[Task] : Add comments + enhance check for index in parent block of reshape.

Author: sahas3

mlir/lib/Dialect/MemRef/IR/MemRefOps.cpp

@@ -1083,17 +1083,34 @@ struct DimOfMemRefReshape : public OpRewritePattern<DimOp> {
       return rewriter.notifyMatchFailure(
           dim, "Dim op is not defined by a reshape op.");
 
+    // dim of a memref reshape can be folded if dim.getIndex() dominates the
+    // reshape. Instead of using `DominanceInfo` (which is usually costly) we
+    // cheaply check that either of the following conditions hold:
+    //      1. dim.getIndex() is defined in the same block as reshape but before
+    //      reshape.
+    //      2. dim.getIndex() is defined in a parent block of
+    //      reshape.
+
+    // Check condition 1
     if (dim.getIndex().getParentBlock() == reshape->getBlock()) {
       if (auto *definingOp = dim.getIndex().getDefiningOp()) {
-        if (reshape->isBeforeInBlock(definingOp))
+        if (reshape->isBeforeInBlock(definingOp)) {
           return rewriter.notifyMatchFailure(
               dim,
               "dim.getIndex is not defined before reshape in the same block.");
-      } // else dim.getIndex is a block argument to reshape->getBlock
-    } else if (!dim.getIndex().getParentRegion()->isProperAncestor(
-                   reshape->getParentRegion()))
+        }
+      } // else dim.getIndex is a block argument to reshape->getBlock and
+        // dominates reshape
+    } // Check condition 2
+    else if (dim->getBlock() != reshape->getBlock() &&
+             !dim.getIndex().getParentRegion()->isProperAncestor(
+                 reshape->getParentRegion())) {
+      // If dim and reshape are in the same block but dim.getIndex() isn't, we
+      // already know dim.getIndex() dominates reshape without calling
+      // `isProperAncestor`
       return rewriter.notifyMatchFailure(
           dim, "dim.getIndex does not dominate reshape.");
+    }
 
     // Place the load directly after the reshape to ensure that the shape memref
     // was not mutated.