(Wangle) Shrink Core to 128 bytes

Summary: I think this makes sense, especially if `interrupt_` is really rare.

Test Plan:
============================================================================
folly/futures/test/Benchmark.cpp                relative  time/iter  iters/s
============================================================================
constantFuture                                             101.25ns    9.88M
promiseAndFuture                                  92.86%   109.03ns    9.17M
withThen                                          28.96%   349.65ns    2.86M
----------------------------------------------------------------------------
oneThen                                                    337.33ns    2.96M
twoThens                                          58.46%   577.06ns    1.73M
fourThens                                         32.42%     1.04us  961.23K
hundredThens                                       1.45%    23.24us   43.04K
----------------------------------------------------------------------------
no_contention                                                4.45ms   224.48
contention                                        72.89%     6.11ms   163.61
----------------------------------------------------------------------------
throwAndCatch                                                6.34us  157.77K
throwAndCatchWrapped                              95.65%     6.63us  150.91K
throwWrappedAndCatch                             142.58%     4.45us  224.95K
throwWrappedAndCatchWrapped                      488.86%     1.30us  771.27K
----------------------------------------------------------------------------
throwAndCatchContended                                        1.01s  987.01m
throwAndCatchWrappedContended                    100.03%      1.01s  987.27m
throwWrappedAndCatchContended                    148.37%   682.86ms     1.46
throwWrappedAndCatchWrappedContended            18765.19%     5.40ms   185.21
============================================================================

Compare to D1843129#31 (seems to be an improvement?)

Reviewed By: hans@fb.com

Subscribers: trunkagent, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1847843

Signature: t1:1847843:1423855151:3602d49df245eda3f85325ab5efd63514213ca44

folly/futures/detail/Core.h

@@ -215,9 +215,9 @@ class Core {
   void raise(exception_wrapper e) {
     std::lock_guard<decltype(interruptLock_)> guard(interruptLock_);
     if (!interrupt_ && !hasResult()) {
-      interrupt_ = std::move(e);
+      interrupt_ = folly::make_unique<exception_wrapper>(std::move(e));
       if (interruptHandler_) {
-        interruptHandler_(interrupt_);
+        interruptHandler_(*interrupt_);
       }
     }
   }
@@ -226,8 +226,8 @@ class Core {
   void setInterruptHandler(std::function<void(exception_wrapper const&)> fn) {
     std::lock_guard<decltype(interruptLock_)> guard(interruptLock_);
     if (!hasResult()) {
-      if (!!interrupt_) {
-        fn(interrupt_);
+      if (interrupt_) {
+        fn(*interrupt_);
       } else {
         interruptHandler_ = std::move(fn);
       }
@@ -282,7 +282,7 @@ class Core {
   std::function<void(Try<T>&&)> callback_ {nullptr};
   std::shared_ptr<RequestContext> context_ {nullptr};
   std::atomic<Executor*> executor_ {nullptr};
-  exception_wrapper interrupt_ {};
+  std::unique_ptr<exception_wrapper> interrupt_ {};
   std::function<void(exception_wrapper const&)> interruptHandler_ {nullptr};
 };
 

folly/futures/test/FutureTest.cpp

@@ -88,12 +88,7 @@ static eggs_t eggs("eggs");
 TEST(Future, coreSize) {
   // If this number goes down, it's fine!
   // If it goes up, please seek professional advice ;-)
-  size_t size = 168;
-  if (sizeof(folly::exception_wrapper) == 80) {
-    // it contains strings, which can be bigger for -fb builds (e.g. fbstring)
-    size = 200;
-  }
-  EXPECT_EQ(size, sizeof(detail::Core<void>));
+  EXPECT_EQ(128, sizeof(detail::Core<void>));
 }
 
 // Future