Introduce getter p on double in the VM to control fractional precision.

tests/standalone/fixed_precision_double_test.dart

+// Copyright (c) 2016, 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.
+
+import "package:expect/expect.dart";
+
+bool TestAssociativity(bool chop) {

[Florian Schneider, 2016/08/03 23:35:58]

For testing here maybe pass in a function chop (or identity) in here as a
parameter f to avoid duplication of the literals/expressions:


bool TestAssociativity(Function f) {
  double a = f(1234.567);
...

[regis, 2016/08/04 18:22:31]

Done.

+  // Example from https://en.wikipedia.org/wiki/Floating_point
+  // Test that (a + b) + c == a + (b + c).
+  double a = chop ? 1234.567.p : 1234.567;  // Chop literals.
+  double b = chop ? 45.67834.p : 45.67834;
+  double c = chop ? 0.0004.p : 0.0004;
+  double x = (a + b) + c;  // Chop result of multiplication or division only.
+  double y = a + (b + c);
+  print("x: $x");
+  print("y: $y");
+  return x == y;
+}
+
+bool TestDistributivity(bool chop) {
+  // Example from https://en.wikipedia.org/wiki/Floating_point
+  // Test that (a + b)*c == a*c + b*c.
+  double a = chop ? 1234.567.p : 1234.567;  // Chop literals.
+  double b = chop ? 1.234567.p : 1.234567;
+  double c = chop ? 3.333333.p : 3.333333;
+  double x = chop ? ((a + b)*c).p : (a + b)*c;  // Chop result of multiplication.
+  double y = (chop ? (a*c).p : (a*c)) + (chop ? (b*c).p : (b*c));
+  print("x: $x");
+  print("y: $y");
+  return x == y;
+}
+
+main() {
+  print("without chopping fractional bits:");
+  Expect.isFalse(TestAssociativity(false));

[Florian Schneider, 2016/08/03 23:35:58]

e.g. Expect.isFalse(TestAssociativity((x) => x));

[regis, 2016/08/04 18:22:31]

Done.

+  Expect.isFalse(TestDistributivity(false));
+  print("with chopping fractional bits:");
+  Expect.isTrue(TestAssociativity(true));

[Florian Schneider, 2016/08/03 23:35:58]

e.g. Expect.isTrue(TestAssociativity((x) => x.p));

[regis, 2016/08/04 18:22:31]

Done.

+  Expect.isTrue(TestDistributivity(true));
+}