[RFC] external compiler support

[Warner Losh]

On Feb 27, 2013, at 2:44 PM, Brooks Davis wrote:

> On Wed, Feb 27, 2013 at 02:01:42PM -0700, Warner Losh wrote:
>> 
>> On Feb 27, 2013, at 12:08 PM, Brooks Davis wrote:
>> 
>>> On Wed, Feb 27, 2013 at 09:08:05AM -0700, Warner Losh wrote:
>>>> Ooops, forgot to add one item..
>>>> 
>>>> 
>>>> On Feb 27, 2013, at 8:57 AM, Warner Losh wrote:
>>>> 
>>>>> 
>>>>> On Feb 26, 2013, at 5:35 PM, Brooks Davis wrote:
>>>>> 
>>>>>> Below (and at http://people.freebsd.org/~brooks/patches/xcc.diff) you
>>>>>> can find an initial patch with proposed commit for external compiler
>>>>>> support.  It relies on the existing cross binutils as I'm finding that
>>>>>> the two are fairly separable.  With this patch I've been able to build
>>>>>> from amd64 to arm, amd64, and i386 using clang from the lang/clang-devel
>>>>>> port.  I've also compiled the tree with a customized clang being
>>>>>> developed at the University of Cambridge.
>>>>> 
>>>>> Cool!
>>>>> 
>>>>>> The patch is untested with gcc.
>>>>> 
>>>>> I'd like to see it tested with gcc :)
>>>>> 
>>>>>> Does this seem like a reasonable approach?  I do plan to look at external
>>>>>> binutils support, but it's not on the critical path for our current work
>>>>>> so I've opted to avoid it for now.
>>>>> 
>>>>> The patches I posted a few months ago had that as well...
>>>>> 
>>>>>> As a bonus for those who don't need an external compiler, but do run
>>>>>> make buildworld frequently, the XCC, XCXX, and XCPP variables can be set
>>>>>> to the location of the installed base system compiler to avoid building
>>>>>> the compiler twice during buildworld.
>>>>> 
>>>>> I think this will work, but it is kludgy.  I had created a __X=<prefix-path> which was prepended to ${CC}, et al, in sys.mk. It was only defined when you set CROSS_COMPILER_PATH (or EXTERNAL_COMPILER_PATH, I forget) during the cross building stage. It also had the advantage of making external cross binutils available. Your patch could fairly easily use this interface instead of having to set 3 different variables, which will morph to 10 when you add binutil support.
>>>> 
>>> 
>>> I think something like this will have to be done for binutils given the
>>> way -B works, but I don't think it's workable in the general compiler
>>> case because I want to be able to use gcc46 or a future clang33 or
>>> similar as CC without changing the system compiler.  Ideally I'd
>>> also like to support clang's method of finding appropriate binutils
>>> by looking first for /binutils/path/${TARGET_TRIPLE}-tool and then
>>> /binutils/path/tool.
>> 
>> I didn't know that clang did this, but that's certainly doable.
> 
> The key is that for it to work we need to set each tool's name
> individually.
> 
>>> As a strawman, let's say we add a CROSS_COMPILER_PATH and a
>>> CROSS_BINUTILS_PATH.  The former will set XCC, XCXX, and XCPP if they
>>> are unset.  The latter will control -B and set the various binutils
>>> variables (XNM, XLD, etc).
>> 
>> I'm not sure I like splitting things like that. It is unnatural.
> 
> That's the traditional view with lots of historic merit.  At least in
> the short term it's not a useful view for me.  I want to be able to
> use our existing infrastructure to build a cross binutils and then use
> it with an external compiler.  In a clang world, we currently have one
> compiler and many binutils unless we gratuitously build many compilers
> as the FreeBSD build system currently does.  Some day we will likely have
> an all-llvm toolchain available and then we will have one toolchain for
> all supported architectures.
> 
> I suppose could hack what I want to do into the traditional single
> toolchain world view by build a mips64 xdev toolchain and then building
> a linkfarm and/or set of wrapper scripts to it and the clang I want to
> include, but that seems problematic from a reproducability perspective
> (not to mention performance if I need wrappers to set -B).
> 
> Having a CROSS_TOOLCHAIN_PATH path that sets both would probably be a
> useful compromise in this regard.

Are you suggesting something like:

CROSS_COMPILER_PATH?=${CROSS_TOOLCHAIN_PATH}
CROSS_BINUTILS_PATH?=${CROSS_TOOLCHAIN_PATH}

If so, I'd agree, that would be a very useful compromise: hits my ease of use issues, and lets you do what you need on the theory that others will likely need it too.

Warner