Let IOBufCompare have a self-compare shortcut

Summary: [Folly] Let `IOBufCompare` have a self-compare shortcut.

Reviewed By: vitaut

Differential Revision: D9981523

fbshipit-source-id: e984cffc2816a0f4f06bf418b16d4d3eb055f6ef

folly/io/IOBuf.cpp

@@ -1066,7 +1066,7 @@ size_t IOBufHash::operator()(const IOBuf& buf) const {
   return static_cast<std::size_t>(h1);
 }
 
-ordering IOBufCompare::operator()(const IOBuf& a, const IOBuf& b) const {
+ordering IOBufCompare::impl(const IOBuf& a, const IOBuf& b) const {
   io::Cursor ca(&a);
   io::Cursor cb(&b);
   for (;;) {

folly/io/IOBuf.h

@@ -1473,7 +1473,9 @@ struct IOBufHash {
  * Ordering for IOBuf objects. Compares data in the entire chain.
  */
 struct IOBufCompare {
-  ordering operator()(const IOBuf& a, const IOBuf& b) const;
+  ordering operator()(const IOBuf& a, const IOBuf& b) const {
+    return &a == &b ? ordering::eq : impl(a, b);
+  }
   ordering operator()(
       const std::unique_ptr<IOBuf>& a,
       const std::unique_ptr<IOBuf>& b) const {
@@ -1485,6 +1487,9 @@ struct IOBufCompare {
         operator()(*a, *b);
     // clang-format on
   }
+
+ private:
+  ordering impl(IOBuf const& a, IOBuf const& b) const;
 };
 
 /**