asm.js uses the "x|0" idiom to coerce the argument to int32, and the "x>>>0" idiom to coerce the argument to uint32. Currently, nashorn compiles these to

<load x as int>
ICONST_0
IOR

and

<load x as int>
ICONST_0
IUSHR
<convert to uint>

the ICONST_0, IOR|IUSHR are unnecessary. In case "x" is already an int local variable, "x|0" nicely just becomes an ILOAD.

Further, <convert to uint> is currently implemented by emitting
I2L
LDC 0xFFFF_FFFFL
LAND
which is 5 bytes long. Encapsulating it in "long JSType.uint32(int)" would result in it needing only 3 bytes for INVOKESTATIC.

Using both these optimizations, we can translate x>>>0 for an already int variable from

ILOAD x
ICONST_0
IUSHR
I2L
LDC 0xFFFF_FFFFL
LAND

to much nicer:

ILOAD x
INVOKESTATIC JSType.toUint32(I)J

For large asm.js-compliant, emscripten/mandreel generated code the savings in bytecode size for emitted methods can be quite significant, e.g. "function a8" in zlib got reduced from ~6400 bytes to ~6000.