JavaScript Cmm backend
I'd like to RFC on the attached patch implementing !JavaScript Cmm backend for GHC.
It adds -fjavascript compilation option. Calling ghc -fjavascript produces JS in the output file.
Otherwise the ghc binary should be fully functional as a native compiler. Thus -fjavascript is
similar to -fllvm in spirit.
The patch adds HscJavaScript constructor to HscTarget. It is used to dispatch code output to
the new JsCodeGen module.
Generated !JavaScript code relies on the built-in JS garbage collection. JsTransforms module
disables GHC Hp and Sp overflow checks.
As !JavaScript has no pointers, we emulate them using JS closures containing arrays and indices.
To distinguish between pointers and scalars we run Hoopl heuristics in PointerMarker module.
As in native world, the generated JS object files are to be linked. In order to do this, there is another project, tentatively called Josh[1]. Josh is regular cabalized Haskell binary which links JS object files using function maps provided by GHC.
The !JavaScript RTS has rts/*.cmm compiled to !JavaScript almost as is. In addition there is a
handful of handwritten JS code residing in Josh distribution[2].
ghc-prim[3], integer-gmp[4] and base[5] are compiled with small changes. Those changes seem to be orthogonal to the GHC patch.
For bootstrap process please see Josh README at github. Josh also includes several tests which work on 32-bit Debian Wheezy.
Generated code is in order of 2 MB uncompressed and un-minified. Most of it is in RTS. It compresses very well though (to 150Kb approx). Plenty of low-hanging fruit optimizations are possible.
There are plenty caveats to the current patch.
- It works on 32 bits only. Math is fishy and Integer support is nonexistant.
- Lots of tests should be imported from GHC, GHCJS, Fay.
- No Handle based IO works at the moment.
- No performance tests were done.
Despite its shortcomings, the patch is fairly non-invasive, IMHO. It should be noted, that the same approach could work for other GC based platforms (e.g. Java, C#).
I'd like to continue working towards merging the patch into GHC, if possible. Could GHC committers provide any guidance of what should be done in order to merge it?