#9188: quot with a power of two is not optimized to a shift --------------------------------------------+------------------------------ Reporter: tibbe | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime performance bug | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: #8311 --------------------------------------------+------------------------------ Changes (by rwbarton): * related: => #8311 Comment: See also #8311. Note that `quot` by 2 rounds towards zero, so for negative `Int`s it's not quite just a shift. The LLVM backend does handle this well, producing this sequence for the division: {{{ 7c: 48 89 d0 mov %rdx,%rax 7f: 48 c1 e8 3f shr $0x3f,%rax 83: 48 01 d0 add %rdx,%rax ; add sign bit to move towards 0 86: 48 d1 f8 sar %rax }}} A `div` by 2 really would be a single arithmetic shift, but at the Cmm level there is still a function call to `divInt#` :( -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9188#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler