[BUFFER] Smarter slice to detect compactness (#587)

* [BUFFER] Smarter slice to detect compactness

* move simplify of begins early

src/lang/buffer.cc

@@ -20,6 +20,13 @@ Array<Expr> GetStrides(Array<Expr> shape) {
   return Array<Expr>(vec.rbegin(), vec.rend());
 }
 
+Array<Expr> SimplifyArray(Array<Expr> array) {
+  for (size_t i = 0; i < array.size(); ++i) {
+    array.Set(i, ir::Simplify(array[i]));
+  }
+  return array;
+}
+
 Buffer decl_buffer(Array<Expr> shape,
                    Type dtype,
                    std::string name) {
@@ -302,6 +309,7 @@ Buffer Buffer::MakeStrideView() const {
 
 Buffer Buffer::MakeSlice(Array<Expr> begins, Array<Expr> extents) const {
   const BufferNode* n = operator->();
+  begins = SimplifyArray(begins);
   Expr elem_offset = ir::Simplify(ElemOffset(n, begins));
   Array<Expr> strides = n->strides;
   if (strides.size() == 0) {

src/pass/storage_flatten.cc

@@ -321,7 +321,8 @@ class StorageFlattener : public IRMutator {
     Buffer slice = be.buffer.MakeSlice(begins, extents);
     if (buffer->strides.size() == 0) {
       CHECK_EQ(slice->strides.size(), 0U)
-          << "Trying to bind compact buffer to strided one";
+          << "Trying to bind compact buffer to strided one strides="
+          << slice->strides;
     } else {
       slice = slice.MakeStrideView();
     }