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Subject [GitHub] [incubator-tvm] hzfan commented on a change in pull request #5092: [TIR][PASS] dtype rewrite for indexing variables
Date Mon, 30 Mar 2020 14:49:39 GMT
hzfan commented on a change in pull request #5092: [TIR][PASS] dtype rewrite for indexing variables
URL: https://github.com/apache/incubator-tvm/pull/5092#discussion_r400252957
 
 

 ##########
 File path: src/tir/pass/narrow_datatype.cc
 ##########
 @@ -0,0 +1,373 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/*!
+ * \file narrow_datatype.cc
+ * \brief narrow the datatype of indexing vars
+ */
+
+#include <tvm/tir/ir_pass.h>
+#include <tvm/tir/op.h>
+#include "../../arith/ir_mutator_with_analyzer.h"
+#include "../../arith/ir_visitor_with_analyzer.h"
+
+namespace tvm {
+namespace tir {
+
+// This pass narrows indexing expressions (like StoreNode::Index)
+// that trivially fit into i32 to i32. Considering that i32 indices
+// may be more efficient on some backends (while i64 may be more
+// efficient on others, like llvm), we may want this pass when i32
+// indices are more efficient.
+//
+// For Var v, we determine its dtype by examining all the PrimExpr
+// that contains v, denoted by E = {e_0 = v, e_1, e_2, ..., e_k}.
+// If all expressions in E fit into i32, then we think v can be narrowed
+// to i32.
+//
+// To make an indexing expression i32, we must make sure that every
+// component of that expression is of dtype i32. So besides Var, we
+// rewrite the following inside an indexing expression
+// - Var
+// - IntImm
+// - Cast
+//
+// Algorithm:
+// - Use DataTypeVisitor to determine whether a Var can be narrowed or not.
+// - Use DataTypeRewritter to rewrite the components of an indexing expression.
+
+using arith::Analyzer;
+using arith::IRMutatorWithAnalyzer;
+using arith::ConstIntBound;
+
+class DataTypeVisitor final : public StmtExprVisitor {
+ public:
+  explicit DataTypeVisitor(int target_bits)
+    : bits_(target_bits), target_bits_(target_bits) {}
+
+  void VisitExpr(const PrimExpr& e) {
+    if (e.dtype().is_int()) {
+      int bits = max_bits_;
+      ConstIntBound bound = analyzer_.const_int_bound(e);
+      int64_t ubound = Downcast<IntImm, PrimExpr>(max_value(DataType::Int(target_bits_)))->value;
+      int64_t lbound = Downcast<IntImm, PrimExpr>(min_value(DataType::Int(target_bits_)))->value;
+      if (e.dtype().bits() <= target_bits_ ||
 
 Review comment:
   Yes, I think this pass aligns with what you said. It only narrows and never promotes. This
code means lower bits fits into higher bits. For example, Consider `e` is `i.i64 <= j.i64`,
a bool expression with only 1 bit. So `e` fits into i32 and does not hinder narrowing i to
i32.
   
   https://github.com/apache/incubator-tvm/pull/5092/files#diff-98ae729cf00e30cff311ed80b4a25df9R129
ensures dtype promotion does not occcur.

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