[llvm] [LoopInterchange] Add metadata to control loop-interchange (PR #127474)

[Michael Kruse via llvm-commits]

================
@@ -569,6 +609,152 @@ struct LoopInterchange {
 
     return true;
   }
+
+  bool processEnabledLoop(SmallVectorImpl<Loop *> &LoopList,
+                          std::vector<std::vector<char>> &DependencyMatrix,
+                          const DenseMap<const Loop *, unsigned> &CostMap) {
+    bool Changed = false;
+
+    // Manage the index so that LoopList[Loop2Index[L]] == L for each loop L.
+    DenseMap<Loop *, unsigned> Loop2Index;
+    for (unsigned I = 0; I != LoopList.size(); I++)
+      Loop2Index[LoopList[I]] = I;
+
+    // Hold outer loops to be exchanged (i.e., loops that have
+    // "llvm.loop.interchange.enable" is true), in the current nest order.
+    SmallVector<Loop *, 4> Worklist;
+
+    // Helper funciton to try to add a new loop into the Worklist. Return false
+    // if there is a duplicate in the loop to be interchanged.
+    auto AddLoopIfEnabled = [&](Loop *L) {
+      if (findMetadata(L) == true) {
+        if (!Worklist.empty()) {
+          // Because the loops are sorted in the order of the current nest, it
+          // is sufficient to compare with the last element.
+          unsigned InnerLoopId = Loop2Index[Worklist.back()] + 1;
+          unsigned OuterLoopId = Loop2Index[L];
+          if (OuterLoopId <= InnerLoopId) {
+            ORE->emit([&]() {
+              return OptimizationRemarkMissed(DEBUG_TYPE, "AmbiguousOrder",
+                                              L->getStartLoc(), L->getHeader())
+                     << "The loops to be interchanged are overlapping.";
+            });
+            return false;
+          }
+        }
+        Worklist.push_back(L);
+      }
+      return true;
+    };
+
+    // Initialize Worklist. To process the loops in inner-loop-first order, add
+    // them to the worklist in the outer-loop-first order.
+    for (unsigned I = 0; I != LoopList.size(); I++)
+      if (!AddLoopIfEnabled(LoopList[I]))
+        return Changed;
+
+    // Set an upper bound of the number of transformations to avoid infinite
+    // loop. There is no deep meaning behind the current value (square of the
+    // size of LoopList).
+    // TODO: Is this really necessary?
----------------
Meinersbur wrote:

If the metadata is properly remove after having applied the interchange, then not. Consider an assertion for debug builds when that happens.

https://github.com/llvm/llvm-project/pull/127474