RFR: 8311630: [s390] Implementation of Foreign Function & Memory API (Preview) [v6]

[sid8606]

On Mon, 31 Jul 2023 08:08:37 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> sid8606 has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Preserve and restore register Z_R6
>>   
>>   Though Z_R6 is argument register it is a saved register
>>   so preserve and restore Z_R6 register
>
> src/hotspot/cpu/s390/upcallLinker_s390.cpp line 45:
> 
>> 43:     if (reg == Z_SP) continue;
>> 44:     // though Z_R6 is argument register it is a saved register
>> 45:     if (!abi.is_volatile_reg(reg) || reg == Z_R6) {
> 
> So, is the prior assumption that all argument registers are also volatile incorrect?
> 
> If so, I think it would be better to change `ABIDescriptor::is_volatile_reg` ([1]) to only look at the `_integer_additional_volatile_registers` list (maybe rename it too), and then list all the volatile regs in  LinuxS390CallArranger explicitly, excluding R6 ([2]). That way all the information about which regs are volatile is in one place (LinuxS390CallArranger).
> 
> [1]: https://github.com/openjdk/jdk/pull/14801/files#diff-7096e1975de20baa3219d616506f26ba2b4500bf5ad28c331e3d6049a32a461eR39-R42
> [2]: https://github.com/openjdk/jdk/pull/14801/files#diff-09da9016992b04bab73b6cc6aad8ca719e86c90c398af4e000583c1f8220a99bR73

Yes, s390x ABI says R6 is an argument register as we as non-volatile register.
It make sense, I'll make suggested changes.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14801#discussion_r1279064461