[Bug libstdc++/64128] New: Let vector take advantage of malloc_usable_size for malloc-using allocators

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64128

            Bug ID: 64128
           Summary: Let vector take advantage of malloc_usable_size for
                    malloc-using allocators
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc-bugzilla at contacts dot eelis.net

Created attachment 34150
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34150&action=edit
testcase

Changing

  this->_M_impl._M_end_of_storage = __new_start + __len;

to

  this->_M_impl._M_end_of_storage = __new_start +
      malloc_usable_size(__new_start) / sizeof(value_type);

in vector.tcc makes the runtime of the attached program drop from 0.35 seconds
to 0.24 seconds on my machine, a 31% reduction.

This shows there is low hanging fruit for a very common operation
(vector::push_back). A proper implementation would of course only do this for
allocators that are known to be malloc frontends. :)