[RR_GRAPH] Fixed Bug in LLVM17 Build

The std::sort function requires operands to be "pointer-like", this
requires that de-referencing a const pointer should be allowed.

This was not an issue in the past, but in LLVM17 this property is
required. The edge_sort_iterator did not follow this property.

Fixing this by naively allowing this property led to a 10% performance
degredation due to requiring pointers to be allocated and deallocated.

To fix this for now, made the sort use std::stable_sort instead. The
performance degredation is quite small and may be worth it.

Author: AlexandreSinger

libs/librrgraph/src/base/rr_graph_storage.cpp

@@ -155,6 +155,13 @@ class edge_sort_iterator {
     using pointer = edge_swapper*;
     using difference_type = ssize_t;
 
+    // In order for this class to be used as an iterator within the std library,
+    // it needs to "act" like a pointer. One thing that it should do is that a
+    // const variable of this type should be de-referenceable. Therefore, this
+    // method should be const method; however, this requires modifying the class
+    // and may yield worst performance. For now the std::stable_sort allows this
+    // but in the future it may not. If this breaks, this is why.
+    // See issue #2517 and PR #2522
     edge_swapper& operator*() {
         return this->swapper_;
     }
@@ -419,7 +426,7 @@ size_t t_rr_graph_storage::count_rr_switches(
     // values.
     //
     // This sort is safe to do because partition_edges() has not been invoked yet.
-    std::sort(
+    std::stable_sort(
         edge_sort_iterator(this, 0),
         edge_sort_iterator(this, edge_dest_node_.size()),
         edge_compare_dest_node());
@@ -527,7 +534,7 @@ void t_rr_graph_storage::partition_edges(const vtr::vector<RRSwitchId, t_rr_swit
     //    by assign_first_edges()
     //  - Edges within a source node have the configurable edges before the
     //    non-configurable edges.
-    std::sort(
+    std::stable_sort(
         edge_sort_iterator(this, 0),
         edge_sort_iterator(this, edge_src_node_.size()),
         edge_compare_src_node_and_configurable_first(rr_switches));