diff mbox series

Use __restrict for __relocate_object_a

Message ID alpine.DEB.2.21.1904271144230.31510@grove.saclay.inria.fr
State New
Headers show
Series Use __restrict for __relocate_object_a | expand

Commit Message

Marc Glisse April 27, 2019, 10:03 a.m. UTC 
Hello,

this small patch feels safe: we are moving an existing object to an 
uninitialized location so the 2 locations can't overlap. And it helps with 
std::vector<std::unique_ptr<int>> where it lets the compiler get rid of 
the branches "if(...) delete ..." when moving elements around, for 
instance in reserve(). We don't get a call to memmove yet (ldist may be 
confused by clobbers, I'll investigate and report that later), but the 
code gets much smaller at -O2, and we vectorize at -O3.

Similarly, I think the call to memmove in __relocate_a_1 could probably be 
memcpy (I don't remember why I chose memmove), but that seems less 
important and I don't want to mix it with this patch.

Bootstrap+regtest on x86_64-pc-linux-gnu.

2019-04-29  Marc Glisse  <marc.glisse@inria.fr>

 	PR libstdc++/87106
 	* include/bits/stl_uninitialized.h (__relocate_object_a): Mark the
 	arguments with __restrict.

Comments

Jonathan Wakely April 27, 2019, 1:09 p.m. UTC | #1 
On 27/04/19 12:03 +0200, Marc Glisse wrote:
>Hello,
>
>this small patch feels safe: we are moving an existing object to an 
>uninitialized location so the 2 locations can't overlap. And it helps 
>with std::vector<std::unique_ptr<int>> where it lets the compiler get 
>rid of the branches "if(...) delete ..." when moving elements around, 
>for instance in reserve(). We don't get a call to memmove yet (ldist 
>may be confused by clobbers, I'll investigate and report that later), 
>but the code gets much smaller at -O2, and we vectorize at -O3.
>
>Similarly, I think the call to memmove in __relocate_a_1 could 
>probably be memcpy (I don't remember why I chose memmove), but that 
>seems less important and I don't want to mix it with this patch.
>
>Bootstrap+regtest on x86_64-pc-linux-gnu.

OK for trunk, thanks.
diff mbox series

Patch

Index: libstdc++-v3/include/bits/stl_uninitialized.h
===================================================================
--- libstdc++-v3/include/bits/stl_uninitialized.h	(revision 270586)
+++ libstdc++-v3/include/bits/stl_uninitialized.h	(working copy)
@@ -877,21 +877,22 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       auto __res = std::__uninitialized_copy_n_pair
 	(_GLIBCXX_MAKE_MOVE_ITERATOR(__first),
 	 __count, __result);
       return {__res.first.base(), __res.second};
     }
 #endif // C++17
 
 #if __cplusplus >= 201103L
   template<typename _Tp, typename _Up, typename _Allocator>
     inline void
-    __relocate_object_a(_Tp* __dest, _Up* __orig, _Allocator& __alloc)
+    __relocate_object_a(_Tp* __restrict __dest, _Up* __restrict __orig,
+			_Allocator& __alloc)
     noexcept(noexcept(std::allocator_traits<_Allocator>::construct(__alloc,
 			 __dest, std::move(*__orig)))
 	     && noexcept(std::allocator_traits<_Allocator>::destroy(
 			    __alloc, std::__addressof(*__orig))))
     {
       typedef std::allocator_traits<_Allocator> __traits;
       __traits::construct(__alloc, __dest, std::move(*__orig));
       __traits::destroy(__alloc, std::__addressof(*__orig));
     }