LLVM Types in JavaScript

This is an overview of how LLVM types are implemented in JS by Emscripten. This stuff has changed quite a bit over time (if you read this and it has not been updated in a while, better to check the source).

Overflow corrections:

i32: | 0
less: & (2^bits-1)

Sign corrections (done right before a math operation if necessary, and to extend):

i32 signed: | 0
i32 unsigned: >>> 0
less signed: << (32-bits) >> (32-bits)
less unsigned: & (2^bits-1)

Truncate:

& (2&bits-1)

Rounding the output of a division:

i32 or less, signed: & -1 (could be |0 as well)
unsigned: Math.floor(.)
>i32 or float-to-int: x >= 0 ? Math.floor(x) : Math.ceil(x)

Bitwise cast of float to int and vice versa:

Write to heap as int, read as float (and vice versa)

Note that there are some ugly optimizations for i64s as emulated doubles, e.g. a sign correction could look like

(x >= 0 ? x : (2^64)+x)

Also, if we do precise i64 (not with emulated doubles), we use [low,high] (i.e. a js array) as the representation in some cases.

LLVM has "non-C struct" types, that we implement as JS objects with fields. These are typically used only in odd things like exception handling, checking for explicit overflows, etc., so not usually performance sensitive.