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.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @skirpichev for commit ac38cb8 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F154796%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[skirpichev]

!buildbot risc

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @skirpichev for commit 6bfff56 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F154796%2Fmerge

The command will test the builders whose names match following regular expression: risc

The builders matched are:

• RISC-V 64-bit Ubuntu PR
• riscv64 Ubuntu PR
• RISC-V 64-bit Ubuntu NoGIL PR
• RISC-V 64-bit Ubuntu Clang PR

[hpkfft]

This looks good.
I tested it using Intel's icx compiler and CFLAGS -mavx512f -mavx512fp16 and test_float passes.