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

[Alain O'Dea]

Got a clear answer about the handling of if defined.

Expanding macros within if defined is non-compliant if cpphs is trying to
be a C99 preprocessor:
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
6.10.1/1 Conditional Inclusion pg 148 indicates that the token after
defined or within defined ( ) is an identifier, not a macro to be expanded.

I'm not sure what's involved in fixing this behavior in cpphs, but I'm
happy to test fixes.

On Sun, Jan 10, 2016 at 3:53 PM Alain O'Dea <alain.odea at gmail.com> wrote:

> 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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