klibc - May 2021 - Debian #943425: [s390x] setjmp/longjmp do not save/restore all registers in use

Dixi quod?
>Jessica Clarke brought out docs saying f8?f15 must be saved, the
>other FPU registers not:
This needs to be fixed in klibc.
>>? klibc does not really support the FPU anyway
>
>? GCC chooses to allocate an FPU register for a pointer value.
This is a curiosity.
>>? the half of v10 that equals f10 just HAPPENS to be saved by
>>  glibc, but what if the upper half, that is outside of the FPU,
>>  is used?
>
>The question here is, does GCC only use the halves of the half
>of the vector registers that match the FPU registers?
04:41??jrtc27:#debian-x32? hephaistor: re s390x vector registers, reading the
gcc and llvm sources they're
     ?    all call-clobbered by default, only the float parts are call-saved
04:41??jrtc27:#debian-x32? so that's why setjmp/longjmp don't need to
save/restore them
04:42??jrtc27:#debian-x32? there *is* a vector calling convention, but it's
not the default for the ABI,
     ?    it's opt-in, and setjmp/longjmp won't be annotated as such

So we indeed need to only save the registers glibc does.
>@klibc list: as indicated earlier, I can provide a patch if needed
>(though it should be obvious).
bye,
//mirabilos
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On 5/5/21 4:45 AM, Thorsten Glaser wrote:
> Dixi quod?
> 
>> Jessica Clarke brought out docs saying f8?f15 must be saved, the
>> other FPU registers not:
I can confirm this. It is f8-f15 for the z/Architecture (64 bit). It is f1, f3,
f5, f7 for the ESA
architecture (32 bit) which is still supported by Glibc and GCC.
> 
> This needs to be fixed in klibc.
> 
>>> ? klibc does not really support the FPU anyway
>>
>> ? GCC chooses to allocate an FPU register for a pointer value.
GCC will put integer values into vector registers for auto-vectorization or for
spilling. We also
use call-clobbered FPRs as save slots for GPRs in leaf-functions if can get rid
of allocating a
stack frame that way.
> 
> This is a curiosity.
> 
>>> ? the half of v10 that equals f10 just HAPPENS to be saved by
>>>  glibc, but what if the upper half, that is outside of the FPU,
>>>  is used?
>>
>> The question here is, does GCC only use the halves of the half
>> of the vector registers that match the FPU registers?
> 
> 04:41??jrtc27:#debian-x32? hephaistor: re s390x vector registers, reading
the gcc and llvm sources they're
>      ?    all call-clobbered by default, only the float parts are
call-saved
> 04:41??jrtc27:#debian-x32? so that's why setjmp/longjmp don't need
to save/restore them
> 04:42??jrtc27:#debian-x32? there *is* a vector calling convention, but
it's not the default for the ABI,
>      ?    it's opt-in, and setjmp/longjmp won't be annotated as
such
> 
> So we indeed need to only save the registers glibc does.
The vector registers are call-clobbered - exactly for the reason of setjmp /
longjmp. Only f8-f15
need to be saved.

You can find the latest version of our ABI here:
https://github.com/IBM/s390x-abi/releases/download/v1.5/lzsabi_s390x.pdf

However, it is still lacking the vector ABI extension. I wrote a document for
that which we use
internally and we are working on integrating it into the publicly available
version.

Andreas
> 
>> @klibc list: as indicated earlier, I can provide a patch if needed
>> (though it should be obvious).
> 
> bye,
> //mirabilos
>