#9400: poor performance when compiling modules with many Text literals at -O1 -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: xnyhps Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: Compile- | Blocked By: time performance bug | Related Tickets: #9370 Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by xnyhps): * owner: => xnyhps Comment: This looked like quite a simple bug I could work on, so I decided to have a look. * Removing the `lengthFS str == 1` case in `mkStringExprFS` does not appear to break anything. * I've managed to modify `CoreSubst.exprIsConApp_maybe` to split calls to `unpackCString#` or `unpackCStringUtf8#`. It'll also recognize when it's a single-character string and split it into `(':', [c, nil])`, to avoid ever creating calls to `unpackCString*# ""#`. Looking at the generated Core, I've verified that it now pushes `unpackCString*#` calls through `case`-statements. This means that, even at -O0: {{{ main = case "abc" of (x:xs) -> putStrLn xs }}} compiles to `main = System.IO.putStrLn GHC.CString.unpackCString# "bc"#` * I haven't timed it, but `src/Text/XmlHtml/HTML/Meta.hs` now compiles quickly and with only ~180MB RAM. * `unpackCString#` and `unpackCStringUtf8#` seem very similar, except that `unpackCStringUtf8#` parses UTF8. At least for `mkStringExprFS` it could work to only use `unpackCStringUtf8#`, with a slight run-time penalty (calling `leChar#` for each character in the string), but the only benefit would be making rewrite rules easier. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9400#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler