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|
module GHC.StgToJS
( stgToJS
)
where
import GHC.StgToJS.CodeGen
-- Note [StgToJS design]
-- ~~~~~~~~~~~~~~~~~~~~~
--
-- StgToJS ("JS backend") is adapted from GHCJS [GHCJS2013].
--
-- Haskell to JavaScript
-- ~~~~~~~~~~~~~~~~~~~~~
-- StgToJS converts STG into a JavaScript AST (in GHC.JS) that has been adapted
-- from JMacro [JMacro].
--
-- Tail calls: translated code is tail call optimized through a trampoline,
-- since JavaScript implementations don't always support tail calls.
--
-- JavaScript ASTs are then optimized. A dataflow analysis is performed and then
-- dead code and redundant assignments are removed.
--
-- Primitives
-- ~~~~~~~~~~
-- Haskell primitives have to be represented as JavaScript values. This is done
-- as follows:
--
-- - Int#/Int32# -> number in Int32 range
-- - Int16# -> number in Int16 range
-- - Int8# -> number in Int8 range
-- - Word#/Word32# -> number in Word32 range
-- - Word16# -> number in Word16 range
-- - Word8# -> number in Word8 range
--
-- - Float#/Double# -> both represented as Javascript Double (no Float!)
--
-- - Int64# -> represented with two fields:
-- high -> number in Int32 range
-- low -> number in Word32 range
-- - Word64# -> represented with two fields: high, low
-- high -> number in Word32 range
-- low -> number in Word32 range
--
-- - Addr# -> represented with two fields: array (used as a namespace) and index
-- - StablePtr# -> similar to Addr# with array fixed to h$stablePtrBuf
--
-- - JSVal# -> any Javascript object (used to pass JS objects via FFI)
--
-- - TVar#, MVar#, etc. are represented with JS objects
--
-- Foreign JavaScript imports
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~
-- StgToJS supports inline JavaScript code. Example:
--
-- > foreign import javascript unsafe
-- > "((x,y) => x + y)"
-- > plus :: Int -> Int -> Int
--
-- Currently the JS backend only supports functions as JS imports.
--
-- In comparison, GHCJS supports JavaScript snippets with $1, $2... variables
-- as placeholders for the arguments. It requires a JavaScript parser that the
-- JS backend lacks. In GHCJS, the parser is inherited from JMacro and supports
-- local variable declarations, loops, etc. Local variables are converted to
-- hygienic names to avoid capture.
--
-- Primitives that are represented as multiple values (Int64#, Word64#, Addr#)
-- are passed to FFI functions with multiple arguments.
--
-- Interruptible convention: FFI imports with the "interruptible" calling
-- convention are passed an extra argument (usually named "$c") that is a
-- continuation function. The FFI function must call this function to return to
-- Haskell code.
--
-- Unboxed tuples: returning an unboxed tuple can be done with the predefined
-- macros RETURN_UBX_TUPn where n is the size of the tuples. Internally it uses
-- predefined "h$retN" global variables to pass additional values; the first
-- element of the tuple is returned normally.
--
-- Memory management
-- ~~~~~~~~~~~~~~~~~
-- Heap objects are represented as JavaScript values.
--
-- Most heap objects are represented generically as JavaScript "objects" (hash
-- maps). However, some Haskell heap objects can use use a more memory efficient
-- JavaScript representation: number, string... An example of a consequence of
-- this is that both Int# and Int are represented the same as a JavaScript
-- number. JavaScript introspection (e.g. typeof) is used to differentiate
-- heap object representations when necessary.
--
-- Generic representation: objects on the heap ("closures") are represented as
-- JavaScript objects with the following fields:
--
-- { f -- (function) entry function + info table
-- , d1 -- two fields of payload
-- , d2
-- , m -- GC mark
-- , cc -- optional cost-center
-- }
--
-- Payload: payload only consists of two fields (d1, d2). When more than two
-- fields of payload are required, the second field is itself an object.
