In users_guide/profiling.xml : -prof flag not to be used with cabal/stack

[robstewartuk]

The documentation for compiling with cost centre profiling support is here: ​https://github.com/ghc/ghc/blob/master/docs/users_guide/profiling.xml#L79

It says:

Compile and run this program as follows:

$ ghc -prof -fprof-auto -rtsopts Main.hs
$ ./Main +RTS -p
121393
$

This is not the case when cabal or stack are used, as -prof should be omitted, i.e.

cabal install --enable-executable-profiling --enable-library-profiling --ghc-options="-fprof-auto -rtsopts"

If the user does not know this and includes -prof in the --ghc-options string, they are likely to see compiler error messages relating to p_dyn, e.g.

Failed to load interface for ‘GHC.Integer.Type’
Perhaps you haven't installed the "p_dyn" libraries for package ‘integer-gmp-1.0.0.0’?
Use -v to see a list of the files searched for.

Given that other parts of the profiling.xml file does refer to cabal, do people agree that it might be helpful to make it explicit in Section "5.1. Cost centres and cost-centre stacks", that using ghc directly -prof should be used (no change to the XML file):

$ ghc -prof -fprof-auto -rtsopts Main.hs

Though if cabal/stack is being used, the -prof should be omitted (an addition to the XML file):

$ cabal install --enable-executable-profiling --enable-library-profiling --ghc-options="-fprof-auto -rtsopts"

[osa1]

I had this problem before and it's quite annoying, but I feel like the right way to solve this is to add a check in Cabal and print a warning if --ghc-options=-prof is used.

[robstewartuk]

Thanks @osa1 , I agree with this suggestion. I have opened this suggestion for both cabal and stack in two mirrored tickets:

​https://github.com/commercialhaskell/stack/issues/1015 ​https://github.com/haskell/cabal/issues/2827

Once both tickets are addressed, I will return here to close this one.

[robstewartuk]

Indeed, these cabal and stack github tickets have been (stack) or are being (cabal) addressed, as a concern of their compile time flags. See the discussions in the respective links in ​https://ghc.haskell.org/trac/ghc/ticket/10894#comment:2. Closing this GHC ticket.