On Wed, Dec 26, 2012 at 9:16 PM, Andi Kleen <a...@firstfloor.org> wrote: >> Can you please post a real-world example, where using %r15 would break >> existing code? > > I used to run into problems like this when porting code to gcc from icc or VC. > A lot of hyper optimized inline assembler snippets wants to use all registers > and icc/VC support that. With gcc usually had to add some manual > push/pops. In older gcc versions usually more than one because reload > tended to error out otherwise. > > So by try and error used the non fixed registers, but let the compiler know > about the others. This case would break. > > In 64bit it was less a problem than on 32bit, but could still happen. > > Admittedly medium is somewhat obscure and rarely used (and very slow), > but someone could be still using it.
The alternative approach is changing cpuid definition in cpuid.h (as in attached patch) to preserve %rbx, but we can't detect various code model settings there. Since the change depends on the definition of __PIC__, we unnecessary preserve %rbx also for -mcmodel=small. However, code that involves cpuid is rarely performance critical, so perhaps we can live with this tradeoff. IMO, this patch can be used on 4.7 branch, too. Uros.
