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.