Dart2js minified vs non-minified treats shifts differently if value < 0

[nex3]

From @stephan-gruen on September 12, 2017 13:0

I ran into this while testing out the BZip2Encoder class from the widely used archive library, which does not work if minified.
At the end of the day this is why: the shift result from unminified and minified JavaScript is not the same if the value is negativ. So this is bad as the behavior should be the same.

print('1 >> 0  = ${1 >> 0}');
print('0 >> 0  = ${0 >> 0}');
print('-1 >> 0  = ${-1 >> 0}');
print('-2 >> 0  = ${-2 >> 0}');

print('1 >> 1  = ${1 >> 1}');
print('0 >> 1  = ${0 >> 1}');
print('-1 >> 1  = ${-1 >> 1}');
print('-2 >> 1  = ${-2 >> 1}');

print('1 << 0  = ${1 << 0}');
print('0 << 0  = ${0 << 0}');
print('-1 << 0  = ${-1 << 0}');
print('-2 << 0  = ${-2 << 0}');

print('1 << 1  = ${1 << 1}');
print('0 << 1  = ${0 << 1}');
print('-1 << 1  = ${-1 << 1}');
print('-2 << 1  = ${-2 << 1}');

pub build mode Debug (uses dart2js to generate unminified JavaScript):

 1 >> 0  = 1
 0 >> 0  = 0
-1 >> 0  = -1
-2 >> 0  = -2
 1 >> 1  = 0
 0 >> 1  = 0
-1 >> 1  = -1
-2 >> 1  = -1
 1 << 0  = 1
 0 << 0  = 0
-1 << 0  = -1
-2 << 0  = -2 
 1 << 1  = 2
 0 << 1  = 0
-1 << 1  = -2
-2 << 1  = -4

pub build mode Release (uses dart2js to generate minified JavaScript):

1 >> 0  = 1
0 >> 0  = 0
-1 >> 0  = 4294967295
-2 >> 0  = 4294967294
1 >> 1  = 0
0 >> 1  = 0
-1 >> 1  = 4294967295
-2 >> 1  = 4294967295
1 << 0  = 1
0 << 0  = 0
-1 << 0  = 4294967295
-2 << 0  = 4294967294
1 << 1  = 2
0 << 1  = 0
-1 << 1  = 4294967294
-2 << 1  = 4294967292

Copied from original issue: dart-lang/pub#1703

[rakudrama]

The difference can't be minification - it does not work in a way that could make this difference - all minification does is choose short names.
When I compile a 'main' program containing the above code with dart2js --checked vs dart2js --minify I get identical outputs.

I suspect that you are running in a browser and the debug run is on Dartium and not compiled by dart2js at all, and the release run is compiled with dart2js and run with Chrome.

Please confirm. If this is the case, it is working as intended.

You can force the sign back by replacing a >> 1 with (a >> 1).toSigned(32).

[stephan-gruen]

So I checked and your are right that this is not a minified vs unminified, but
more a development vs. release issue.

My point is that the behavior of the shift during development and release is different.

Using dart-sdk 1.25.0-dev.16.3 and dart-sdk 2.0.0-dev.2.0 (Results are the same)

Run from WebStorm as Debug in
Dartium 50.0.2661.108
-1 >> 0 = -1

Run a test:

group('shift', () {
   test('zero', () {
   	expect(-1 >> 0, 4294967295);
   });
 });

Test failed:

Expected: <4294967295>
  Actual: <-1>

Pub: Build mode Debug, uploaded to Webserver
Dartium 50.0.2661.108 32-Bit:
-1 >> 0 = -1

Chrome 61.0.3163.10 64-Bit:
-1 >> 0 = 4294967295

Firefox 56.0 32-Bit:
-1 >> 0 = 4294967295

Pub: Build mode Release, uploaded to Webserver
Dartium 50.0.2661.108 32-Bit:
-1 >> 0 = 4294967295

Chrome 61.0.3163.10 64-Bit:
-1 >> 0 = 4294967295

Firefox 56.0 32-Bit:
-1 >> 0 = 4294967295

[rakudrama]

I'm closing this issue:
(1) There is no difference between minified and unminified, or between DDC and dart2js
(2) The reference of Dartium is obsolete
(3) There is work underway to better describe the necessary differences between VM and web implementations (@vsmenon )

[stephan-gruen]

This is 3 years old and was not fixed? Oh me, oh my...

Server:
"D:\dart-sdk\dart-sdk 2.13.0-12.0.dev\bin\dart.exe" --enable-asserts --pause_isolates_on_start --enable-vm-service:55630 D:\dart-apps\webV\vaderService\bin\server.dart

1 >> 0  = 1
0 >> 0  = 0
-1 >> 0  = -1
-2 >> 0  = -2
1 >> 1  = 0
0 >> 1  = 0
-1 >> 1  = -1
-2 >> 1  = -1
1 << 0  = 1
0 << 0  = 0
-1 << 0  = -1
-2 << 0  = -2
1 << 1  = 2
0 << 1  = 0
-1 << 1  = -2
-2 << 1  = -4

Client:
"D:\dart-sdk\dart-sdk 2.13.0-12.0.dev\bin\pub.bat" global run webdev serve web:53325
"D:\dart-sdk\dart-sdk 2.13.0-12.0.dev\bin\pub.bat" global run webdev build --output=web:build

1 >> 0  = 1
0 >> 0  = 0
-1 >> 0  = 4294967295
-2 >> 0  = 4294967294
1 >> 1  = 0
0 >> 1  = 0
-1 >> 1  = 4294967295
-2 >> 1  = 4294967295
1 << 0  = 1
0 << 0  = 0
-1 << 0  = 4294967295
-2 << 0  = 4294967294
1 << 1  = 2
0 << 1  = 0
-1 << 1  = 4294967294
-2 << 1  = 4294967292

// still not possible
// e.g. in a loop to unpack data
b = b << 1;
if (b < 0) {
  // No more data
}

[vsmenon]

Unfortunately, it's working as intended for the VM and Web to differ on int operations. We're going to better document this ( #42924 ), but the gist is that we need to make a tradeoff between precisely equivalent int semantics on the one hand and performance/code size on the other. We've chosen to go with the latter.

You can see more discussion on why >> behaves as it does on the web here: dart-lang/language#1434

In your example above, you can do b = (b << 1).toSigned(32); to force the semantics you want.