Re: minor optimization bug in basic_string move assignment

["François Dumont" <frs.dumont@gmail.com>]

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On 04/01/23 00:11, waffl3x via Libstdc++ wrote:
> Example: https://godbolt.org/z/sKhGqG1qK
> https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libstdc%2B%2B-v3/include/bits/basic_string.h;hb=HEAD#l880
> When move assigning to a basic_string, the allocated memory of the moved into string is stored into the source string instead of deallocating it, a good optimization when everything is compatible. However in the case of a stateful allocator (is_always_true() evaluating as false) this optimization is never taken. Unless there is some reason I can't think of that makes equal stateful allocators incompatible here, I believe the if statement on line 880 of basic_string.h should also compare the equality of each strings allocator. The first condition in the function seems to indicate to me that this scenario was being considered and just forgotten about, as the memory doesn't get deallocated immediately if the two allocators are equal. I'll note that because of how everything is handled, this doesn't result in a leak so this bug is still only a minor missed optimization.
>
> mailto:libstdc++@gcc.gnu.org

Hmmm, I don't know, at least it is not as simple as you present it.

You cannot add a check on allocator equality as you are proposing 
because it is too late. __str allocator might have already been 
propagated to *this on the previous call to std::__alloc_on_move. Note 
that current check is done only if 
!_Alloc_traits::_S_propagate_on_move_assign().

This patch might do the job but I wonder if equal allocators can become 
un-equal after the propagate-on-move-assignment ?



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diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 109915653af..2627631e102 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -844,9 +844,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       operator=(basic_string&& __str)
       noexcept(_Alloc_traits::_S_nothrow_move())
       {
+	const bool __equal_allocs = _Alloc_traits::_S_always_equal()
+	    || _M_get_allocator() == __str._M_get_allocator()
 	if (!_M_is_local() && _Alloc_traits::_S_propagate_on_move_assign()
-	    && !_Alloc_traits::_S_always_equal()
-	    && _M_get_allocator() != __str._M_get_allocator())
+	    && !__equal_allocs)
 	  {
 	    // Destroy existing storage before replacing allocator.
 	    _M_destroy(_M_allocated_capacity);
@@ -868,16 +869,14 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 		_M_set_length(__str.size());
 	      }
 	  }
-	else if (_Alloc_traits::_S_propagate_on_move_assign()
-	    || _Alloc_traits::_S_always_equal()
-	    || _M_get_allocator() == __str._M_get_allocator())
+	else if (_Alloc_traits::_S_propagate_on_move_assign() || __equal_allocs)
 	  {
 	    // Just move the allocated pointer, our allocator can free it.
 	    pointer __data = nullptr;
 	    size_type __capacity;
 	    if (!_M_is_local())
 	      {
-		if (_Alloc_traits::_S_always_equal())
+		if (__equal_allocs)
 		  {
 		    // __str can reuse our existing storage.
 		    __data = _M_data();