Pass unions by value with the C ABI's register classes - #10
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union_spec.rb and UnionTest.c match the ffi gem's March 2026 versions without its JRuby skips; union_by_value_spec.rb adds mixed-class, nested, oversize and callback shapes.
Pick the libffi filler from the union's leaves: the float type for a homogeneous floating aggregate, per-eightbyte SSE/INTEGER cells on SysV x86_64, an integer of the union's alignment otherwise.
A 4-aligned union whose cells differ in class, nested at offset 4, takes the enclosing struct's eightbytes: (INTEGER, SSE) on SysV x86_64. Argument, return and callback forms.
libffi merges the cells into eightbytes at the union's offset inside an enclosing struct; a per-cell class is exact there and unchanged for a union passed on its own.
No behaviour change: the one-argument newUnion hands the host platform to the overload, which tests can call with any CPU and OS.
A union of one floating type is a floating-point aggregate on AArch64, ARM, PPC64 ELFv2 and x86_64; riscv64, loongarch64 and s390x pass every union in integer registers, so the integer filler must stay there.
riscv64, loongarch64 and s390x pass unions in integer registers while libffi would put a struct of doubles into FP registers; keep the integer filler there and the float one on AArch64, ARM, PPC64 ELFv2 and x86_64.
JRuby's FFI rejects :long_double layout members, so no union has a long double leaf or an alignment of 16; the integer filler list is now what its name says.
master names riscv64 as a CPU of its own (jruby-10.0 reports it as UNKNOWN); like s390x and loongarch64 it must keep the integer filler.
A failing assertion printed the jffi Type objects by identity; it now prints the cell names, so a wrong filler reads as SINT64 versus DOUBLE.
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Makes JRuby pass C unions by value the way a C compiler does.
Before, every 8-byte union went in an integer register, so a union of floats came back as garbage and a callback receiving one crashed the JVM.
Now the union's members decide the register class, on x86_64 and ARM64.