gh-153740: use native _Float16 in PyFloat_Pack/Unpack2

[skirpichev]

• Issue: Conditionally support native half-precision IEEE types in struct/array/ctypes modules and memoryview #153740

[skirpichev]

Not sure how to fix this error:

/usr/bin/ld: /usr/bin/ld: DWARF error: invalid or unhandled FORM value: 0x25
Objects/floatobject.o: in function `PyFloat_Pack2':
floatobject.c:(.text.PyFloat_Pack2[PyFloat_Pack2]+0x32): undefined reference to `__truncdfhf2'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

CC @StanFromIreland (fuzzers)

[StanFromIreland]

The configure probe only tests a compile-time _Float16 constant, so it reports "yes" in the OSS-Fuzz Docker container where clang accepts the type but the container's pre-GCC-12 libgcc lacks __truncdfhf2 which is the runtime helper the PyFloat_Pack2 double to _Float16 conversion links against. I suggest you make configure do a runtime double to _Float16 conversion.

[skirpichev]

I suggest you make configure do a runtime double to _Float16 conversion.

Good idea, that works.

[hpkfft]

Do you need/want -mf16c?
I think GCC will call __truncdfhf2 without it.

[skirpichev]

You can get support for half-floats without real hardware support. This happened on my old laptop. I don't think it's useful.

[hpkfft]

The F16C extensions (VCVTPH2PS and VCVTPS2PH) convert to/from binary32, which doesn't help you if you're trying to convert directly from binary64. So, I think you might as well eliminate the -mf16c from the configuration test above. (Also, it doesn't apply to AArch64, for which I use -march=armv8.4-a+sve when using half precsion.)
I hope you do get this PR merged. If CPython is compiled with -mavx512fp16 or -mavx10.1 or -mavx10.2, then the compiler will generate vcvtsd2sh (scalar double to scalar half) without calling the library's __truncdfhf2 function.

[bedevere-bot]

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