js_runtime: Improve hashCode for double values
Change-Id: Ibe97c130eb6658fadec1010dd9ff3292488ffc1d
Reviewed-on: https://dart-review.googlesource.com/c/88760
Reviewed-by: Jenny Messerly <jmesserly@google.com>
Commit-Queue: Stephen Adams <sra@google.com>
diff --git a/pkg/dev_compiler/tool/input_sdk/private/interceptors.dart b/pkg/dev_compiler/tool/input_sdk/private/interceptors.dart
index fb34f25..445f0cb 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/interceptors.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/interceptors.dart
@@ -8,7 +8,7 @@
import 'dart:_internal' hide Symbol;
import 'dart:_js_helper';
import 'dart:_foreign_helper' show JS, JSExportName;
-import 'dart:math' show Random;
+import 'dart:math' show Random, ln2;
import 'dart:_runtime' as dart;
part 'js_array.dart';
diff --git a/pkg/dev_compiler/tool/input_sdk/private/js_number.dart b/pkg/dev_compiler/tool/input_sdk/private/js_number.dart
index f539a10..2a1fb74 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/js_number.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/js_number.dart
@@ -218,7 +218,40 @@
}
@notNull
- int get hashCode => JS('int', '# & 0x1FFFFFFF', this);
+ int get hashCode {
+ int intValue = JS('int', '# | 0', this);
+ // Fast exit for integers in signed 32-bit range. Masking converts -0.0 to 0
+ // and ensures that result fits in JavaScript engine's Smi range.
+ if (this == intValue) return 0x1FFFFFFF & intValue;
+
+ // We would like to access the exponent and mantissa as integers but there
+ // are no JavaScript operations that do this, so use log2-floor-pow-divide
+ // to extract the values.
+ num absolute = JS('num', 'Math.abs(#)', this);
+ num lnAbsolute = JS('num', 'Math.log(#)', absolute);
+ num log2 = lnAbsolute / ln2;
+ // Floor via '# | 0' converts NaN to zero so the final result is not NaN.
+ int floorLog2 = JS('int', '# | 0', log2);
+ num factor = JS('num', 'Math.pow(2, #)', floorLog2);
+ num scaled = absolute < 1 ? absolute / factor : factor / absolute;
+ // [scaled] is in the range [0.5, 1].
+
+ // Multiply and truncate to pick up all the mantissa bits. Multiplying by
+ // 0x20000000000000 (which has 53 zero bits) converts the mantissa into an
+ // integer. There are interesting subsets where all the bit variance is in
+ // the most significant bits of the mantissa (e.g. 0.5, 0.625, 0.75), so we
+ // need to mix in the most significant bits. We do this by scaling with a
+ // constant that has many bits set to use the multiplier to mix in bits from
+ // all over the mantissa into low positions.
+ num rescaled1 = scaled * 0x20000000000000;
+ num rescaled2 = scaled * 0x0C95A6C285A6C9;
+ int d1 = JS('int', '# | 0', rescaled1);
+ int d2 = JS('int', '# | 0', rescaled2);
+ // Mix in exponent to distinguish e.g. 1.25 from 2.5.
+ int d3 = floorLog2;
+ int h = 0x1FFFFFFF & ((d1 + d2) * (601 * 997) + d3 * (1259));
+ return h;
+ }
@notNull
JSNumber operator -() => JS('num', r'-#', this);
diff --git a/sdk/lib/_internal/js_runtime/lib/interceptors.dart b/sdk/lib/_internal/js_runtime/lib/interceptors.dart
index a9fcb75..287ec00 100644
--- a/sdk/lib/_internal/js_runtime/lib/interceptors.dart
+++ b/sdk/lib/_internal/js_runtime/lib/interceptors.dart
@@ -52,7 +52,7 @@
JS_EMBEDDED_GLOBAL,
JS_INTERCEPTOR_CONSTANT,
JS_STRING_CONCAT;
-import 'dart:math' show Random;
+import 'dart:math' show Random, ln2;
part 'js_array.dart';
part 'js_number.dart';
diff --git a/sdk/lib/_internal/js_runtime/lib/js_number.dart b/sdk/lib/_internal/js_runtime/lib/js_number.dart
index 58e5e55..fed5dd8 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_number.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_number.dart
@@ -241,7 +241,40 @@
}
}
- int get hashCode => JS('int', '# & 0x1FFFFFFF', this);
+ int get hashCode {
+ int intValue = JS('int', '# | 0', this);
+ // Fast exit for integers in signed 32-bit range. Masking converts -0.0 to 0
+ // and ensures that result fits in JavaScript engine's Smi range.
+ if (this == intValue) return 0x1FFFFFFF & intValue;
+
+ // We would like to access the exponent and mantissa as integers but there
+ // are no JavaScript operations that do this, so use log2-floor-pow-divide
+ // to extract the values.
+ num absolute = JS('num', 'Math.abs(#)', this);
+ num lnAbsolute = JS('num', 'Math.log(#)', absolute);
+ num log2 = lnAbsolute / ln2;
+ // Floor via '# | 0' converts NaN to zero so the final result is not NaN.
+ int floorLog2 = JS('int', '# | 0', log2);
+ num factor = JS('num', 'Math.pow(2, #)', floorLog2);
+ num scaled = absolute < 1 ? absolute / factor : factor / absolute;
+ // [scaled] is in the range [0.5, 1].
+
+ // Multiply and truncate to pick up all the mantissa bits. Multiplying by
+ // 0x20000000000000 (which has 53 zero bits) converts the mantissa into an
+ // integer. There are interesting subsets where all the bit variance is in
+ // the most significant bits of the mantissa (e.g. 0.5, 0.625, 0.75), so we
+ // need to mix in the most significant bits. We do this by scaling with a
+ // constant that has many bits set to use the multiplier to mix in bits from
+ // all over the mantissa into low positions.
+ num rescaled1 = scaled * 0x20000000000000;
+ num rescaled2 = scaled * 0x0C95A6C285A6C9;
+ int d1 = JS('int', '# | 0', rescaled1);
+ int d2 = JS('int', '# | 0', rescaled2);
+ // Mix in exponent to distinguish e.g. 1.25 from 2.5.
+ int d3 = floorLog2;
+ int h = 0x1FFFFFFF & ((d1 + d2) * (601 * 997) + d3 * (1259));
+ return h;
+ }
JSNumber operator -() => JS('num', r'-#', this);
