On Tue, 23 May 2023 07:46:08 GMT, Richard Reingruber <rr...@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
