5c582f82f0
`int` variables can attain NaN values because web int arithmetic
implemented by JavaScript numbers (doubles) is not closed under many
operations. It is possible get NaN using only addition:
int a = 1, b = -1;
while (a + a != a) { a += a; b += b; }
int nan = a + b;
On the VM, a, b and nan are all zero.
On the web, a, b and nan are Infinity, -Infinity and NaN, respectively.
Since NaN can leak into int arithmetic, is it helpful if bounds checks
catch NaN indexes. NaN compares false in any comparison, so a test
of the form
if (index < 0 || index >= a.length) throw ioore(a, index);
fails to detect a NaN value of `index`.
This is fixed by negating the comparisons, and applying De Morgan's law:
if (!(index >= 0 && index < a.length)) throw ioore(a, index);
These changes have been applied to JSArray.[], JSArray.[]= and String.[]
For dart2js the change is a little more involved. Primitive indexing is
lowered to code with a HBoundsCheck check instruction. The code generated
for the instruction now uses, e.g. `!(i>=0)` instead of `i<0`.
This leads to a small code size regression.
There is no regression at -O4 since bounds checks are omitted at -O4.
At -O3 (where the regression is largest) the regression is
0.01% for cm
0.06% for flutter gallery -- array-heavy diff and layout
0.21% for Meteor -- array-heavy code
0.30% for Box2DOctane -- array-heavy code
I believe the regression can be largely alleviated by determining if
NaN is impossible at the index check, and if so, reverting to the smaller
code pattern. The analysis could be global, incorporating NaN into the
global abstract value domain, or a much simpler a local dataflow
analysis. Many indexes are loop driven and cannot reach infinity because
they are incremented by a small bump and eventually (even without a loop
guard) the index would stop growing when the increment falls below the
rounding error in O(2^53) iterations.
Change-Id: I23ab1eb779f1d0c9c6655e13d69f65d453db9284
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/210321
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Mayank Patke <fishythefish@google.com>
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| .github | ||
| benchmarks | ||
| build | ||
| client | ||
| docs | ||
| pkg | ||
| runtime | ||
| samples | ||
| samples-dev/swarm | ||
| samples_2 | ||
| sdk | ||
| tests | ||
| third_party | ||
| tools | ||
| utils | ||
| .clang-format | ||
| .gitattributes | ||
| .gitconfig | ||
| .gitignore | ||
| .gn | ||
| .mailmap | ||
| .packages | ||
| .style.yapf | ||
| .vpython | ||
| AUTHORS | ||
| BUILD.gn | ||
| CHANGELOG.md | ||
| codereview.settings | ||
| CONTRIBUTING.md | ||
| DEPS | ||
| LICENSE | ||
| PATENT_GRANT | ||
| PRESUBMIT.py | ||
| README.dart-sdk | ||
| README.md | ||
| sdk_args.gni | ||
| WATCHLISTS | ||
Dart
A client-optimized language for fast apps on any platform
Dart is:
-
Optimized for UI: Develop with a programming language specialized around the needs of user interface creation.
-
Productive: Make changes iteratively: use hot reload to see the result instantly in your running app.
-
Fast on all platforms: Compile to ARM & x64 machine code for mobile, desktop, and backend. Or compile to JavaScript for the web.
Dart's flexible compiler technology lets you run Dart code in different ways, depending on your target platform and goals:
-
Dart Native: For programs targeting devices (mobile, desktop, server, and more), Dart Native includes both a Dart VM with JIT (just-in-time) compilation and an AOT (ahead-of-time) compiler for producing machine code.
-
Dart Web: For programs targeting the web, Dart Web includes both a development time compiler (dartdevc) and a production time compiler (dart2js).
License & patents
Dart is free and open source.
See LICENSE and PATENT_GRANT.
Using Dart
Visit dart.dev to learn more about the language, tools, and to find codelabs.
Browse pub.dev for more packages and libraries contributed by the community and the Dart team.
Our API reference documentation is published at api.dart.dev, based on the stable release. (We also publish docs from our beta and dev channels, as well as from the primary development branch).
Building Dart
If you want to build Dart yourself, here is a guide to getting the source, preparing your machine to build the SDK, and building.
There are more documents on our wiki.
Contributing to Dart
The easiest way to contribute to Dart is to file issues.
You can also contribute patches, as described in Contributing.