Commit ad3fe1dd authored by Phil Willoughby's avatar Phil Willoughby Committed by Facebook Github Bot

Mark constructing an Unexpected as cold

Summary:
By marking the main constructors of `Unexpected` cold we are telling the compiler that we don't expect to execute them often. This can improve the locality and performance of the `Expected` branch, and reduce the code-size for the `Unexpected` branch.

The constructors we have marked cold here are only the ones used to originate an `Unexpected`. We don't mark the copy- move- or conversion-constructors as `cold` because they will typically be used in error-handling branches and it is not correct to say that they are colder than their surroundings in that case. Note that the entire error-handling branch is likely to be deduced to be cold because we told the compiler that it is likely that hasValue() is true and hasError() is false.

Because `makeUnexpected` unconditionally invokes one of these cold constructors it will inherit the property of being cold.

The GCC function attribute reference, which describes what `cold` means, is here: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes

Reviewed By: yfeldblum

Differential Revision: D6234305

fbshipit-source-id: 6876073e92ce54936ba7152175d76be653f5f463
parent 05a56e4c
......@@ -103,3 +103,15 @@
#define FOLLY_NULLABLE
#define FOLLY_NONNULL
#endif
/**
* "Cold" indicates to the compiler that a function is only expected to be
* called from unlikely code paths. It can affect decisions made by the
* optimizer both when processing the function body and when analyzing
* call-sites.
*/
#if __GNUC__
#define FOLLY_COLD __attribute__((__cold__))
#else
#define FOLLY_COLD
#endif
......@@ -31,6 +31,7 @@
#include <glog/logging.h>
#include <folly/CppAttributes.h>
#include <folly/Likely.h>
#include <folly/Optional.h>
#include <folly/Portability.h>
......@@ -711,8 +712,10 @@ class Unexpected final {
Unexpected(Unexpected&&) = default;
Unexpected& operator=(const Unexpected&) = default;
Unexpected& operator=(Unexpected&&) = default;
constexpr /* implicit */ Unexpected(const Error& err) : error_(err) {}
constexpr /* implicit */ Unexpected(Error&& err) : error_(std::move(err)) {}
FOLLY_COLD constexpr /* implicit */ Unexpected(const Error& err)
: error_(err) {}
FOLLY_COLD constexpr /* implicit */ Unexpected(Error&& err)
: error_(std::move(err)) {}
template <class Other FOLLY_REQUIRES_TRAILING(
std::is_constructible<Error, Other&&>::value)>
......
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