cross-compiling for arm with clang

[Warner Losh]

On Jun 21, 2011, at 3:12 AM, Damjan Marion wrote:

> 
> On Jun 17, 2011, at 9:16 PM, Warner Losh wrote:
> 
>> 
>> On Jun 17, 2011, at 12:38 PM, Damjan Marion wrote:
>> 
>>> Now, I'm back on my original problem, clang invokes /usr/bin/as which is i386 AS instead of ARM version in obj tree.
>> 
>> That's a bogus assumption on the part of clang.  At the very least, it should be adding the magic gas flags to use the correct triple for the platform.  But if you did that, your next complaint would be that we only build as for the native architecture and not anything else.
>> 
>>> How this works on other platforms (i.e. cross-compiling for amd64)?
>> 
>> How it normally works is that we build a compiler that invokes the right as, ld, etc for the architecture in question.  Clang is clearly broken here in assuming that as can cope with anything other than native assembler :)  We normally either build gcc that knows where to find these files.  In the buildworld case, this is communicate with weird paths and shell variables.  With the whole 'xdev' series of targets (which are instructive to look at), we build into a specific location /usr/freebsd-xdev-arm/bin, etc).
>> 
>> It works by accident for i386 on amd64, I think.  as treats the more or less as the same.
> 
> I submitted a patch[1] to clang mailer which invokes xxx-yyy-zzz-as/ld based on -ccc-host-triple. Patch is copy of netbsd code which is already in clang.
> 
> For me it works well, even when I use clang on Darwin host, it calls proper as/ld.
> 
> # /opt/llvm/bin/clang -ccc-host-triple arm-unknown-freebsd -mfloat-abi=soft  test.c -o test -v -c
> clang version 3.0 (http://llvm.org/git/clang.git 98138cdfdee05c0afbab2b209ce8cfe4a52474e1)
> Target: arm-unknown-freebsd
> Thread model: posix
> [snip]
> End of search list.
> "/opt/llvm/bin/arm-unknown-freebsd-as" -o test /var/folders/kb/kbDaYaFqFQaCg3mXbTLH5U+16gE/-Tmp-/cc-RMHOwf.s
> 
> 
> Is this what we need to support cross-compiling freebsd with clang?

As we do it today?  No.  Today we build the binaries as 'as' which are part of the buildworld.  These binaries aren't called arm-unknown-freebsd-as, but instead 'as'.

Do we want to switch? Maybe.

There's a make xdev target that installs both the /usr/arm-freebsd/usr/bin/as type files as well as symbolic links in /usr/local/bin/arm-freebsd-as.  But that target isn't part of buildworld.  It has been tested recently (it was broken for a while in 8, and I'm not sure I've merged the patches over yet).  However, it doesn't include the extra bogus 'unknown' which if you were to have included it breaks many of the autoconf scripts that I ran into when I did this patch.  Also, I install things as /usr/local/bin/arm-freebsd9-as as well, since that's what we needed for gcc compatibility.  I suppose we could include a unknown thing.

However, you'd need to install symlinks to make this work in a build-world setting.

> Is there any other comment to this patch[1]?
> 
> 
> [1] http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20110620/043072.html

Not enough of a llvm guy to know if this patch is good or bad.

Warner