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Subject: Re: [boost] [clang][preprocessor] Testing of clang emualting the VC++ preprocessor on Windows
From: Edward Diener (eldiener_at_[hidden])
Date: 2016-03-31 07:28:28

On 3/30/2016 5:05 PM, Edward Diener wrote:
> I have made local changes in the Boost preprocessor config.hpp on the
> 'develop' branch to have clang's emulation of the non-standard VC++
> preprocessor act like the VC++ preprocessor as far as Boost PP is
> concerned. Boost PP has a large number of workarounds for VC++ without
> which Boost PP preprocessor code would not work when compiling using any
> version of VC++.
> I have tested the usual compilers extensively to make sure they are
> unaffected by these changes as well as the normal clang-linux
> implementation. Everything remains the same for these compilers with my
> changes so I am assured the changes will not affect any other compiler
> implementation.
> I was able to test both clang-3.8 and the latest build of clang, which
> is clang-3.9, by invoking clang-cl. My user-config.jam file for these
> entries looks like:
> using clang-win : 3.8 : SomePath/clang-cl : <compatibility>vc12
> using clang-win : 3.9 : SomePath/clang-cl : <compatibility>vc14
> By passing toolset=clang-win-3.8 or toolset=clang-win-3.9 I was able to
> successfully test these implementations against the Boost PP test code.
> There are quite a few failures in this clang emulation of the VC++
> preprocessor when tested against the preprocessor test code. In fact
> testing clang emulation of VC++ preprocessor when I don't treat clang as
> VC++ in Boost PP, but as its normal C++ standard conforming
> preprocessor, still fails a few cases but far less than when clang is
> treated as VC++ in Boost PP. Needless to say when invoking clang-linux,
> which tests the normal clang implementation whether on Windows or Linux,
> there are no failures in the Boost PP tests.
> I am perfectly willing to push my changes to 'develop' for the
> preprocessor. The changes are essentially just in the Boost PP
> config.hpp file, which sets macros which control the compiler
> workarounds in Boost PP. As I have previously stated I am not personally
> willing to try to figure out or report to clang why their VC++
> preprocessor emulation in Boost PP is failing, but others who are
> interested can do so if they wish. My personal reasons for this is that
> I think it is a very bad thing to attempt to replicate an already flawed
> C++ preprocessor in another compiler in general code. I also have spent
> enough time working with the VC++ preprocessor trying to find
> workarounds for its non-standard behavior that I do not want to do so
> for another preprocessor which attempts to be non-standard.
> I posted this to alert others to my plans to update Boost PP on
> 'develop' with this change, in case anybody is greatly opposed to it or
> wants to discuss it further. The change will not affect any other
> compiler but clang in VC++ preprocessor emulation mode. Unless I hear
> objections etc. I plan to push my update within the next day although I
> will wait awhile for any replies to this post.

I pushed my update to Boost PP on 'develop' so anybody using clang
targeting VC++ for Windows can try it out. It is possible that the clang
emulating the non-standard VC++ preprocessor errors I see when running
the preprocessor tests with clang-win toolset are few but are being
propagated in many different cases. However I don't even think there is
a way to see, for anybody interested, how clang is expanding macros in
its VC++ emulation as opposed to normal clang-linux where macro
expansion is shown when clang preprocessor erros are detected.

Note on clang targeting on Windows:

The LLVM 3.8 binary distribution of clang for Windows defaults to
clang/clang++ targeting VC++, while all previous LLVM binary
distributions of clang for Windows defaulted to clang/clang++ targeting

The clang-cl executable always targets VC++ AFAIK. When you build from
source using VC++ on Windows then clang/clang++ targets VC++ by default
and when you build from source using mingw(-64)/gcc on Windows then
clang/clang++ targets mingw(-64)/gcc by default.

You can always change the targeting of clang through the use of the
clang/clang++ '-target' option. It's details are poorly documented at I chided
clang developers about this incomplete documentation but without effect.

I was given the targets '-target i686-pc-windows-gnu' and '-target
x86_64-pc-windows-gnu' as a way of targeting mingw(-64)/gcc for 32-bit
and 64-bit clang/clang++ versions respectively, both of which I have
tested and do work. I believe the equivalent targets for 32-bit and
64-bit versions of clang/clang++ targeting VC++ are
'-target=i686-pc-windows-msvc' and '-target=x86_64-pc-windows-msvc'
respectively but I have not tried these out.

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