[libcxx-commits] [libcxx] ada2a8e - Remove the try/catch codepath if `swap` is `noexcept`.

[Michael Park via libcxx-commits]

Author: Michael Park
Date: 2020-06-16T14:51:22-07:00
New Revision: ada2a8ea4a9c5b2141c30c77b80b85945e346999

URL: https://github.com/llvm/llvm-project/commit/ada2a8ea4a9c5b2141c30c77b80b85945e346999
DIFF: https://github.com/llvm/llvm-project/commit/ada2a8ea4a9c5b2141c30c77b80b85945e346999.diff

LOG: Remove the try/catch codepath if `swap` is `noexcept`.

Summary:
In the case where `swap` is `noexcept`, we should avoid the extension to provide strong-exception guarantee.

Fixes https://bugs.llvm.org/show_bug.cgi?id=46342

Reviewers: #libc, ldionne

Reviewed By: #libc, ldionne

Subscribers: dexonsmith, mclow.lists, miscco, ldionne, zoecarver, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D81954

Added: 
    

Modified: 
    libcxx/include/variant

Removed: 
    


################################################################################
diff  --git a/libcxx/include/variant b/libcxx/include/variant
index 98a62c992fa1..897361fc3be3 100644
--- a/libcxx/include/variant
+++ b/libcxx/include/variant
@@ -1062,21 +1062,28 @@ public:
         _VSTD::swap(__lhs, __rhs);
       }
       __impl __tmp(_VSTD::move(*__rhs));
-#ifndef _LIBCPP_NO_EXCEPTIONS
-      // EXTENSION: When the move construction of `__lhs` into `__rhs` throws
-      // and `__tmp` is nothrow move constructible then we move `__tmp` back
-      // into `__rhs` and provide the strong exception safety guarantee.
-      try {
+      static constexpr bool __is_noexcept =
+#ifdef _LIBCPP_NO_EXCEPTIONS
+          true;
+#else
+          __all<(is_nothrow_move_constructible_v<_Types> &&
+                 is_nothrow_swappable_v<_Types>)...>::value;
+#endif
+      if constexpr (__is_noexcept) {
         this->__generic_construct(*__rhs, _VSTD::move(*__lhs));
-      } catch (...) {
-        if (__tmp.__move_nothrow()) {
-          this->__generic_construct(*__rhs, _VSTD::move(__tmp));
+      } else {
+        // EXTENSION: When the move construction of `__lhs` into `__rhs` throws
+        // and `__tmp` is nothrow move constructible then we move `__tmp` back
+        // into `__rhs` and provide the strong exception safety guarantee.
+        try {
+          this->__generic_construct(*__rhs, _VSTD::move(*__lhs));
+        } catch (...) {
+          if (__tmp.__move_nothrow()) {
+            this->__generic_construct(*__rhs, _VSTD::move(__tmp));
+          }
+          throw;
         }
-        throw;
       }
-#else
-      this->__generic_construct(*__rhs, _VSTD::move(*__lhs));
-#endif
       this->__generic_construct(*__lhs, _VSTD::move(__tmp));
     }
   }