Fix problem with SCC sort

[andrew.w.kaylor]

Summary

This patch fixes a minor problem with SCC sorting wherein a value is compared to itself. The problem has been present since this code was introduced in D114204. It appears to be a simple typo.

[arsenm]

lgtm but is a test possible?

[andrew.w.kaylor]

In D157938#4587091, @arsenm wrote:

lgtm but is a test possible?

I would guess that it is possible to write a test for this, but I don't know this code well enough to do it. When this code was added, llvm/test/Transforms/SampleProfile/profile-context-order.ll was updated to test it, but clearly that update to the test didn't depend on this comparison. Perhaps @hoy could suggest a test?

llvm/include/llvm/ADT/SCCIterator.h

bool unionGroups(const EdgeType *Edge) {
     NodeInfo *G1 = find(&NodeInfoMap[Edge->Source]);
     NodeInfo *G2 = find(&NodeInfoMap[Edge->Target]);
 
     // If the edge forms a cycle, do not add it to MST
     if (G1 == G2)
       return false;
 
     // Make the smaller rank tree a direct child or the root of high rank tree.
-    if (G1->Rank < G1->Rank)
+    if (G1->Rank < G2->Rank)
       G1->Group = G2;
     else {
       G2->Group = G1;
       // If the ranks are the same, increment root of one tree by one.
       if (G1->Rank == G2->Rank)
         G2->Rank++;
     }
     return true;