[vm/tfa] Infer type for indexing into a const list

Before:
DartNTZ.NTZ32(RunTime): 351.9357010381841 us.
DartNTZ.NTZ64(RunTime): 351.99127771911304 us.

After:
DartNTZ.NTZ32(RunTime): 150.19256867162275 us.
DartNTZ.NTZ64(RunTime): 149.48622578475337 us.

Issue: #37789
Change-Id: Ie8b16b2602a15140df9f0f21199ee9eb2bbf158c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/115012
Reviewed-by: Aart Bik <ajcbik@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Samir Jindel <sjindel@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

pkg/vm/lib/transformations/type_flow/summary_collector.dart

@@ -945,9 +945,16 @@ class SummaryCollector extends RecursiveVisitor<TypeExpr> {
 
   @override
   TypeExpr visitMethodInvocation(MethodInvocation node) {
-    final receiver = _visit(node.receiver);
+    final receiverNode = node.receiver;
+    final receiver = _visit(receiverNode);
     final args = _visitArguments(receiver, node.arguments);
     final target = node.interfaceTarget;
+    if (receiverNode is ConstantExpression && node.name.name == '[]') {
+      Constant constant = receiverNode.constant;
+      if (constant is ListConstant) {
+        return _handleIndexingIntoListConstant(constant);
+      }
+    }
     if (target == null) {
       if (node.name.name == '==') {
         assertx(args.values.length == 2);
@@ -993,6 +1000,24 @@ class SummaryCollector extends RecursiveVisitor<TypeExpr> {
     }
   }
 
+  TypeExpr _handleIndexingIntoListConstant(ListConstant list) {
+    final elementTypes = new Set<Type>();
+    for (var element in list.entries) {
+      elementTypes.add(constantAllocationCollector.typeFor(element));
+    }
+    switch (elementTypes.length) {
+      case 0:
+        return new Type.empty();
+      case 1:
+        return elementTypes.single;
+      default:
+        final join = new Join(null, list.typeArgument);
+        join.values.addAll(elementTypes);
+        _summary.add(join);
+        return join;
+    }
+  }
+
   @override
   TypeExpr visitPropertyGet(PropertyGet node) {
     var receiver = _visit(node.receiver);

pkg/vm/testcases/transformations/type_flow/summary_collector/constants.dart

@@ -0,0 +1,17 @@
+// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+smiLiteral() => 42;
+
+intLiteral() => 0x8000000000000000;
+
+strLiteral() => 'abc';
+
+const _constList1 = [1, 2, 3];
+indexingIntoConstantList1(int i) => _constList1[i];
+
+const _constList2 = ['hi', 33, null, -5];
+indexingIntoConstantList2(int i) => _constList2[i];
+
+main() {}

pkg/vm/testcases/transformations/type_flow/summary_collector/constants.dart.expect

@@ -0,0 +1,25 @@
+------------ #lib::smiLiteral ------------
+
+RESULT: _T (dart.core::_Smi)
+------------ #lib::intLiteral ------------
+
+RESULT: _T (dart.core::int*)+
+------------ #lib::strLiteral ------------
+
+RESULT: _T (dart.core::_OneByteString)
+------------ #lib::indexingIntoConstantList1 ------------
+%i = _Parameter #0 [_T (dart.core::int*)+?]
+RESULT: _T (dart.core::_Smi)
+------------ #lib::indexingIntoConstantList2 ------------
+%i = _Parameter #0 [_T (dart.core::int*)+?]
+t1 = _Join [dart.core::Object*] (_T (dart.core::_OneByteString), _T (dart.core::_Smi), _T {}?)
+RESULT: t1
+------------ #lib::main ------------
+
+RESULT: _T {}?
+------------ #lib::_constList1 ------------
+
+RESULT: _T (dart.core::_ImmutableList)
+------------ #lib::_constList2 ------------
+
+RESULT: _T (dart.core::_ImmutableList)