Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC Commits: f69c5f14 by Cheng Shao at 2025-12-19T03:19:45-05:00 wasm: fix handling of ByteArray#/MutableByteArray# arguments in JSFFI imports This patch fixes the handling of ByteArray#/MutableByteArray# arguments in JSFFI imports, see the amended note and manual for explanation. Also adds a test to witness the fix. Co-authored-by: Codex <codex@openai.com> - - - - - 7 changed files: - compiler/GHC/HsToCore/Foreign/Wasm.hs - docs/users_guide/wasm.rst - testsuite/tests/jsffi/all.T - + testsuite/tests/jsffi/bytearrayarg.hs - + testsuite/tests/jsffi/bytearrayarg.mjs - + testsuite/tests/jsffi/bytearrayarg.stdout - testsuite/tests/perf/should_run/all.T Changes: ===================================== compiler/GHC/HsToCore/Foreign/Wasm.hs ===================================== @@ -224,6 +224,25 @@ especially since leaving all the boxing/unboxing business to C unifies the implementation of JSFFI imports and exports (rts_mkJSVal/rts_getJSVal). +We don't support unboxed FFI types like Int# etc. But we do support +one kind of unlifted FFI type for JSFFI import arguments: +ByteArray#/MutableByteArray#. The semantics is the same in C: the +pointer to the ByteArray# payload is passed instead of the ByteArray# +closure itself. This allows efficient zero-copy data exchange between +Haskell and JavaScript using unpinned ByteArray#, and the following +conditions must be met: + +- The JSFFI import itself must be a sync import marked as unsafe +- The JavaScript code must not re-enter Haskell when a ByteArray# is + passed as argument + +There's no magic in the handling of ByteArray#/MutableByteArray# +arguments. When generating C stub, we treat them like Ptr that points +to the payload, just without the rts_getPtr() unboxing call. After +lowering to C import, the backend takes care of adding the offset, see +add_shim in GHC.StgToCmm.Foreign and +Note [Unlifted boxed arguments to foreign calls]. + Now, each sync import calls a generated C function with a unique symbol. The C function uses rts_get* to unbox the arguments, call into JavaScript, then boxes the result with rts_mk* and returns it to @@ -517,8 +536,9 @@ importCStub sync cfun_name arg_tys res_ty js_src = CStub c_doc [] [] cfun_ret | res_ty `eqType` unitTy = cfun_call_import <> semi | otherwise = text "return" <+> cfun_call_import <> semi - cfun_make_arg arg_ty arg_val = - text ("rts_get" ++ ffiType arg_ty) <> parens arg_val + cfun_make_arg arg_ty arg_val + | isByteArrayPrimTy arg_ty = arg_val + | otherwise = text ("rts_get" ++ ffiType arg_ty) <> parens arg_val cfun_make_ret ret_val | res_ty `eqType` unitTy = ret_val | otherwise = @@ -543,7 +563,11 @@ importCStub sync cfun_name arg_tys res_ty js_src = CStub c_doc [] [] | res_ty `eqType` unitTy = text "void" | otherwise = text "HaskellObj" cfun_arg_list = - [text "HaskellObj" <+> char 'a' <> int n | n <- [1 .. length arg_tys]] + [ text (if isByteArrayPrimTy arg_ty then "HsPtr" else "HaskellObj") + <+> char 'a' + <> int n + | (arg_ty, n) <- zip arg_tys [1 ..] + ] cfun_args = case cfun_arg_list of [] -> text "void" _ -> hsep $ punctuate comma cfun_arg_list @@ -746,8 +770,18 @@ lookupGhcInternalTyCon m t = do n <- lookupOrig (mkGhcInternalModule m) (mkTcOcc t) dsLookupTyCon n +isByteArrayPrimTy :: Type -> Bool +isByteArrayPrimTy ty + | Just tc <- tyConAppTyCon_maybe ty, + tc == byteArrayPrimTyCon || tc == mutableByteArrayPrimTyCon = + True + | otherwise = + False + ffiType :: Type -> String -ffiType = occNameString . getOccName . fst . splitTyConApp +ffiType ty + | isByteArrayPrimTy ty = "Ptr" + | otherwise = occNameString $ getOccName $ tyConAppTyCon ty commonCDecls :: SDoc commonCDecls = ===================================== docs/users_guide/wasm.rst ===================================== @@ -265,7 +265,7 @@ backend’s JavaScript FFI, which we’ll now abbreviate as JSFFI. Marshalable types and ``JSVal`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -JSFFI supports all boxed marshalable foreign types in C FFI: +JSFFI supports all lifted