Add tests for roundTiesToEven rounding in Math.fround (#2021)

test/built-ins/Math/fround/ties.js

+// Copyright (C) 2019 Tiancheng "Timothy" Gu. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-math.fround
+description: Math.fround should use roundTiesToEven for conversion to binary32.
+---*/
+
+// We test five values against Math.fround, with their binary64 representation
+// shown:
+// a0 := 1.0                = 0x1p+0
+// a1 := 1.0000000596046448 = 0x1.000001p+0
+// a2 := 1.0000001192092896 = 0x1.000002p+0
+// a3 := 1.0000001788139343 = 0x1.000003p+0
+// a4 := 1.000000238418579  = 0x1.000004p+0
+// a5 := 1.0000002980232239 = 0x1.000005p+0
+// a6 := 1.0000003576278687 = 0x1.000006p+0
+// (Note: they are separated by 2 ** -24.)
+//
+// a0, a2, a4, and a6 are all representable exactly in binary32; however, while
+// a0 and a4 have even mantissas in binary32, a2 and a6 have an odd mantissa
+// when represented in that way.
+//
+// a1 is exactly halfway between a0 and a2, a3 between a2 and a4, and a5
+// between a4 and a6. By roundTiesToEven, Math.fround should favor a0 and a4
+// over a2 when they are equally close, and a4 over a6 when they are equally
+// close.
+
+var a0 = 1.0;
+var a1 = 1.0000000596046448;
+var a2 = 1.0000001192092896;
+var a3 = 1.0000001788139343;
+var a4 = 1.000000238418579;
+var a5 = 1.0000002980232239;
+var a6 = 1.0000003576278687;
+
+assert.sameValue(Math.fround(a0), a0, 'Math.fround(a0)');
+assert.sameValue(Math.fround(a1), a0, 'Math.fround(a1)');
+assert.sameValue(Math.fround(a2), a2, 'Math.fround(a2)');
+assert.sameValue(Math.fround(a3), a4, 'Math.fround(a3)');
+assert.sameValue(Math.fround(a4), a4, 'Math.fround(a4)');
+assert.sameValue(Math.fround(a5), a4, 'Math.fround(a5)');
+assert.sameValue(Math.fround(a6), a6, 'Math.fround(a6)');
+
+assert.sameValue(Math.fround(-a0), -a0, 'Math.fround(-a0)');
+assert.sameValue(Math.fround(-a1), -a0, 'Math.fround(-a1)');
+assert.sameValue(Math.fround(-a2), -a2, 'Math.fround(-a2)');
+assert.sameValue(Math.fround(-a3), -a4, 'Math.fround(-a3)');
+assert.sameValue(Math.fround(-a4), -a4, 'Math.fround(-a4)');
+assert.sameValue(Math.fround(-a5), -a4, 'Math.fround(-a5)');
+assert.sameValue(Math.fround(-a6), -a6, 'Math.fround(-a6)');