Re: [Qemu-devel] User mode emulation and TCG_OPF_CALL_CLOBBER

["Edgar E. Iglesias" <edgar.iglesias@axis.com>]

On Mon, Dec 29, 2008 at 11:46:07AM +0100, Edgar E. Iglesias wrote:
> On Fri, Dec 26, 2008 at 03:32:06PM +0100, Laurent Desnogues wrote:
> > Hello,
> > 
> > while looking at generated code for a user mode emulated program
> > I noticed some registers were saved/restored for qemu_{ld,st}
> > operations.  My understanding is that this is only needed for softmmu
> > (and even in that case for the slow path as a comment in tcg.c says)
> > since in that case, a call to a helper might be generated.
> > 
> > This register save & restore behavior is enabled by the op flag
> > TCG_OPF_CALL_CLOBBER.
> > 
> > A quick test on ARM target and x86_64 host for a SPEC2000 test
> > shows removing that flag speeds up execution by about 15%.
> > 
> > Did I understand things correctly?  If so what would be the best
> 
> Hello Laurent,
> 
> I think you did and I think what you propose kind of makes sense but
> unfortunately your patch exposes errors on my setup.
> 
> The i386 backend's ld64 seems to clobber registers (eax/edx) behind
> tcg's back and with your patch at least CRIS no longer passes it's testsuite
> on i386 hosts. (Actually, I can't see how the plain tcg_gen_ld_i64 can work
> reliably with the i386 backend from svn.)

I see now. AFAICT, only qemu_ld64 has issues and only if you remove the
clobber flag.

Cheers