[MLIR] Fix/check for aliases for escaping memrefs in affine fusion

[bondhugula]

Summary

Check for aliases for escaping memrefs check in affine fusion pass. Fix
isEscapingMemRef to handle unknown defining ops for the memref.

[bondhugula]

Test cases fix.

[dcaballe]

LGTM

mlir/lib/Dialect/Affine/Transforms/LoopFusion.cpp

gatherProducerConsumerMemrefs(unsigned srcId, unsigned dstId,
                               DenseSet<Value> &producerConsumerMemrefs) {
   auto *dstNode = mdg->getNode(dstId);
   auto *srcNode = mdg->getNode(srcId);
   gatherProducerConsumerMemrefs(srcNode->stores, dstNode->loads,
                                 producerConsumerMemrefs);
 }
 
 /// A memref escapes the function if either:
-///   1. it is a function argument, or
-///   2. it is used by a non-affine op (e.g., std load/store, std
-///   call, etc.)
-/// FIXME: Support alias creating ops like memref view ops.
+///   1. it (or its alias) is a block argument, or
+///   2. created by an op not known to guarantee alias freedom,
+///   3. it (or its alias) is used by a non-affine op (e.g., call op, memref
+///      load/store ops, alias creating ops, unknown ops, etc.); such ops
+///      do not deference the memref in an affine way.
 static bool isEscapingMemref(Value memref) {
-  // Check if 'memref' escapes because it's a block argument.
-  if (memref.isa<BlockArgument>())
+  Operation *defOp = memref.getDefiningOp();
+  // Check if 'memref' is a block argument.
+  if (!defOp)
     return true;
 
-  // Check if 'memref' escapes through a non-affine op (e.g., std load/store,
-  // call op, etc.). This already covers aliases created from this.
+  // Check if this is defined to be an alias of another memref.
+  if (auto viewOp = dyn_cast<mlir::ViewLikeOpInterface>(defOp))
+    if (isEscapingMemref(viewOp.getViewSource()))
+      return true;
+
+  // Any op besides allocating ops wouldn't guarantee alias freedom
+  if (!hasSingleEffect<mlir::MemoryEffects::Allocate>(defOp, memref))
+    return true;
+
+  // Check if 'memref' is used by a non-deferencing op (including unknown ones)
+  // (e.g., call ops, alias creating ops, etc.).
   for (Operation *user : memref.getUsers())
     if (!isa<AffineMapAccessInterface>(*user))
       return true;
   return false;
 }
 
 /// Returns in 'escapingMemRefs' the memrefs from affine store ops in node 'id'
-/// that escape the function.
+/// that escape the function or are accessed by non-affine ops.
 void gatherEscapingMemrefs(unsigned id, MemRefDependenceGraph *mdg,
                            DenseSet<Value> &escapingMemRefs) {
   auto *node = mdg->getNode(id);
   for (Operation *storeOp : node->stores) {
     auto memref = cast<AffineWriteOpInterface>(storeOp).getMemRef();
     if (escapingMemRefs.count(memref))
       continue;
     if (isEscapingMemref(memref))

mlir/test/Transforms/loop-fusion-3.mlir

affine.for %arg13 = 0 to 20 {
     }
   }
   return
 }
 // CHECK:         affine.for
 // CHECK:         affine.for
 // CHECK-NOT:     affine.for
 
+// CHECK-LABEL: func @alias_escaping_memref
+func.func @alias_escaping_memref(%a : memref<2x5xf32>) {
+  %cst = arith.constant 0.000000e+00 : f32
+  %alias = memref.reinterpret_cast %a to offset: [0], sizes: [10], strides: [1] : memref<2x5xf32> to memref<10xf32>
+  affine.for %i0 = 0 to 10 {
+    affine.store %cst, %alias[%i0] : memref<10xf32>
+  }
+
+  affine.for %i1 = 0 to 10 {
+    %0 = affine.load %alias[%i1] : memref<10xf32>
+  }
+  // Fusion happens, but memref isn't privatized since %alias is an alias of a
+  // function argument.
+  // CHECK:       memref.reinterpret_cast
+  // CHECK-NEXT:  affine.for
+  // CHECK-NEXT:    affine.store %{{.*}}, %{{.*}}[%{{.*}}] : memref<10xf32>
+  // CHECK-NEXT:    affine.load %{{.*}}[%{{.*}}] : memref<10xf32>
+  // CHECK-NEXT:  }
+  // CHECK-NOT:   affine.for
+
+  return
+}
+
+// CHECK-LABEL: func @unknown_memref_def_op
+func.func @unknown_memref_def_op() {
+  %cst = arith.constant 0.000000e+00 : f32
+  %may_alias = call @bar() : () -> memref<10xf32>
+  affine.for %i0 = 0 to 10 {
+    affine.store %cst, %may_alias[%i0] : memref<10xf32>
+  }
+
+  affine.for %i1 = 0 to 10 {
+    %0 = affine.load %may_alias[%i1] : memref<10xf32>
+  }
+  // Fusion happens, but memref isn't privatized since %may_alias's origin is
+  // unknown.
+  // CHECK:       call
+  // CHECK-NEXT:  affine.for
+  // CHECK-NEXT:    affine.store %{{.*}}, %{{.*}}[%{{.*}}] : memref<10xf32>
+  // CHECK-NEXT:    affine.load %{{.*}}[%{{.*}}] : memref<10xf32>
+  // CHECK-NEXT:  }
+  // CHECK-NOT:   affine.for
+
+  return
+}
+func.func private @bar() -> memref<10xf32>
+
+
 // Add further tests in mlir/test/Transforms/loop-fusion-4.mlir