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Cheng Shao pushed to branch wip/wasm-minor-improvements at Glasgow Haskell Compiler / GHC

Commits:

f098a7b5

by Cheng Shao at 2025-05-09T21:24:20+00:00

wasm: add zero length fast path for fromJSString

This patch adds a zero length fast path for `fromJSString`; when
marshaling a zero-length `JSString` we don't need to allocate an empty
`ByteArray#` at all.

1 changed file:

libraries/ghc-internal/src/GHC/Internal/Wasm/Prim/Types.hs

Changes:

libraries/ghc-internal/src/GHC/Internal/Wasm/Prim/Types.hs

@@ -184,15 +184,16 @@ newtype JSString
  -- eagerly once the resulting 'String' is forced, and the argument
  -- 'JSString' may be explicitly freed if no longer used.
  fromJSString :: JSString -> String
 -fromJSString s = unsafeDupablePerformIO $ do
 -  l <- js_stringLength s
 -  fp <- mallocPlainForeignPtrBytes $ l * 3
 -  withForeignPtr fp $ \buf -> do
 -    l' <- js_encodeInto s buf $ l * 3
 -    peekCStringLen utf8 (buf, l')
 +fromJSString s = case js_stringLength s * 3 of
 +  0 -> ""
 +  max_len -> unsafePerformIO $ do
 +    fptr <- mallocPlainForeignPtrBytes max_len
 +    withForeignPtr fptr $ \ptr -> do
 +      len <- js_encodeInto s ptr max_len
 +      peekCStringLen utf8 (ptr, len)
  foreign import javascript unsafe "$1.length"
 -  js_stringLength :: JSString -> IO Int
 +  js_stringLength :: JSString -> Int
  foreign import javascript unsafe "(new TextEncoder()).encodeInto($1, new Uint8Array(__exports.memory.buffer, $2, $3)).written"
    js_encodeInto :: JSString -> Ptr a -> Int -> IO Int