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

[Laurent Desnogues]

On Mon, Dec 29, 2008 at 12:35 PM, Edgar E. Iglesias
<address@hidden> wrote:
> 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.

Well even if that din't break qemu_ld64 on i386 (which it does),
lack of saves before doing a memory access that could
generate a signal is a killer.

Trash the idea :)


Laurent