As per the Itanium ABI gcc mangles the return types of function
templates, so that we can see mangled names like
_Z1hIiEDTplcl1fIT_EEcl1gIS0_EEEv
which demangle to
decltype (((f<int>)())+((g<int>)())) h<int>()
In seastar [1] this causes serious compile-time performance problems.
Replacing complicated template argument dependent return types improves
compile time and object size by around 10%.
What is the reason that the ABI mandates mangling the return type into
the function name?
The patch below (replacing return types with auto) gave me about 10%
compile time improvement:
--- a/core/future.hh
+++ b/core/future.hh
@@ -460,7 +460,8 @@ private:
}
template <typename Ret, typename Func, typename Param>
- futurize_t<Ret> then(Func&& func, Param&& param) noexcept {
+ auto // futurize_t<Ret>
+ then(Func&& func, Param&& param) noexcept {
using futurator = futurize<Ret>;
using P = typename futurator::promise_type;
if (state()->available() && (++future_avail_count % 256)) {
@@ -526,12 +527,14 @@ public:
}
template <typename Func>
- futurize_t<std::result_of_t<Func(T&&...)>> then(Func&& func) noexcept {
+ auto // futurize_t<std::result_of_t<Func(T&&...)>>
+ then(Func&& func) noexcept {
return
then<std::result_of_t<Func(T&&...)>>(std::forward<Func>(func), []
(future_state<T...>&& state) { return state.get(); });
}
template <typename Func>
- futurize_t<std::result_of_t<Func(future<T...>)>>
+ auto
+ //futurize_t<std::result_of_t<Func(future<T...>)>>
then_wrapped(Func&& func) noexcept {
return
then<std::result_of_t<Func(future<T...>)>>(std::forward<Func>(func), []
(future_state<T...>&& state) { return future(std::move(state)); });
}
[1]https://github.com/cloudius-systems/seastar
<https://github.com/cloudius-systems/seastar>, specifically
future::then() in core/future.hh
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