Re: Building world with clang

On Tue, 17 Aug 2010 13:32:39 +0200
Dimitry Andric <dimitry_at_andric.com> wrote:

> Hi,
> 
> Since clang has gone into the tree, there has been an effort to get
> head in a state where world can optionally be built with it.  A
> number of changes required for this have already been committed,
> mainly thanks to Ed Schouten, Roman Divacky and Rui Paulo.  Most of
> these changes were adapted from the clangbsd project branch.
> 
> There are several other changes in the queue, pending review and/or
> further enhancement, but I would like to solicit your comments about
> one particular set: the changes required to let buildworld use clang
> as the compiler.
> 
> Probably the most logical way to specify that you want world built
> with clang, is to put CC=clang and CXX=clang++ in /etc/src.conf.  Any
> necessary modifications to Makefile.inc1 or bsd.*.mk can then be put
> between conditionals like:
> 
> .if ${CC:T:Mclang} == "clang"
> [...stuff specific to clang...]
> .endif
> 
> so nothing will change for non-clang builds.
> 
> Now, for building clang as the bootstrap compiler, there are basically
> two methods to make it use the correct headers, C startup objects, and
> libraries from the object tree (${WORLDTMP}):
> 
> 1) The "isysroot" method: build a regular version of clang, and make
>    sure WMAKEENV contains something like:
> 
>    CC="${CC} -isysroot ${WORLDTMP} -B${WORLDTMP}/usr/lib/ \
>                                    -L${WORLDTMP}/usr/lib/"
> 
> 2) The "tools-prefix" method: build a special version of clang, that
>    has its default search paths for headers, startup objects and
>    libraries modified, to look for everything under ${WORLDTMP}.
> 
> Method 1) is used in the clangbsd project branch.
> 
> Method 2) is similar to what is used for building the in-tree gcc as
> the bootstrap compiler.  During the cross-tools stage, TOOLS_PREFIX is
> defined to point at ${WORLDTMP}, and the bootstrap gcc's built-in
> search paths get prefixed with it.
> 
> An advantage of method 1) is that it does not require any
> modifications to the compiler, and basically just a few extra command
> line arguments. The same method could probably even be made to work
> for gcc.
> 
> However, a disadvantage is that the built-in search paths of the
> bootstrap compiler are not entirely disabled by using the -isysroot,
> -B and -L flags, so there is still a chance that headers, objects or
> libraries outside of ${WORLDTMP} will be picked up during the world
> stage.
> 
> An advantage of method 2) is that you can be 100% sure all built-in
> search paths of the bootstrap compiler point to directories under
> ${WORLDTMP}.  Of course, a disadvantage is that you have to make some
> modifications to the compiler source itself.
> 
> I would like to hear your opinions about which method is preferred, or
> if there may be another good way to solve the bootstrap compiler
> issue.
> 
> I have also attached two patches to this mail, which can be applied to
> head, to show the exact set of changes required for each method.
> 

Does method 1) work fine with 'make buildenv'? I doubt that. I would
strongly suggest we should not lose this feature. I do not like the
idea of having to depend on -isystem in CFLAGS in such an environment. 

-- 
Alexander Kabaev