- Glasgow-haskell-users

Optimization & Destructive Updates
by Lajos Nagy 14 Apr '06

14 Apr '06

I was just musing the other day about the possibility of allowing (efficient and transparent) destructive updates in certain situations. Take the following (giberish) example: f xs = g xs [] where g [] ac = ac g (x1:x2:xs) ac = g xs (ac ++ [x2,x1]) It seems to me that the list concatenation in the tail recursion call can be safely performed destructively. Does anybody know about any research going on in this area? (Mind you: no linear types, no monads, only `under the hood' compiler optimization.) I'm aware of the fact that this would imply another kind of overloading (destructive vs. non-destructive) for functions which also seems an interesting research area. Regards, -- Lajos Nagy Computer Science Ph.D. Student, Florida Institute of Technology

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GHC and Cygwin/MinGW
by Rich Fought 14 Apr '06

14 Apr '06

Hello, I'm trying to port a linux-based Haskell application over to Win32. I am fiddling with both MinGW and Cygwin with varying degrees of bafflement. This is a server app that utilizes secure connections with the GnuTLS libraries. >From what I understand, the Win32 version of GHC targets MinGW. Does this mean that the GnuTLS libraries must be compiled for MinGW as well, or is it possible to link in cygwin libraries via the cygwin.dll somehow? Also, any ideas how difficult a Cygwin port of GHC would be? Tips would be much appreciated. Thanks, Rich

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Possible runtime overhead of wrapping the IO monad?
by Brian Hulley 13 Apr '06

13 Apr '06

Hi, I'm designing an API for a simple graphics window, and am trying to make the correct usage of the API functions explicit and visible to the type system by using different monads which ultimately just wrap the IO monad. For example, in a callback for rendering stuff to the screen, only operations in the RenderM monad are allowed, and when specifying vertex info for a primitive, only VertexM operations are allowed. However I'm wondering if I can rely on all this monad stuff being optimized out at compile time. A sample monad is below: newtype VertexM a = VertexM (IO a) instance Monad VertexM where VertexM x >>= fry = VertexM $ do ax <- x let VertexM y = fry ax y return x = VertexM $ return x instance MonadIO VertexM where liftIO = VertexM The monad doesn't do anything interesting apart from allowing the type checker to reject programs that don't use the API the way it was intended (all these things you have to keep in your head in C programs), but I don't want to use it if I'm going to get a performance hit. Also, in: foreign import ccall duma_vertex3f :: Float -> Float -> Float -> IO () vertex3f :: Float -> Float -> Float -> VertexM () vertex3f x y z = liftIO $ duma_vertex3f x y z is there a penalty involved in calling vertex3f (from another module) or will the worker/wrapper optimization ensure that machine code in the other module just calls duma_vertex3f directly since the liftIO operation is just an irrelevance at the machine code level? So far I've just been using ghc --make and not bothering about what kind of code is generated. Is there a flag I can use to get ghc to output the stg code (or something higher level than just x86 machine code itself) so I can look at the output to see what optimizations are being done? Thanks, Brian.

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Serious bug with ghc & FC5
by Alain Cremieux 13 Apr '06

13 Apr '06

(resent after being indefinitely held in fedora-haskell validation queue) Hi, 1) I have installed FC5 on 2 different machines. On my Athlon1800+ everything works perfectly. My other machine is a Pentium IV with hyperthreading, considered by Linux as SMP (x86 32). This is where problems occur 2) I have installed GHC-6.4.1 from Fedora Extras When I compile 'Omega' with it, Omega (which has a read-eval system) produces a 'mallocBytesRWX: failed to protect 0x' message I have compiled ghc-6.5... from the head. Compiled with GHC-6.4.1, I get the 'mallocBytesRWX...' message when I run 'ghci' 3) I have then patched 'RtsUtils.c' to suppress the 'barf("mallocBytesRWX...', and rerun- the compilation of ghc-6.5... 'Omega' compiled with the resulting ghc-6.5 works OK. ghci still produces the 'malloc...' error 4) When I compile ghc-6.5... with my patched ghc-6.5..., ghci does not produce the 'mallocBytesRWX' error (which is logical), but in some cases creates a 'segfault' So obviously the ghc-6.4.1 rpm is incorrect, as is the source ghc HEAD version. Probably an incorrect #ifdef here : #elif defined(openbsd_HOST_OS) || defined(linux_HOST_OS) || defined(darwin_HOST_OS) on a Pentium IV machine. And there is probably something else somewhere. Alain

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unexpected ambiguity
by Niklas Sorensson 10 Apr '06

10 Apr '06

Hello, Trying to compile the attached modules gives the following error message: Main.hs:8:15: Ambiguous occurrence `lift' It could refer to either `Control.Monad.Trans.lift', ... or `Apa.lift', imported from Apa at ... Is this correct? I would have expected that an unqualified lift could only refer to the one exported from Apa in this case. /Niklas

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RE: Is there a way to optimize out some foreign calls?
by Simon Peyton-Jones 10 Apr '06

