicatable() const {
   if (isa<BeginAccessInst>(this))
     return false;
 
+  // All users of builtin "once" should directly operate on it (no phis etc)
+  if (auto *bi = dyn_cast<BuiltinInst>(this)) {
+    if (bi->getBuiltinInfo().ID == BuiltinValueKind::Once)
+      return false;
+  }
+
   // begin_apply creates a token that has to be directly used by the
   // corresponding end_apply and abort_apply.
   if (isa<BeginApplyInst>(this))

lib/SILOptimizer/Utils/LoopUtils.cpp

@@ -309,6 +309,11 @@ bool swift::canDuplicateLoopInstruction(SILLoop *L, SILInstruction *I) {
     return true;
   }
 
+  if (auto *bi = dyn_cast<BuiltinInst>(I)) {
+    if (bi->getBuiltinInfo().ID == BuiltinValueKind::Once)
+      return false;
+  }
+
   if (isa<DynamicMethodBranchInst>(I))
     return false;
 

test/SILOptimizer/simplify_cfg_ossa_jump_threading.sil

@@ -229,3 +229,49 @@ bb4:
   destroy_value %4 : $FakeOptional<Klass>
   return %5 : $Builtin.Int1
 }
+
+sil_global @my_global : $S
+
+sil @globalinit_func11 :  $@convention(thin) () -> ()
+
+struct S {
+}
+
+// This test is not optimized because the dest bb of the branch has a single predecessor
+// CHECK-LABEL: sil [ossa] @test_simplify_switch_enum_jump_threading_sil_token :
+// CHECK: switch_enum
+// CHECK-NOT: switch_enum
+// CHECK-LABEL: } // end sil function 'test_simplify_switch_enum_jump_threading_sil_token'
+sil [ossa] @test_simplify_switch_enum_jump_threading_sil_token : $@convention(thin) (@owned Klass, Builtin.RawPointer) -> () {
+bb0(%0 : @owned $Klass, %1 : $Builtin.RawPointer):
+  cond_br undef, bb1, bb2
+
+bb1:
+  specify_test "simplify-cfg-try-jump-threading @instruction[2]"
+  %s = enum $FakeOptional<Klass>, #FakeOptional.some!enumelt, %0 : $Klass
+  br bb3(%s : $FakeOptional<Klass>)
+
+bb2:
+  destroy_value %0 : $Klass
+  %n = enum $FakeOptional<Klass>, #FakeOptional.none!enumelt
+  br bb3(%n : $FakeOptional<Klass>)
+
+bb3(%3 : @owned $FakeOptional<Klass>):
+  %f = function_ref @globalinit_func11 : $@convention(thin) () -> ()
+  %b = builtin "once"(%1 : $Builtin.RawPointer, %f : $@convention(thin) () -> ()) : $Builtin.SILToken 
+  %g = global_addr @my_global : $*S depends_on %b
+  switch_enum %3 : $FakeOptional<Klass>, case #FakeOptional.some!enumelt: bb5, case #FakeOptional.none!enumelt: bb4
+
+bb4:
+  br bb6
+
+bb5(%6 : @owned $Klass):
+  %ld = load [trivial] %g : $*S
+  apply undef(%ld) : $@convention(thin) (S) -> () 
+  destroy_value %6 : $Klass
+  br bb6
+
+bb6:
+  %t = tuple ()
+  return %t : $()
+}