lsr(-123, 0) not equiv. to -123 >>> 0

[leoliu]

While converting a JS program to rescript I discovered that lsr(-123,0) is compiled to just -123 which is not equivalent to -123 >>> 0. I wonder if this is a bug?

https://rescript-lang.org/try?version=v11.1.4&module=esmodule&code=DYUwLgBAHhC8HAM4CcAUBaAjAJgMwBoAGASgChRIBPOCAUmQEMB3VAIizwgD4eJDWypAFKIAdMAD2Ac1QxY8SsSA

[jagadam97]

i think these #7171 enhancements will fix what ever issue we are having here.

But i found an inconstency from OCaml Lib here.

-123 lsr 0 is giving output equivalent to -123 >> 0
where as
-123 lsr 1 is giving output equivalent to -123 >>> 1

[shulhi]

So it's not a bug because JS converts it to an unsigned 32-bit integer before shifting.

-123 converted two's complement is 0b1111111_11111111_11111111_10000101. lsr 0 doesn't shift, so the result remains.

However, in JS, >>> forces the number to be interpreted as an unsigned 32-bit integer, thus 2³² + (-123) = 4294967173.

@cometkim if our goal is to match javascript, should we change the implementation of bitwise to match the javascript?

[cometkim]

@cometkim if our goal is to match javascript, should we change the implementation of bitwise to match the javascript?

We can discuss it further at rescript-lang/rfcs#1, but as already mentioned it is not the current goal.

https://github.com/rescript-lang/rfcs/blob/int-semantic/text/0001-int.md#can-we-somehow-make-it-match-javascripts-number

One day we might be able to treat int32 like a specialized second-class citizen, but the migration path won't be simple.

[leoliu]

So it's not a bug because JS converts it to an unsigned 32-bit integer before shifting.

-123 converted two's complement is 0b1111111_11111111_11111111_10000101. lsr 0 doesn't shift, so the result remains.

However, in JS, >>> forces the number to be interpreted as an unsigned 32-bit integer, thus 2³² + (-123) = 4294967173.

@cometkim if our goal is to match javascript, should we change the implementation of bitwise to match the javascript?

That explains it. Sometimes we forget how crazy JS can be. Maybe a heads up is enough when documenting lsr.