Re: [Qemu-devel] [PATCH] fix compilation/link with clang, target-i386/cpu.c

[Andreas Tobler <andreast@FreeBSD.org>]

On 11.11.13 22:02, Peter Maydell wrote:
> On 11 November 2013 20:10, Andreas Tobler <andreast@freebsd.org> wrote:
>> Paolo asked me to test and submit the below patch to fix compilation and
>> link with clang.
>>
>> Paolo reduced the issue to a clang bug where dead code is not properly
>> eliminated before linktime. (the clang bug ID: 17882)
> 
> Thanks for the patch. However, it looks a bit odd to me. Can
> you quote the error message clang produces, please?

[tcx58:build/qemu/objdir] andreast% gmake
  CC    x86_64-softmmu/target-i386/cpu.o
  LINK  x86_64-softmmu/qemu-system-x86_64
target-i386/cpu.o: In function `cpu_x86_cpuid':
/export/devel/net/src/qemu/qemu-master/target-i386/cpu.c:2203: undefined
reference to `kvm_arch_get_supported_cpuid'
/export/devel/net/src/qemu/qemu-master/target-i386/cpu.c:2204: undefined
reference to `kvm_arch_get_supported_cpuid'
/export/devel/net/src/qemu/qemu-master/target-i386/cpu.c:2223: undefined
reference to `kvm_arch_get_supported_cpuid'
cc: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[1]: *** [qemu-system-x86_64] Error 1
gmake: *** [subdir-x86_64-softmmu] Error 2

> I think I would agree with the commenter in the bug report you
> reference (http://llvm.org/bugs/show_bug.cgi?id=17882)
> that this is not a clang bug. We shouldn't be relying on the
> compiler's dead code elimination to get rid of references to
> functions that don't exist in certain configurations. This will
> always be unreliable (especially if compiling without optimization).
> Instead we should either be using ifdefs or stub functions (probably
> the latter in this case).

I know it is a difficult business. And probably you're right, but from a
dump users point of view I do not agree.

I'm used to gcc, which is able to compile this, and I expect other
compilers to be able to do the same. The compiler should work for me not
vice versa :)

> If you put a stub implementation of kvm_arch_get_supported_cpuid()
> into target-i386/kvm-stub.c does this fix the compilation issue?

Will try and let you know.
Thanks,
Andreas