marshalable foreign types in C FFI: - ``Bool`` - ``Char`` @@ -298,8 +298,14 @@ types in JSFFI. Some caveats to keep in mind: results in type errors, so keep this in mind. As for ``Int`` / ``Word``, they are 32-bit since the GHC wasm backend is based on ``wasm32`` . -- JSFFI doesn’t support unboxed foreign types like ``Int#``, - ``ByteArray#``, etc, even when ``UnliftedFFITypes`` is enabled. +- JSFFI doesn’t support unboxed foreign types like ``Int#``, even + when ``UnliftedFFITypes`` is enabled. The only supported unlifted + types are ``ByteArray#`` and ``MutableByteArray#``, they may only + be used as JSFFI import argument types, with the same semantics in + C FFI: the pointer to the payload is passed to JavaScript. Be + careful and avoid calling back into Haskell in such cases, + otherwise GC may occur and the pointer may be invalidated if it's + unpinned! In addition to the above types, JSFFI supports the ``JSVal`` type and its ``newtype``\ s as argument/result types. ``JSVal`` is defined in ===================================== testsuite/tests/jsffi/all.T ===================================== @@ -25,4 +25,6 @@ test('jsffion', [], compile_and_run, ['-optl-Wl,--export=main']) test('jsffisleep', [], compile_and_run, ['-optl-Wl,--export=testWouldBlock,--export=testLazySleep,--export=testThreadDelay,--export=testInterruptingSleep']) +test('bytearrayarg', [], compile_and_run, ['-optl-Wl,--export=main']) + test('textconv', [], compile_and_run, ['-optl-Wl,--export=main']) ===================================== testsuite/tests/jsffi/bytearrayarg.hs ===================================== @@ -0,0 +1,39 @@ +{-# LANGUAGE MagicHash #-} +{-# LANGUAGE UnboxedTuples #-} +{-# LANGUAGE UnliftedFFITypes #-} + +module Test where + +import GHC.Exts +import GHC.IO +import GHC.Word (Word8(W8#)) + +foreign import javascript unsafe "(() => { const u8 = new Uint8Array(__exports.memory.buffer, $1, 4); return (u8[0] === 0x12 && u8[1] === 0x34 && u8[2] === 0x56 && u8[3] === 0x78) ? 1 : 0; })()" + js_check_mba :: MutableByteArray# RealWorld -> IO Int + +foreign import javascript unsafe "(() => { const u8 = new Uint8Array(__exports.memory.buffer, $1, 4); return (u8[0] === 0x12 && u8[1] === 0x34 && u8[2] === 0x56 && u8[3] === 0x78) ? 1 : 0; })()" + js_check_ba :: ByteArray# -> IO Int + +foreign export javascript "main" + main :: IO () + +main :: IO () +main = + IO $ \s0 -> + case newPinnedByteArray# 4# s0 of + (# s1, mba# #) -> + case (0x12 :: Word8) of { W8# b0# -> + case (0x34 :: Word8) of { W8# b1# -> + case (0x56 :: Word8) of { W8# b2# -> + case (0x78 :: Word8) of { W8# b3# -> + let s2 = writeWord8Array# mba# 0# b0# s1 + s3 = writeWord8Array# mba# 1# b1# s2 + s4 = writeWord8Array# mba# 2# b2# s3 + s5 = writeWord8Array# mba# 3# b3# s4 + in case unIO (js_check_mba mba#) s5 of + (# s6, ok_mba #) -> case unsafeFreezeByteArray# mba# s6 of + (# s7, ba# #) -> case unIO (js_check_ba ba#) s7 of + (# s8, ok_ba #) -> case unIO (print ok_mba) s8 of + (# s9, _ #) -> case unIO (print ok_ba) s9 of + (# s10, _ #) -> (# s10, () #) + }}}} ===================================== testsuite/tests/jsffi/bytearrayarg.mjs ===================================== @@ -0,0 +1,4 @@ +export default async (__exports) => { + await __exports.main(); + process.exit(); +} ===================================== testsuite/tests/jsffi/bytearrayarg.stdout ===================================== @@ -0,0 +1,2 @@ +1 +1 ===================================== testsuite/tests/perf/should_run/all.T ===================================== @@ -420,6 +420,7 @@ test('T17949', [collect_stats('bytes allocated', 1), only_ways(['normal'])], com test('ByteCodeAsm', [ extra_run_opts('"' + config.libdir + '"') , js_broken(22261) + , when(arch('wasm32'), run_timeout_multiplier(10)) , collect_stats('bytes allocated', 10), ], compile_and_run, View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/f69c5f1492b275da7d2947612574d5c5... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/f69c5f1492b275da7d2947612574d5c5... 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