While looking into LWG 3741 I came up with this small change to
chrono::abs, which reduces how much work the compiler does to compile
it, but makes the code less clear. The current implementation is very
easy to understand, and compiling chrono::abs probably isn't a hotspot
in anybody's build. Is this worth it?
-- >8 --
This change manually inlines the call to duration::zero, the comparison
using chrono::operator< with duration arguments, the call to
duration::operator- (and the common_type instantiation it does). This
also avoids calling the duration(const duration<R2,P2>&) constructor
(and its constraint checks).
By performing the arithmetic operations directly on the Rep value we
improve compilation throughput and also runtime performance for
unoptimized builds.
libstdc++-v3/ChangeLog:
* include/bits/chrono.h (chrono::abs): Optimize.
---
libstdc++-v3/include/bits/chrono.h | 42 +++++++++++++++---------------
1 file changed, 21 insertions(+), 21 deletions(-)
diff --git a/libstdc++-v3/include/bits/chrono.h
b/libstdc++-v3/include/bits/chrono.h
index 05987ca09df..99d47503af3 100644
--- a/libstdc++-v3/include/bits/chrono.h
+++ b/libstdc++-v3/include/bits/chrono.h
@@ -317,6 +317,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ };
#endif // C++20
+ /// duration_values
+ template<typename _Rep>
+ struct duration_values
+ {
+ static constexpr _Rep
+ zero() noexcept
+ { return _Rep(0); }
+
+ static constexpr _Rep
+ max() noexcept
+ { return numeric_limits<_Rep>::max(); }
+
+ static constexpr _Rep
+ min() noexcept
+ { return numeric_limits<_Rep>::lowest(); }
+ };
+
#if __cplusplus >= 201703L
# define __cpp_lib_chrono 201611L
@@ -365,11 +382,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Rep, typename _Period>
constexpr
enable_if_t<numeric_limits<_Rep>::is_signed, duration<_Rep, _Period>>
- abs(duration<_Rep, _Period> __d)
+ abs(duration<_Rep, _Period> __d) noexcept(is_arithmetic_v<_Rep>)
{
- if (__d >= __d.zero())
- return __d;
- return -__d;
+ if (_Rep __c = __d.count(); __c < duration_values<_Rep>::zero())
+ return duration<_Rep, _Period>(-__c);
+ return __d;
}
// Make chrono::ceil<D> also usable as chrono::__detail::ceil<D>.
@@ -399,23 +416,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
#endif // C++17
- /// duration_values
- template<typename _Rep>
- struct duration_values
- {
- static constexpr _Rep
- zero() noexcept
- { return _Rep(0); }
-
- static constexpr _Rep
- max() noexcept
- { return numeric_limits<_Rep>::max(); }
-
- static constexpr _Rep
- min() noexcept
- { return numeric_limits<_Rep>::lowest(); }
- };
-
/// @cond undocumented
template<typename _Tp>
--
2.37.2
