RFR: 8303040: linux PPC64le: Implementation of Foreign Function & Memory API (Preview) [v33]

[Richard Reingruber]

On Mon, 22 May 2023 16:06:02 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

>> Implementation of "Foreign Function & Memory API" for linux on Power (Little Endian) according to "Power Architecture 64-Bit ELF V2 ABI Specification".
>> 
>> This PR does not include code for VaList support because it's supposed to get removed by [JDK-8299736](https://bugs.openjdk.org/browse/JDK-8299736). I've kept the related tests disabled for this platform and throw an exception instead. Note that the ABI doesn't precisely specify variable argument lists. Instead, it refers to `<stdarg.h>` (2.2.4 Variable Argument Lists).
>> 
>> Big Endian support is implemented to some extend, but not complete. E.g. structs with size not divisible by 8 are not passed correctly (see `useABIv2` in CallArranger.java). Big Endian is excluded by selecting `ARCH.equals("ppc64le")` (CABI.java) only.
>> 
>> There's another limitation: This PR only accepts structures with size divisible by 4. (An `IllegalArgumentException` gets thrown otherwise.) I think arbitrary sizes are not usable on other platforms, either, because `SharedUtils.primitiveCarrierForSize` only accepts powers of 2. Update: Resolved after merging of [JDK-8303017](https://bugs.openjdk.org/browse/JDK-8303017)
>> 
>> The ABI has some tricky corner cases related to HFA (Homogeneous Float Aggregate). The same argument may need to get passed in both, a FP reg and a GP reg or stack slot (see "no partial DW rule"). This cases are not covered by the existing tests.
>> 
>> I had to make changes to shared code and code for other platforms:
>> 1. Pass type information when creating `VMStorage` objects from `VMReg`. This is needed for the following reasons:
>> - PPC64 ABI requires integer types to get extended to 64 bit (also see CCallingConventionRequiresIntsAsLongs in existing hotspot code). We need to know the type or at least the bit width for that.
>> - Floating point load / store instructions need the correct width to select between the correct IEEE 754 formats. The register representation in single FP registers is always IEEE 754 double precision on PPC64.
>> - Big Endian also needs usage of the precise size. Storing 8 Bytes and loading 4 Bytes yields different values than on Little Endian!
>> 2. It happens that a `NativeMemorySegmentImpl` is used as a raw pointer (with byteSize() == 0) while running TestUpcallScope. Hence, existing size checks don't work (see MemorySegment.java). As a workaround, I'm just skipping the check in this particular case. Please check if this makes sense or if there's a better fix (possibly as separat...
>
> Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add comment about Register Save Area.

src/hotspot/cpu/ppc/downcallLinker_ppc.cpp line 161:

> 159:   // (native_abi_reg_args is native_abi_minframe plus space for 8 argument register spill slots)
> 160:   assert(_abi._shadow_space_bytes == frame::native_abi_minframe_size, "expected space according to ABI");
> 161:   // Note: For ABIv2, we only need (_input_registers.length() > 8) ? _input_registers.length() : 0

This is hard to understand. It should be explained that we allocate a PSA even though ABI V2 only requires it if not all parameters can be passed in registers.

src/hotspot/cpu/ppc/downcallLinker_ppc.cpp line 162:

> 160:   assert(_abi._shadow_space_bytes == frame::native_abi_minframe_size, "expected space according to ABI");
> 161:   // Note: For ABIv2, we only need (_input_registers.length() > 8) ? _input_registers.length() : 0
> 162:   int register_save_area_slots = MAX2(_input_registers.length(), 8);

Both specs, ABI V1 and V2, call this "Parameter Save Area" we should use the same name.
Suggestion:

  int parameter_save_area_slots = MAX2(_input_registers.length(), 8);

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PR Review Comment: https://git.openjdk.org/jdk/pull/12708#discussion_r1201132931
PR Review Comment: https://git.openjdk.org/jdk/pull/12708#discussion_r1201128718