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

[Andreas Krebbel]

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
>