From: Stefan Liebler <stli@linux.ibm.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [PATCH] S390: Remove not needed stack frame in syscall function.
Date: Tue, 22 Oct 2019 11:38:00 -0000 [thread overview]
Message-ID: <0ca333b6-3faf-df73-bb24-d03aa155dfb9@linux.ibm.com> (raw)
In-Reply-To: <87o8y9krf9.fsf@oldenburg2.str.redhat.com>
On 10/22/19 12:25 PM, Florian Weimer wrote:
> * Stefan Liebler:
>
>> As an svc invocation does not clobber any user space registers
>> despite of the return value r2 and it does not need a special
>> stack frame. This patch gets rid of the extra frame.
>> We just have to save and restore r6 and r7 as those are
>> preserved across function calls.
>
> Looks okay to me. Would it be possible to save r6 and r7 in
> caller-saved registers not clobbered by the system call? That might
> provide another small benefit.
>
> Thanks,
> Florian
>
The syscall itself just clobbers the return value in r2. But for its
parameters we have to clobber r6 and r7.
According to the ABI, r0-r5 and r14 are volatile.
We need r1 for the syscall number for "svc 0", r2-r7 as parameters for
svc and r14 is the return-address.
Thus we could use r0 for saving/restoring r6.
For r7 we have the option to either use the register save area on the
stack-frame prepared by the caller or one of the volatile fprs. But the
instructions for transferring gpr <-> fpr are not available with all
architecture level sets. Thus we would need something like #ifdef /
#else to provide alternative implementations.
Therefore I think just storing/restoring both registers at once with one
stmg/lmg instruction is okay.
Thanks,
Stefan
next prev parent reply other threads:[~2019-10-22 11:38 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-22 8:00 Stefan Liebler
2019-10-22 10:25 ` Florian Weimer
2019-10-22 11:38 ` Stefan Liebler [this message]
2019-10-22 11:44 ` Florian Weimer
2019-10-23 12:54 ` Stefan Liebler
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