#9688: Improve the interaction between CSE and the join point transformation -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: CSE Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Runtime | Related Tickets: performance bug | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): Indeed! It looks like a classic code-specialisation question to me. In `digitToIntMaybe` you use some code `isHexDigit` that just happens to use the same tests as the RHS, `digitToInt`. Moreover there are three ways for `isHexDigit` to succeed. If you duplicate the RHS into those three outcomes you'll get the result you want; this amounts to inlining `$j_s2BC`. But that entails code duplication -- and might gain nothing whatsoever. Or, to put it another way, if the user wrote the above `Core`, complete with the local function `$j_s2BC`, would you expect it to be optimised? It would be cool if so. But I don't yet see how to achieve that at reasonable cost. To put it another way, if you wrote this in C: {{{ if blah then x = e1 else x = e2 if similar-blah then s1 else s2 }}} you'd usually expect to test `blah`, assign to `x`, and then test `similar-blah` and do `s1` or `s2`. But it might be better to duplicate (i.e. specialise) the first `if` into the branches: {{{ if similar-blah then { if blah then x=e1 else x=e2 s1 } else { if blah then x=e1 else x=e2 s2 } }}} and now the inner `if` might be optimised away entirely. These are not simple choices. By representing them as simple inlining choices ("shall I inline `$j_s2Bc`?") GHC piggy backs on a lot of careful heuristics for inlining. But, I agree, it does not always work well. I have taken time to explain here, in the hope that someone can do better! Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9688#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler