1. The Design Space
2. GHCJS’s FFI
3. Lightweight safety checks
4. Returning multiple values
5. Changes in the FFI System for the JS Backend

Users of GHCJS enjoyed a rich FFI system for foreign JavaScript imports. However, this has changed during our adaptation of GHCJS to GHC 9.x. This short post goes over the GHCJS FFI system, the motivation for these changes and what the changes are. First, we must consider the design space of an FFI system.

The Design Space

FFI code is typically employed in high performance scenarios. Additionally, users of the FFI do not want to deal with the object language the compiler is compiling to. Instead, users want a simple way to call functions from the object language and use them in their own code as normal Haskell functions. However, users of the FFI system do tend to be power users, and so as a design principle we want to expose the tools they need to achieve their performance needs, whatever those needs may be. We can summarize these constraints as follows:

1. The FFI must abstract the JavaScript backend’s infidelities away as much as possible. That is, users of the FFI should need to worry about the Int64# representation, but should also be able to simply follow standard patterns we have written in base.
2. The FFI must provide tools to achieve high performance code, even if those tools require up front knowledge of the runtime system to use. However, these tools should not be in the path of least resistance to use the FFI system.
3. The FFI must provide a lightweight specification that user’s program against for the JS backend to optimize the imported function and for good error messages for users.

GHCJS’s FFI sets a high (qualitative) benchmark on these three constraints. Let’s inspect them each in detail, in no particular order.

GHCJS’s FFI

In GHCJS, a user could take advantage of JavaScript functions in their Haskell code using the GHCJS’s FFI. However, the syntax was unique to GHCJS with place holder variables like one might see in perl, nix, or bash. For example, here is a foreign import from the base library for st_size:

-- base/System/Posix/Internal.hs
-- the JS FFI version
foreign import javascript unsafe "$r1 = h$base_st_size($1_1,$1_2); $r2 = h$ret1;"
   st_size :: Ptr CStat -> IO Int64

The syntax is different from what we know and love in the normal Haskell world but the grammar is straightforward. We declare a foreign import from javascript, state that the import is unsafe or interruptible and then provide a string, h$base_fstat(...) for the code generator to use when compiling. Compare this with the C version:

-- base/System/Posix/Internal.hs
-- the C FFI version
foreign import ccall unsafe "HsBase.h __hscore_st_size"
   st_size :: Ptr CStat -> IO Int64

And we see that they are similar. The only difference is the strange $n symbols in the referrent string. Contrast this with the C version, which simply declares a name.

These symbols are place holder variables with special meaning in GHCJS. There are two intractable reasons for the placeholder patterns. First, we require these patterns to work around the limitations of JavaScript as a backend (1). For example, consider the case where we need to return an Int64# from an imported foreign function. In C and Haskell this is not a problem because both can represent Int64# natively, however JavaScript only has native support for 32-bit values. Thus, to be able to return an Int64# we need to have a method to return two 32-bit numbers. Similarly, in order to apply a function to an Int64# that function must take at least two arguments, one for the high bits and one for the low. Second, the referrent string is untyped and can contain arbritrary JavaScript code. So placeholder patterns provide a simply and lightweight way for safety checks and eliminate classes of untyped, hard to understand errors. For example, consider an arity mismatch error between a function definition and call site. When this happens JavaScript happily continues processing with the return value from the function application defined as NaN (of course). Such arity conflicts can easily occur, especially when dealing with 64-bit values which require function arity assumptions.

Lightweight safety checks

Lightweight safety checks (3) are done by GHCJS by parsing the names of the place holder variables; each of which follows a specific naming convention. This convention is:

• Argument types:
  • $n: Used for unary arguments, i.e., arguments which require only a single register.
  • $n_n: Used for binary arguments, i.e., arguments which require two registers.
  • $c: A continuation argument, only valid for interruptible foreign functions.
• Return types:
  • $r: a unary return
  • $r1, $r2: a binary return
  • $r1, $r2, $r3_1, $r3_2: unboxed tuple return
• Top level patterns:
  • "&value": simply emitted as value by the code generator
  • "someFunction": emitted as ret = someFunction(...), i.e., map the FFI to the result of the function call.
  • "$r = $1.f($2)": emitted as r1 = a1.f(a2), i.e., a combination of a function call and a property access.

With this standard GHCJS then parses the FFI referrent string to ensure that it conforms to this standard. If not then GHCJS can at least respond to the user with an ill-formatted FFI message and say precisely where the issue is. For example, it could respond that only half of an Int64# is returned based on the referrent string and the function type.

Returning multiple values

But what of performant code? GHCJS achieves performant FFI by not trying to abstract away from the runtime system. Instead, an advantage of GHCJS’s FFI is that we can specify exactly which registers the foreign function should dump its results or even arbitrary global variables. This places more burden on the user of the FFI in specific scenarios, but crucially allows the FFI system to get out of the way of the user. The FFI system also exploits this capability to return multiple values from a single function call, which is a common need when compiling to JavaScript. For example, in the above code st_size is declared to return an IO Int64, the JavaScript handler h$base_st_size returns the Int64 using two registers $r1 and $r2, but does so through the use of a special purpose global variable called h$ret1:

function h$base_st_size(stat, stat_off) {
    h$ret1 = (stat.i3[(stat_off>>2)+2]);
    return (stat.i3[(stat_off>>2)+1]);
}

The function inputs a pointer and an offset. Pointers in GHCJS are simply pointers to ByteArrays so the function indexes into the ByteArray and retrieves and stores the lower 32-bits in h$ret1, then returns the higher 32-bits directly. These results are picked up by the FFI code, which performs assignment to set $r1 to the result of the function call (the higher 32-bits), and set $r2 to the value of h$ret1 (the lower 32-bits). Crucially, the runtime system needs to do nothing. The registers are already handled ready to be consumed by whatever the caller of the foreign function will do.

One might consider using a simpler design, which trades register juggling for a more straightforward representation such as a ByteArray which stores the Int64#. However, such a design would trade speed for implementation simplicity. If we passed ByteArrays then each foreign function would spend time wrapping and unwrapping the array to get the payload; clearly an undesirable outcome for high performance code.

Changes in the FFI System for the JS Backend

So we see that GHCJS’s FFI system actually performs quite well in the design space. Power users are well supported and can leverage enough unsafety to bind global variables like h$ret1 and specific registers such as $r1. The system provides some lightweight checking through parsing. The nuances of the JavaScript platform are generally abstracted over and the FFI system is tuned for performance critical scenarios. So why change it?

The short answer is to hit deadlines. By skipping the FFI parsing the JS Backend team was able to produce a working (can output “Hello World!”, and compile GHC’s boot libraries), integrated, JS backend in GHC faster than had we finished the FFI system.

For the time being, we have opted to replaced each foreign function call with a JavaScript fat arrow, for example:

foreign import javascript unsafe "(($1_1,$1_2) => { return h$base_st_size($1_1,$1_2); })"
   st_size :: Ptr CStat -> IO Int64

Of course, this situation is untenable, as argued above, FFI code is assumed to be used in performance critical code, and thus any extra overhead, such as a function closure and consequent indirection, must be avoided. But fear not! In the near future we’ll be overhauling the FFI system and returning it to its former glory.