RFR: 8303040: linux PPC64le: Implementation of Foreign Function & Memory API (Preview) [v35]

[Martin Doerr]

On Tue, 23 May 2023 07:46:08 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> src/hotspot/cpu/ppc/downcallLinker_ppc.cpp line 163:
>> 
>>> 161:   // The Parameter Save Area needs to be at least 8 slots for ABIv1.
>>> 162:   // ABIv2 allows omitting it when all parameters can get passed in registers. We currently don't optimize this.
>>> 163:   // For ABIv2, we only need (_input_registers.length() > 8) ? _input_registers.length() : 0
>> 
>> The PSA is also needed if the parameter list is variable in length. Is the expression `(_input_registers.length() > 8) ? _input_registers.length() : 0` correct in that case too?
>> Otherwise: `ABIv2 allows omitting it if the callee's prototype indicates that stack parameters are not expected. We currently don't optimize this.`
>
> Ok, I see now. This is not obvious though. There are a few layers of abstraction at play which hide this. A comment is needed. Maybe like this:
> ```c++
>             // With ABIv1 a Parameter Save Area of at least 8 double words is always needed.
>             // ABIv2 allows omitting it if the callee's prototype indicates that stack parameters are not expected.
>             // We currently don't optimize this (see DowncallStubGenerator in the backend).
>             if (reg == null) return stack;

I believe omitting the PSA is wrong for varargs, but we don't have this information in the backend. So, I think we should simply not optimize it. Reserving 64 Byte stack space should be affordable for a downcall even if it's not always needed. The Java side could compute it, but there's no way to pass this information to the backend. I've improved the comments. Please take a look.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/12708#discussion_r1202235085