-- payload [] ==> { d1 = null, d2 = null }
-- payload [a] ==> { d1 = a , d2 = null }
-- payload [a,b] ==> { d1 = a , d2 = b }
-- payload [a,b,c] ==> { d1 = a , d2 = { d1 = b, d2 = c} }
-- payload [a,b,c...] ==> { d1 = a , d2 = { d1 = b, d2 = c, ...} }
--
-- Entry function/ info tables: JavaScript functions are JavaScript objects. If
-- "f" is a function, we can:
-- - call it, e.g. "f(arg0,arg1...)"
-- - get/set its fields, e.g. "f.xyz = abc"
-- This is used to implement the equivalent of tables-next-to-code in
-- JavaScript: every heap object has an entry function "f" that also contains
-- some metadata (info table) about the Haskell object:
-- { t -- object type
-- , size -- number of fields in the payload (-1 if variable layout)
-- , i -- (array) fields layout (empty if variable layout)
-- , n -- (string) object name for easier dubugging
-- , a -- constructor tag / fun arity
-- , r -- ??
-- , s -- static references?
-- , m -- GC mark?
-- }
--
-- Payloads for each kind of heap object:
--
-- THUNK =
-- { f = returns the object reduced to WHNF
-- , m = ?
-- , d1 = ?
-- , d2 = ?
-- }
--
-- FUN =
-- { f = function itself
-- , m = ?
-- , d1 = free variable 1
-- , d2 = free variable 2
-- }
--
-- There are two different kinds of partial application:
-- - pap_r : pre-generated PAP that contains r registers
-- - pap_gen : generic PAP, contains any number of registers
--
-- PAP =
-- { f = ?
-- , m = ?
-- , d1 = function
-- , d2 =
-- { d1 & 0xff = number of args (PAP arity)
-- , d1 >> 8 = number of registers (r for h$pap_r)
-- , d2, d3... = args (r)
-- }
-- }
--
-- CON =
-- { f = entry function of the datacon worker
-- , m = 0
-- , d1 = first arg
-- , d2 = arity = 2: second arg
-- arity > 2: { d1, d2, ...} object with remaining args (starts with "d1 = x2"!)
-- }
--
-- BLACKHOLE =
-- { f = h$blackhole
-- , m = ?
-- , d1 = owning TSO
-- , d2 = waiters array
-- }
--
-- StackFrame closures are *not* represented as JS objects. Instead they are
-- "unpacked" in the stack, i.e. a stack frame occupies a few slots in the JS
-- array representing the stack ("h$stack").
--
-- When a shared thunk is entered, it is overriden with a black hole ("eager
-- blackholing") and an update frame is pushed on the stack.
--
-- Stack: the Haskell stack is implemented with a dynamically growing JavaScript
-- array ("h$stack").
-- TODO: does it shrink sometimes?
-- TODO: what are the elements of the stack? one JS object per stack frame?
--
--
-- Interaction with JavaScript's garbage collector
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- Using JS objects to represent Haskell heap objects means that JS's GC does
-- most of the memory management work.
--
-- However, GHC extends Haskell with features that rely on GC layer violation
-- (weak references, finalizers, etc.). To support these features, a heap scan
-- is can be performed (using TSOs, StablePtr, etc. as roots) to mark reachable
-- objects. Scanning the heap is an expensive operation, but fortunately it
-- doesn't need to happen too often and it can be disabled.
--
-- TODO: importance of eager blackholing
--
-- Concurrency
-- ~~~~~~~~~~~
-- The scheduler is implemented in JS and runs in a single JavaScript thread
-- (similarly to the C RTS not using `-threaded`).
--
-- The scheduler relies on callbacks/continuations to interact with other JS
-- codes (user interface, etc.). In particular, safe foreign import can use "$c"
-- as a continuation function to return to Haskell code.
--
-- TODO: is this still true since 2013 or are we using more recent JS features now?
-- TODO: synchronous threads
--
--
-- REFERENCES
-- * [GHCJS2013] "Demo Proposal: GHCJS, Concurrent Haskell in the Browser", Luite Stegeman,
-- 2013 (https://www.haskell.org/haskell-symposium/2013/ghcjs.pdf)
-- * [JMacro] https://hackage.haskell.org/package/jmacro
|
