vtr_memory: add operator== for aligned_allocator

This fixes building with GCC11, which previously errored:

In file included from /usr/include/c++/11.1.0/bits/stl_algobase.h:59,
                 from /usr/include/c++/11.1.0/vector:60,
                 from /builddir/vtr-verilog-to-routing/libs/libvtrutil/src/vtr_linear_map.h:3,
                 from /builddir/vtr-verilog-to-routing/vpr/src/timing/PostClusterDelayCalculator.h:3,
                 from /builddir/vtr-verilog-to-routing/vpr/src/timing/AnalysisDelayCalculator.h:4,
                 from /builddir/vtr-verilog-to-routing/vpr/src/analysis/timing_reports.h:5,
                 from /builddir/vtr-verilog-to-routing/vpr/src/analysis/timing_reports.cpp:1:
/usr/include/c++/11.1.0/bits/stl_vector.h: In instantiation of ‘void std::vector<_Tp, _Alloc>::swap(std::vector<_Tp, _Alloc>&) [with _Tp = t_rr_node_data; _Alloc = vtr::aligned_allocator<t_rr_node_data>]’:
/usr/include/c++/11.1.0/bits/allocator.h:308:28:   required from ‘static bool std::__shrink_to_fit_aux<_Tp, true>::_S_do_it(_Tp&) [with _Tp = std::vector<t_rr_node_data, vtr::aligned_allocator<t_rr_node_data> >]’
/usr/include/c++/11.1.0/bits/vector.tcc:693:56:   required from ‘bool std::vector<_Tp, _Alloc>::_M_shrink_to_fit() [with _Tp = t_rr_node_data; _Alloc = vtr::aligned_allocator<t_rr_node_data>]’
/usr/include/c++/11.1.0/bits/stl_vector.h:990:9:   required from ‘void std::vector<_Tp, _Alloc>::shrink_to_fit() [with _Tp = t_rr_node_data; _Alloc = vtr::aligned_allocator<t_rr_node_data>]’
/builddir/vtr-verilog-to-routing/vpr/src/route/rr_graph_storage.h:442:36:   required from here
/usr/include/c++/11.1.0/bits/stl_vector.h:1483:9: error: no match for ‘operator==’ (operand types are ‘std::_Vector_base<t_rr_node_data, vtr::aligned_allocator<t_rr_node_data> >::_Tp_alloc_type’ {aka ‘std::allocator_traits<vtr::aligned_allocator<t_rr_node_data> >::rebind_alloc<t_rr_node_data>’} and ‘std::_Vector_base<t_rr_node_data, vtr::aligned_allocator<t_rr_node_data> >::_Tp_alloc_type’ {aka ‘std::allocator_traits<vtr::aligned_allocator<t_rr_node_data> >::rebind_alloc<t_rr_node_data>’})
 1483 |         __glibcxx_assert(_Alloc_traits::propagate_on_container_swap::value
      |         ^~~~~~~~~~~~~~~~

This is caused by gcc commit 10f51543b, which adds compile-time checking
to __glibcxx_assert regardless of the _GLIBCXX_ASSERTIONS preprocessor
definition.

An assert in std::vector::swap that compares the two vectors' allocators
was previously completely removed by the preprocessor, but now expands
to a static assert. Since no equality operator is defined for
aligned_allocator, any instantiation of a vector with this allocator
fails to compile.

Author: Xiretza

libs/libvtrutil/src/vtr_memory.h

@@ -136,6 +136,16 @@ struct aligned_allocator {
     }
 };
 
+/**
+ * @brief compare two aligned_allocators.
+ *
+ * Since the allocator doesn't have any internal state, all allocators for a given type are the same.
+ */
+template<typename T>
+bool operator==(const aligned_allocator<T>&, const aligned_allocator<T>&) {
+    return true;
+}
+
 } // namespace vtr
 
 #endif