Subject: Re: linux-next: manual merge of the xen-tip tree with the tip tree

From: Peter Zijlstra <[email-masked]>
Date: Wed, 12 Aug 2015 19:38:23 +0200

On Wed, Aug 12, 2015 at 07:21:05PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 12, 2015 at 09:27:38AM -0400, Boris Ostrovsky wrote:
> 
> > Incidentally, 11276d53 ("locking/static_keys: Add a new static_key
> > interface") breaks old-ish compilers (gcc version 4.4.4 20100503 (Red Hat
> > 4.4.4-2) (GCC)):
> > 
> > 
> > 
> >   CC      arch/x86/kernel/nmi.o
> > In file included from
> > /home/build/linux-boris/include/linux/jump_label.h:109,
> >                  from
> > /home/build/linux-boris/arch/x86/include/asm/spinlock.h:5,
> >                  from /home/build/linux-boris/include/linux/spinlock.h:88,
> >                  from /home/build/linux-boris/arch/x86/kernel/nmi.c:14:
> > /home/build/linux-boris/arch/x86/include/asm/jump_label.h: In function
> > ‘nmi_handle’:
> > /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: warning: asm
> > operand 0 probably doesn’t match constraints
> > /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: error:
> > impossible constraint in ‘asm’
> > make[3]: *** [arch/x86/kernel/nmi.o] Error 1
> > make[2]: *** [arch/x86/kernel] Error 2
> > make[1]: *** [arch/x86] Error 2
> 
> Ugh bugger.
> 
> I bet its that: &((char *)key)[branch] business, an earlier variant
> thereof tripped up more recent GCCs too.
> 
> So its an __always_inline function, and both argument are always compile
> time constants, @key is the address of an object in static storage (a
> global) and @branch is a simple 0/1 at the call site.
> 
> Now we wish to compute (unsigned long)key + branch at compile/link time
> to feed to the assembler as an immediate, which should be possible,
> given its all 'constants'.
> 
> It just appears GCC is having a hard time with this.
> 
> Let me see if I have a sufficiently old GCC around to play with.

So both my 4.4 compilers:

gcc-4.4 (Ubuntu/Linaro 4.4.7-8ubuntu1) 4.4.7
gcc-4.4 (Debian 4.4.7-2) 4.4.7

Do not have CC_HAVE_ASM_GOTO at all, and my gcc-4.6 (my next oldest
compiler) doesn't have trouble building this.