[foreign-memaccess+abi] RFR: 8300294: Add tests for by-value unions and structs with nested fixed-length arrays

[Maurizio Cimadamore]

On Fri, 3 Feb 2023 17:52:23 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> I've added some tests for by-value unions, structs/unions nested into other structs, and aggregates with nested inline arrays.
> 
> These test cases are taken from a fuzzer I wrote a while ago. So, they are a bit random in terms of the generated layouts, but should nonetheless  serve as a basic test for the cases that we miss with TestDowncall/TestUpcall today. This mostly helps give some coverage of the linker's classification code for these cases.
> 
> I've also pulled in some test value generation code, which can generate random test values when given a MemoryLayout, allocator and random generator. (if it seems useful, I can re-write the existing tests to use this generation code as well, but I'd like to save that for a followup PR).
> 
> Unfortunately, the fallback linker doesn't seem to be able to reliably handle by-value unions. It's a know issue with libffi. I've filed: https://bugs.openjdk.org/browse/JDK-8301800 for now. I've changed the implementation to throw an IAE, which was already specified for `Linker::downcallHandle/upcallStub`: 
> 
> https://github.com/openjdk/panama-foreign/blob/f61f3a31af4976d0e64d3bfa72cda95b501e2a7d/src/java.base/share/classes/java/lang/foreign/Linker.java#L232-L235
> 
> There was also a bug in the classification of HFAs on AArch64 which I've fixed. The issue was that structs with float/double fields that are nested into arrays or other structs/unions should also be counted as HFAs. But, the correct classification logic only accounted for the 'flat' case, where a struct only contains ValueLayouts.

Nice test!

src/java.base/share/classes/jdk/internal/foreign/abi/aarch64/CallArranger.java line 269:

> 267:             int regType = forHFA ? StorageType.VECTOR : StorageType.INTEGER;
> 268:             int numChunks = (int)Utils.alignUp(layout.byteSize(), MAX_COPY_SIZE) / MAX_COPY_SIZE;
> 269:             List<MemoryLayout> scalarLayouts = forHFA ? TypeClass.scalarLayouts(layout) : null;

Perhaps, instead of two conditionals, it would be better to use a simple `if` ? I'm a bit worried about one path being `null` which then works in the subsequent conditional because the leading condition is the same. Seems a bit brittle.

src/java.base/share/classes/jdk/internal/foreign/abi/aarch64/TypeClass.java line 70:

> 68:     }
> 69: 
> 70:     private static void scalarLayoutsR(List<MemoryLayout> out, GroupLayout gl) {

where does the `R` in the name comes from?

test/jdk/java/foreign/nested/TestNested.java line 86:

> 84:     }
> 85: 
> 86:     static final StructLayout S1 = MemoryLayout.structLayout(

Wouldn't it be better to have these structs defined inline inside the `List.of` call in the provider? That should make it a little easier to just add a new case.

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PR: https://git.openjdk.org/panama-foreign/pull/780