non-throwing, non-allocating exception_wrapper

Summary:
The purpose of this reimplementation of `exception_wrapper` is threefold:

- Make `exception_wrapper` smaller. It goes from 48 bytes to 24.
- Give it `noexcept` ~~copy and~~ move
- Store small exception objects in an internal buffer; i.e., with zero allocations.

The ultimate goal is to change `folly::Try<T>` to a thin wrapper over `folly::Expected<T, exception_wrapper>`. (Currently, it stores the `exception_wrapper` on the heap.)

As part of this redesign, I:

- Remove `exception_wrapper::getCopied`. The user shouldn't care how the `exception_wrapper` stores the exception.
- Remove `exception_wrapper::operator==`. It was only used in 2 places in test code. The existing semantics (return true IFF two `exception_wrapper`s point to the //same// exception object) prevented the small-object optimization.
- Add new `handle()` API that behaves like cascading `catch` clauses. For instance:
```lang=c++
exception_wrapper ew = ...;
ew.handle(
    [&](const SomeException& e) { /*...*/ },
    [&](const AnotherException& e) { /*...*/ },
    [&](...) { /* catch all*/ }, // yes, lambda with ellipses works!
```
- Add a `type()` member for accessing the `typeid` of the wrapped exception, if it's known or can be determined with a `catch(std::exception&)`.

This table shows the percent improvement for the exception_wrapper_benchmark test:

| Test  | Percent improvement (gcc-5)  | Percent improvement (gcc-4)
| -----  | -----  | -----
| exception_wrapper_create_and_test  | 14.33%    | -6.50%
| exception_wrapper_create_and_test_concurrent | 11.91% | 20.15%
| exception_wrapper_create_and_throw | -0.82% | -0.25%
| exception_wrapper_create_and_cast | 15.02% | 14.31%
| exception_wrapper_create_and_throw_concurrent | 18.37% | 8.03%
| exception_wrapper_create_and_cast_concurrent | 28.18% | -10.77%

The percent win for gcc-5 is 15% on average. The non-throwing tests show a greater win since the cost of actually throwing an exception drowns out the other improvements. (One of the reasons to use `exception_wrapper` is to not need to throw in the first place.) On gcc-4, there is roughly no change since the gcc-4 standard exceptions (`std::runtime_error`, std::logic_error`) are non-conforming since they have throwing copy operations.

Reviewed By: yfeldblum

Differential Revision: D4385822

fbshipit-source-id: 63a8316c2923b29a79f8fa446126a8c37aa32989

folly/CPortability.h

@@ -126,3 +126,12 @@
 #else
 # define FOLLY_ALWAYS_INLINE inline
 #endif
+
+// attribute hidden
+#if _MSC_VER
+#define FOLLY_ATTR_VISIBILITY_HIDDEN
+#elif defined(__clang__) || defined(__GNUC__)
+#define FOLLY_ATTR_VISIBILITY_HIDDEN __attribute__((__visibility__("hidden")))
+#else
+#define FOLLY_ATTR_VISIBILITY_HIDDEN
+#endif

folly/ExceptionWrapper.cpp

@@ -15,50 +15,53 @@
  */
 #include <folly/ExceptionWrapper.h>
 
-#include <exception>
 #include <iostream>
 
+#include <folly/Logging.h>
+
 namespace folly {
 
-[[noreturn]] void exception_wrapper::throwException() const {
-  if (throwfn_) {
-    throwfn_(*item_);
-  } else if (eptr_) {
-    std::rethrow_exception(eptr_);
+constexpr exception_wrapper::VTable const exception_wrapper::uninit_;
+constexpr exception_wrapper::VTable const exception_wrapper::ExceptionPtr::ops_;
+constexpr exception_wrapper::VTable const exception_wrapper::SharedPtr::ops_;
+
+namespace {
+std::exception const* get_std_exception_(std::exception_ptr eptr) noexcept {
+  try {
+    std::rethrow_exception(eptr);
+  } catch (const std::exception& ex) {
+    return &ex;
+  } catch (...) {
+    return nullptr;
   }
-  std::ios_base::Init ioinit_; // ensure std::cerr is alive
-  std::cerr
-      << "Cannot use `throwException` with an empty folly::exception_wrapper"
-      << std::endl;
-  std::terminate();
+}
 }
 
-fbstring exception_wrapper::class_name() const {
-  if (item_) {
-    auto& i = *item_;
-    return demangle(typeid(i));
-  } else if (eptr_ && eobj_) {
-    return demangle(typeid(*eobj_));
-  } else if (eptr_ && etype_) {
-    return demangle(*etype_);
+exception_wrapper::exception_wrapper(std::exception_ptr ptr) noexcept
+    : exception_wrapper{} {
+  if (ptr) {
+    if (auto e = get_std_exception_(ptr)) {
+      LOG(DFATAL)
+          << "Performance error: Please construct exception_wrapper with a "
+             "reference to the std::exception along with the "
+             "std::exception_ptr.";
+      *this = exception_wrapper{std::move(ptr), *e};
     } else {
-    return fbstring();
+      Unknown uk;
+      *this = exception_wrapper{ptr, uk};
+    }
   }
 }
 
-fbstring exception_wrapper::what() const {
-  if (item_) {
-    return exceptionStr(*item_);
-  } else if (eptr_ && eobj_) {
-    return class_name() + ": " + eobj_->what();
-  } else if (eptr_ && etype_) {
-    return class_name();
-  } else {
-    return class_name();
-  }
+[[noreturn]] void exception_wrapper::onNoExceptionError() {
+  std::ios_base::Init ioinit_; // ensure std::cerr is alive
+  std::cerr
+      << "Cannot use `throwException` with an empty folly::exception_wrapper"
+      << std::endl;
+  std::terminate();
 }
 
-fbstring exceptionStr(const exception_wrapper& ew) {
+fbstring exceptionStr(exception_wrapper const& ew) {
   return ew.what();
 }
 

folly/Makefile.am

@@ -93,6 +93,7 @@ nobase_follyinclude_HEADERS = \
 	Exception.h \
 	ExceptionString.h \
 	ExceptionWrapper.h \
+	ExceptionWrapper-inl.h \
 	Executor.h \
 	Expected.h \
 	experimental/AsymmetricMemoryBarrier.h \

folly/futures/test/FutureSplitterTest.cpp

@@ -126,8 +126,8 @@ TEST(FutureSplitter, splitFutureFailure) {
   EXPECT_FALSE(f1.isReady());
   try {
     throw std::runtime_error("Oops");
-  } catch (...) {
-    p.setException(exception_wrapper(std::current_exception()));
+  } catch (std::exception& e) {
+    p.setException(exception_wrapper(std::current_exception(), e));
   }
   EXPECT_TRUE(f1.isReady());
   EXPECT_TRUE(f1.hasException());