S390: Remove not needed stack frame in syscall function.
Commit Message
Hi,
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.
Bye,
Stefan
Comments
* 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
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
* Stefan Liebler:
> 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.
I see. I didn't know the ABI details.
Thanks,
Florian
On 10/22/19 1:44 PM, Florian Weimer wrote:
> * Stefan Liebler:
>
>> 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.
>
> I see. I didn't know the ABI details.
>
> Thanks,
> Florian
>
Committed.
Thanks
commit 1349ca6793627f62d0fe96d5e0891e2e0166ba90
Author: Stefan Liebler <stli@linux.ibm.com>
Date: Thu Oct 17 13:49:03 2019 +0200
S390: Remove not needed stack frame in syscall function.
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.
@@ -22,37 +22,19 @@
more information about the value -4095 used below.*/
ENTRY (syscall)
- /* Save registers and setup stack. */
- stm %r6,%r15,24(%r15) /* save registers */
- cfi_offset (%r15, -36)
- cfi_offset (%r14, -40)
- cfi_offset (%r13, -44)
- cfi_offset (%r12, -48)
- cfi_offset (%r11, -52)
- cfi_offset (%r10, -56)
- cfi_offset (%r9, -60)
- cfi_offset (%r8, -64)
+ stm %r6,%r7,24(%r15) /* save registers */
cfi_offset (%r7, -68)
cfi_offset (%r6, -72)
- lr %r1,%r15
- l %r0,4(0,%r15) /* load eos */
- ahi %r15,-96 /* buy stack space */
- cfi_adjust_cfa_offset (96)
- st %r1,0(0,%r15) /* store back chain */
- st %r0,4(0,%r15) /* store eos */
lr %r1,%r2 /* move syscall number */
lr %r2,%r3 /* first parameter */
lr %r3,%r4 /* second parameter */
lr %r4,%r5 /* third parameter */
lr %r5,%r6 /* fourth parameter */
- l %r6,192(%r15) /* fifth parameter */
- l %r7,196(%r15) /* sixth parameter */
-
+ lm %r6,%r7,96(%r15) /* fifth / sixth parameter */
svc 0
- l %r15,0(%r15) /* load back chain. */
- cfi_adjust_cfa_offset (-96)
- lm %r6,%r15,24(%r15) /* load registers. */
+
+ lm %r6,%r7,24(%r15) /* load registers. */
lhi %r0,-4095
clr %r2,%r0 /* check R2 for error */
@@ -22,37 +22,19 @@
more information about the value -4095 used below.*/
ENTRY (syscall)
- /* Save registers and setup stack. */
- stmg %r6,%r15,48(%r15) /* Save registers. */
- cfi_offset (%r15,-40)
- cfi_offset (%r14,-48)
- cfi_offset (%r13,-56)
- cfi_offset (%r12,-64)
- cfi_offset (%r11,-72)
- cfi_offset (%r10,-80)
- cfi_offset (%r9,-88)
- cfi_offset (%r8,-96)
+ stmg %r6,%r7,48(%r15) /* Save registers. */
cfi_offset (%r7,-104)
cfi_offset (%r6,-112)
- lgr %r1,%r15
- lg %r0,8(%r15) /* Load eos. */
- aghi %r15,-160 /* Buy stack space. */
- cfi_adjust_cfa_offset (160)
- stg %r1,0(%r15) /* Store back chain. */
- stg %r0,8(%r15) /* Store eos. */
lgr %r1,%r2 /* Move syscall number. */
lgr %r2,%r3 /* First parameter. */
lgr %r3,%r4 /* Second parameter. */
lgr %r4,%r5 /* Third parameter. */
lgr %r5,%r6 /* Fourth parameter. */
- lg %r6,320(%r15) /* Fifth parameter. */
- lg %r7,328(%r15) /* Sixth parameter. */
-
+ lmg %r6,%r7,160(%r15) /* Fifth / Sixth parameter. */
svc 0
- lg %r15,0(%r15) /* Load back chain. */
- cfi_adjust_cfa_offset (-160)
- lmg %r6,%r15,48(%r15) /* Load registers. */
+
+ lmg %r6,%r7,48(%r15) /* Load registers. */
lghi %r0,-4095
clgr %r2,%r0 /* Check R2 for error. */