10 Apr '06

| I'm thinking in terms of some way of marking foreign functions to specify | how they can be optimized out or specifying rewrite sequences eg | | x >> api_sync >> api_pop >> api_sync >> y = x >> api_pop >> | api_sync >> y You should be able to say something like RULE api_sync >>= (\_ -> api_pop >>= rest) = api_pop >>= rest api_pop >>= (\_ -> api_sync >>= rest) = api_pop >>= rest Do-notation is desugared to (>>=) stuff. Give it a try on a small example and see if it works SImon | | I've read the manual (7.10) on rewrite rules, but I'm having difficulty | understanding how to express the lhs of the above equation as a rewrite rule | so that it matches a sequence anywhere in a do block (to deal with nested | parentheses and the use of >>= instead of >>) | | Any ideas? | | Thanks, Brian. | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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darcsweb
by John Meacham 10 Apr '06

10 Apr '06

is there a darcsweb browser for the ghc repos anywhere? a la http://repetae.net/dw/darcsweb.cgi?r=jhc;a=summary John -- John Meacham - ⑆repetae.net⑆john⑈

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FFI Bug - Finalizer always called with NULL pointer?
by Brian Hulley 10 Apr '06

10 Apr '06

Hi - I've discovered that the finalizer in the code below is always called with a NULL pointer even though the pointer returned by the allocation routine is always not NULL. Is this a known bug, and if so, are there any workarounds? I'm using ghc version 6.4: data RawFont newtype Font = Font (ForeignPtr RawFont) foreign import ccall duma_createFont :: CString -> Int -> Bool -> Bool -> Bool -> IO (Ptr RawFont) foreign import ccall duma_releaseFont :: FunPtr (Ptr RawFont -> IO ()) createFont :: String -> Int -> Bool -> Bool -> Bool -> IO Font createFont name pointSize bold italic underline = block $ do f <- (withCString name $ \cname -> duma_createFont cname pointSize bold italic underline) >>= newForeignPtr duma_releaseFont -- Gets a NULL pointer!!! return $ Font f Thanks, Brian.

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ghc command line evaluation
by Pedro Miguel Duarte 06 Apr '06

06 Apr '06

I am writing a Java program with a call to a Haskell module M.hs, in order to evaluate some expression expr. A very simple idea, which I got somewhere in the net, is to create a Process object p which executes a GHC command-line instruction: Process p = Runtime.getRuntime(); p.exec( " ghc M.hs -e \"expr\" " ); This would be very simple, if it worked... My problem is that expressions i want to evaluate involve strings, and GHC command-line 'ghc' misinterprets some special symbols when it parses double quoted strings. For instance, ghc -e " reverse \"2<3\" " gives an error! Thus any one know a simple way around this? The only one obvious to me is to create a temporary Haskell module M_tmp.hs, with the expression to be evaluated, which is then executed through a command-line instruction like ghc M_tmp.hs -e main

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Re[2]: ghc releasing memory during compilation
by Bulat Ziganshin 06 Apr '06

06 Apr '06

Hello Simon, Monday, March 13, 2006, 3:40:21 PM, you wrote: >> just to check available PHYSICAL memory he answered that he don't know >> that is physical memory, only virtual memory matters SM> I think that's a mischaracterisation of what I said. Actually I said SM> that I didn't understand your comment about physical vs. virtual memory, SM> and in fact, looking back at the message, I still don't understand it :) SM> http://www.haskell.org//pipermail/glasgow-haskell-users/2005-April/008373.h… SM> I think what you're suggesting is that the runtime should detect the SM> amount of physical memory on the system and auto-tune itself to switch SM> to compacting collection when its residency reaches that amount. This SM> is certainly something we could do. Bear in mind that GHC is not SM> necessarily the only process running on the machine, though, and what SM> about running multiple GHCs? i suggest checking of AVAILABLE physical ram, that is perfectly possible in windows SM> Also, you can do this by setting your GHC_RTS environment variable. SM> Suppose you have 1G of physical mem. You want GHC to give up when it SM> reaches 1.5G, and you want to switch to compacting collection when the SM> residency reaches 300M. You could do this: SM> export GHC_RTS='-M1.5G -c20' SM> because 300M is 20% of 1.5G. Perhaps a better interface would be to SM> allow you to specify exactly the residency at which to switch to compaction. you mix up two questions: 1) switching from copying to compacting algorithm 2) return memory to OS after GC the only commonality between them is what we want to do both these things only when we are short on physical memory (1) is possible in GHC. (2) can be implemented, i think. so, the questions is how to trigger each thing. GHC has mechanism to switching GC algorithm although this mechanism is not enough flexible. i personally will prefer to have API that allow to control all GC parameters (although at this moment we also have this api unofficially - we have access to all these variables). so the main question at this time is implementation of returning memory to OS and providing some simple mechanism of enabling this (say, in percents of maximal heap size like this is done "-c"). then someone like me can roll out some API to allow programs better control the entire mechanism -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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