On 21 September 2011 17:33, Jeremy O'Donoghue
jeremy.odonog...@gmail.com wrote:
Hi Dave,
On 20 September 2011 17:58, Dave Tapley duked...@gmail.com wrote:
On 16 September 2011 23:30, Dave Tapley duked...@gmail.com wrote:
I presume everyone has a very long compile time when building wxcore?
Specifically rebuilding everything under src/cpp/ every time..
Has anyone ever looked into avoiding this complete rebuild?
I've just spent a few hours looking deeper in to this and come across two
issues:
1. There is a very informative blog post[1] written by Jeremy, which deals
with the subject of Compiling C or C++ code from within Cabal.
Unfortunately I can't find any of the code mentioned in the post in the
project, specifically I tried to find a myBuildHook in ./wxcore/Setup.hs
(I also looked in previous revisions using a trackdown grep[2]) but I didn't
find anything. Perhaps someone with better knowledge of the project can
comment on if/where/when the code in the post was used?
The code sits on my development machine. It works, but with a fairly serious
limitation.
The limitation is that I do not do any dependency tracking. It works by
checking if an object file is older than its corresponding source file. This
works fine for .cpp files but breaks if you change a header. Short of
writing a complete dependency tracker, this is hard to fix, and in truth I
think it belongs in Cabal.
If Cabal wants to say that it can compile C/C++ code, it should be the one
to do so correctly, especially as dependency tracking for C/C++ is vile and
compiler dependent(*).
Firstly welcome back, and thanks for taking the time to reply :)
Ah, this all makes more sense now.
Do you know if there is any documentation on cabal's C/C++ compiling abilities?
I still have no idea how the confHook stuff actually results in code
being built, because I don't see any reference to the C++ code (i.e.
wxcore/src/cpp/) in Setup.hs, only the calling of 'wxdirect' to
generate them, and use of 'libBuildInfo' to set up GCC.
I would be very happy to put the code out there as a GitHub gist or similar
- it's only in one or two files, and quite easy to follow, but I don't feel
it is ready for prime time due to the limitations noted above.
Well, for me, the compile time of the C++ component is around four
minutes, which proves to be a real pain when you miss one semi-colon.
I'd certainly like to give your code a go, and I shall do so whilst
heeding your warning :)
(*) one option might be to do this only for GCC, as in practice we only
really support GCC anyway.
2. Inspecting the wxdirect code you can see that System.IO.writeFile is
used to write all the generated code[3], but no test is performed to see if
the output file has actually changed. Thus the file is always opened for
write, and so its modification time is changed, and so everything is
recompiled every time wxcore is built. I have have written a local patch
which replaces the writeFile function with one which first checks whether
the string to be written differs (aside from date/time stamp) to the current
one; it only performs the writeFile if there has been a change.
Using this patch none of the Haskell code is re-built, but unfortunately
all the C++ code is.
This is a nice patch - I think we should apply it.
Excellent, well at the moment it's a rather 'quick' implementation,
comprising of the following function:
writeFileIfRequired :: FilePath - String - IO ()
writeFileIfRequired f str = readFile f = evaluate = \str' - when
(not $ and $ zipWith isSame (lines str) (lines str')) (writeFile f
str)
where isSame l1 l2 = (UTC `isInfixOf` l1) || (UTC `isInfixOf`
l2) || (l1 == l2)
You'll note the the use of (UTC `isInfixOf`), this is my nasty hack
to get around the fact that the time of generation is written in to
the file, and this obviously changes every time. Unless anyone has a
good reason not to, I'd like to suggest we remove this generated
on... lines (but leave in the don't edit this blurb), if someone
really wants to know they can always check the file modification time?
Dave,
Jeremy
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