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Re: [Qemu-devel] User mode emulation and TCG_OPF_CALL_CLOBBER
From: |
Edgar E. Iglesias |
Subject: |
Re: [Qemu-devel] User mode emulation and TCG_OPF_CALL_CLOBBER |
Date: |
Mon, 29 Dec 2008 12:35:53 +0100 |
User-agent: |
Mutt/1.5.16 (2007-06-09) |
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