expose strlen from Foreign.C.String

[chessai]

I forgot about that addition. In that case we would just need the lifted
wrapper

On Wed, Jan 20, 2021, 17:01 Viktor Dukhovni <ietf-dane at dukhovni.org> wrote:

> On Wed, Jan 20, 2021 at 09:54:30AM -0800, chessai wrote:
>
> > I've wanted the following before:
> >
> > foreign import ccall unsafe "strlen"
> >   cstringLength# :: Addr# -> Int#
> >
> > cstringLength :: CString -> Int
> > cstringLength (Ptr s) = I# (cstringLength# s)
> >
> > A natural place for this seems to be Foreign.C.String.
>
> Why a new FFI call, rather than `cstringLength#` from ghc-prim: GHC.CString
> (as of GHC 9.0.1):
>
>     9.0.1-notes.rst:  ``ghc-prim`` library
>     9.0.1-notes.rst:  ~~~~~~~~~~~~~~~~~~~~
>     9.0.1-notes.rst:
>     9.0.1-notes.rst:  - Add a known-key ``cstringLength#`` to
> ``GHC.CString`` that is eligible
>     9.0.1-notes.rst:    for constant folding by a built-in rule.
>
>     ghc-prim/changelog.md:  - Add known-key `cstringLength#` to
> `GHC.CString`. This is just the
>     ghc-prim/changelog.md:    C function `strlen`, but a built-in rewrite
> rule allows GHC to
>     ghc-prim/changelog.md:    compute the result at compile time when the
> argument is known.
>
>     CString.hs:  -- | Compute the length of a NUL-terminated string. This
> address
>     CString.hs:  -- must refer to immutable memory. GHC includes a
> built-in rule for
>     CString.hs:  -- constant folding when the argument is a
> statically-known literal.
>     CString.hs:  -- That is, a core-to-core pass reduces the expression
>     CString.hs:  -- @cstringLength# "hello"#@ to the constant @5#@.
>     CString.hs:  cstringLength# :: Addr# -> Int#
>     CString.hs:  {-# INLINE[0] cstringLength# #-}
>     CString.hs:  cstringLength# = c_strlen
>
> Which is in turn re-exported by GHC.Exts:
>
>     GHC/Exts.hs:  -- * CString
>     GHC/Exts.hs:  unpackCString#,
>     GHC/Exts.hs:  unpackAppendCString#,
>     GHC/Exts.hs:  unpackFoldrCString#,
>     GHC/Exts.hs:  unpackCStringUtf8#,
>     GHC/Exts.hs:  unpackNBytes#,
>     GHC/Exts.hs:  cstringLength#,
>
> It is perhaps somewhat disappointing that the cstringLength#
> optimisations for `bytestring` (in master) aren't included in the
> `bytestring` version in 9.0.1.
>
> --
>     Viktor.
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