cpphs: can't be used to build GHC 7.10.3. Bug?

[Alain O'Dea]

I've isolated the issue to the handling of if defined on multi-argument
macros.

I took a crack at interpreting the cpphs source for this and I think it may
be a bug in the conversion of defined expressions here in
Language.Preprocessor.CppIfdef here:

    convert "defined" [arg] =
      case lookupST arg st of
        Nothing | all isDigit arg    -> return arg
        Nothing                      -> return "0"
        Just (a at AntiDefined{})       -> return "0"
        Just (a at SymbolReplacement{}) -> return "1"
        Just (a at MacroExpansion{})    -> return "1"

It looks like it will macro expand the contents of a defined expression
which isn't what GCC does.  I don't know if GCC is wrong or if using
parameterized macros within

if defined works on single-argument macros.

working1.hs:

    {-# LANGUAGE CPP #-}

    #define EXAMPLE_MACRO(arg) (\
                                   arg)

    #if defined(EXAMPLE_MACRO)
    #endif

preprocess it (it works!):

    $ cpphs --cpp working1.hs -o $tempfile
    $

ifdef works on multiple-argument macros.

working2.hs:

    {-# LANGUAGE CPP #-}

    #define EXAMPLE_MACRO(arg1,arg2) (\
                                   arg1 > arg2)

    #ifdef EXAMPLE_MACRO
    #endif

preprocess it (it works!):

    $ cpphs --cpp working2.hs -o $tempfile
    $

if defined fails on multi-argument macros.

broken2.hs:

    {-# LANGUAGE CPP #-}

    #define EXAMPLE_MACRO(arg1,arg2) (\
                                   arg1 > arg2)

    #if defined(EXAMPLE_MACRO)
    #endif

preprocess it (it fails!):

    $ cpphs --cpp broken2.hs -o $tempfile
    cpphs: macro EXAMPLE_MACRO expected 2 arguments, but was given 0
    $

I've posted a StackOverflow question to see if any of them know if this is
undefined behavior:
http://stackoverflow.com/questions/34709769/is-cpphs-wrong-or-is-the-behavior-of-macros-with-arguments-in-if-defined-express

If it is undefined behavior we should stop relying on it in GHC sources.
Either way the behavior is inconsistent with GCC which complicates things.

Best,
Alain

On Sun, Jan 10, 2016 at 2:04 PM Alain O'Dea <alain.odea at gmail.com> wrote:

> Hi Malcolm:
>
> cpphs is under consideration as a replacement for GCC's C preprocessor in
> the GHC toolchain:
> https://ghc.haskell.org/trac/ghc/wiki/Proposal/NativeCpp
>
> GHC 7.10.3's build fails when cpphs is used as the C preprocessor
> (--with-hs-cpp=cpphs --with-hs-cpp-flags="--cpp").
>
> It runs into this error when preprocessing libraries/base/GHC/Natural.hs:
>
> cpphs: macro MIN_VERSION_integer_gmp expected 3 arguments, but was given 0
>
> I've reproduced this issue on Ubuntu 14.04 x86-64 and SmartOS 15.3.0
> x86-64.
>
> Interestingly the error seems to arise only when preprocessing Natural.hs
> while the autogenerated cabal-macros.h is present.  Removing that include
> from the cpphs flags leads to a clean preprocessing run.
>
> I have more details of this investigation here:
> https://gist.github.com/AlainODea/bd5b3e0e6f7c4227f009
>
> Is this a bug?
>
> Best,
> Alain
>
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