[llvm] [LoopInterchange] Consider forward/backward dependency in vectorize heuristic (PR #133672)

[Michael Kruse via llvm-commits]

================
@@ -228,9 +260,47 @@ static bool populateDependencyMatrix(CharMatrix &DepMatrix, unsigned Level,
           Dep.push_back('I');
         }
 
+        // Test whether the dependency is forward or not.
+        bool IsKnownForward = true;
+        if (Src->getParent() != Dst->getParent()) {
+          // In general, when Src and Dst are in different BBs, the execution
+          // order of them within a single iteration is not guaranteed. Treat
+          // conservatively as not-forward dependency in this case.
+          IsKnownForward = false;
+        } else {
+          // Src and Dst are in the same BB. If they are the different
+          // instructions, Src should appear before Dst in the BB as they are
+          // stored to MemInstr in that order.
+          assert((Src == Dst || inThisOrder(Src, Dst)) &&
+                 "Unexpected instructions");
+
+          // If the Dependence object is reversed (due to normalization), it
+          // represents the dependency from Dst to Src, meaning it is a backward
+          // dependency. Otherwise it should be a forward dependency.
+          bool IsReversed = D->getSrc() != Src;
+          if (IsReversed)
+            IsKnownForward = false;
+        }
+
+        // Initialize the last element.
+        Dep.push_back('<');
+
+        // The last element should express the "summary" among one or more
+        // direction vectors whose first N elements are the same (where N is
+        // the depth of the loop nest). Hence we exclude the last element from
+        // the Seen map.
+        auto [Ite, Inserted] = Seen.try_emplace(
+            StringRef(Dep.data(), Dep.size() - 1), DepMatrix.size());
+
         // Make sure we only add unique entries to the dependency matrix.
-        if (Seen.insert(StringRef(Dep.data(), Dep.size())).second)
+        if (Inserted)
           DepMatrix.push_back(Dep);
+
+        // If we cannot prove that this dependency is forward, change the last
+        // element of the corresponding entry. Note that the existing entry in
----------------
Meinersbur wrote:

```suggestion
        // If we cannot prove that this dependency is forward, change the last
        // element of the corresponding entry. Since a `[... *]` dependency includes a `[... <]` dependency, we do not need to keep both and change the existing entry instead. 
```
Just some additional explanation

https://github.com/llvm/llvm-project/pull/133672