[llvm] [IndVars] Fix strict weak ordering violation (PR #108947)

[Florian Hahn via llvm-commits]

================
@@ -1788,28 +1788,37 @@ bool IndVarSimplify::predicateLoopExits(Loop *L, SCEVExpander &Rewriter) {
     return false;
   };
 
+  // Make sure all exits dominate the latch. This means there is a linear chain
+  // of exits. We check this before sorting so we have a total order.
+  BasicBlock *Latch = L->getLoopLatch();
+  for (BasicBlock *ExitingBB : ExitingBlocks)
+    if (!DT->dominates(ExitingBB, Latch))
+      return false;
+
   // If we have any exits which can't be predicated themselves, than we can't
   // predicate any exit which isn't guaranteed to execute before it.  Consider
   // two exits (a) and (b) which would both exit on the same iteration.  If we
   // can predicate (b), but not (a), and (a) preceeds (b) along some path, then
   // we could convert a loop from exiting through (a) to one exiting through
   // (b).  Note that this problem exists only for exits with the same exit
   // count, and we could be more aggressive when exit counts are known inequal.
-  llvm::sort(ExitingBlocks,
-            [&](BasicBlock *A, BasicBlock *B) {
-              // std::sort sorts in ascending order, so we want the inverse of
-              // the normal dominance relation, plus a tie breaker for blocks
-              // unordered by dominance.
-              if (DT->properlyDominates(A, B)) return true;
-              if (DT->properlyDominates(B, A)) return false;
-              return A->getName() < B->getName();
-            });
-  // Check to see if our exit blocks are a total order (i.e. a linear chain of
-  // exits before the backedge).  If they aren't, reasoning about reachability
-  // is complicated and we choose not to for now.
-  for (unsigned i = 1; i < ExitingBlocks.size(); i++)
-    if (!DT->dominates(ExitingBlocks[i-1], ExitingBlocks[i]))
+  llvm::sort(ExitingBlocks, [&](BasicBlock *A, BasicBlock *B) {
+    // std::sort sorts in ascending order, so we want the inverse of
----------------
fhahn wrote:

```suggestion
    // llvm::sort sorts in ascending order, so we want the inverse of
```
?


https://github.com/llvm/llvm-project/pull/108947