#9340: Implement new `clz` inline primop -------------------------------------+------------------------------------- Reporter: hvr | Owner: Type: feature request | Status: new Priority: normal | Milestone: 7.10.1 Component: Compiler | Version: 7.8.2 (CodeGen) | Differential Revisions: Keywords: primop clz | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------- From a draft for #9281, I had to use a C FFI and using a gcc/clang `__builtin` to get access to the [http://en.wikipedia.org/wiki/Find_first_set CLZ instruction]: {{{#!hs -- | Compute base-2 log of 'Word#' -- -- This is internally implemented as count-leading-zeros machine instruction. foreign import ccall unsafe "integer_gmp_word_log2" wordLog2# :: Word# -> Int# }}} {{{#!c HsWord integer_gmp_word_log2(HsWord x) { #if (SIZEOF_HSWORD) == (SIZEOF_INT) return x ? (WORD_SIZE_IN_BITS-1) - __builtin_clz(x) : -1; #elif (SIZEOF_HSWORD) == (SIZEOF_LONG) return x ? (WORD_SIZE_IN_BITS-1) - __builtin_clzl(x) : -1; #elif (SIZEOF_HSWORD) == (SIZEOF_LONG_LONG) return x ? (WORD_SIZE_IN_BITS-1) - __builtin_clzll(x) : -1; #else # error unsupported SIZEOF_HSWORD #endif } }}} Since a `clz`-like operation should be available on most cpus GHC should expose that as a primop ({{{clz# :: Word# -> Int#}}}). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9340 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler