register_pass_v and usage in Function

Summary: Whether a value is passed in registers also depends on its size, not just whether it is trivially copyable. (It also depends on its position in the arg-list but that is another step in complexity.)

Reviewed By: luciang

Differential Revision: D33079529

fbshipit-source-id: ae7383d61491001e54892cf077e45556801bdcf4

folly/Function.h

@@ -229,6 +229,7 @@
 #include <folly/Portability.h>
 #include <folly/Traits.h>
 #include <folly/functional/Invoke.h>
+#include <folly/lang/Align.h>
 #include <folly/lang/Exception.h>
 
 namespace folly {
@@ -303,7 +304,7 @@ template <typename T>
 using CallArg = T&&;
 #else
 template <typename T>
-using CallArg = conditional_t<is_trivially_copyable<T>::value, T, T&&>;
+using CallArg = conditional_t<is_register_pass_v<T>, T, T&&>;
 #endif
 
 template <typename F, typename R, typename... A>

folly/lang/Align.h

@@ -20,9 +20,39 @@
 #include <cstdint>
 
 #include <folly/Portability.h>
+#include <folly/Traits.h>
 
 namespace folly {
 
+//  register_pass_max_size
+//
+//  The platform-specific maximum size of a value which may be passed by-value
+//  in registers.
+//
+//  According to each platform ABI, trivially-copyable types up to this maximum
+//  size may, if the stars align, be passed by-value in registers rather than
+//  implicitly by-reference to stack copies.
+//
+//  Approximate. Accuracy is not promised.
+constexpr std::size_t register_pass_max_size = kMscVer ? 8u : 16u;
+
+//  register_pass_v
+//
+//  Whether a value may be passed in a register.
+//
+//  Trivially-copyable values up to register_pass_max_size in width may be
+//  passed by-value in registers rather than implicitly by-reference to stack
+//  copies.
+//
+//  Approximate. Accuracy is not promised.
+template <typename T>
+constexpr bool is_register_pass_v =
+    (sizeof(T) <= register_pass_max_size) && is_trivially_copyable_v<T>;
+template <typename T>
+constexpr bool is_register_pass_v<T&> = true;
+template <typename T>
+constexpr bool is_register_pass_v<T&&> = true;
+
 //  has_extended_alignment
 //
 //  True if it may be presumed that the platform has static extended alignment;