[NVPTX] Add support for atomic add for f16 type

[akuegel]

atom.add.noftz.f16 is supported since SM 7.0

[Artem-B]

LGTM in general, modulo missing constrain on PTX version.

[Artem-B]

For this test it might be convenient to autogenerate the checks with llvm/utils/update_test_checks.py

[akuegel]

Thanks for the suggestion. Done

[justinfargnoli]

The ordering of the ld.param.*s relative to the atom.* instructions isn't relevant to this test, correct? If so, we may not want to include those CHECK-NEXT:s in the test.

[akuegel]

By now I have created a reproducer for what caused the revert. If I adjust the test slightly, and change the line with %r1:

%r1 = atomicrmw fadd ptr %dp0, half 1.0 seq_cst, align 2

The codegen for this becomes:

ld.param.u64 %rd1, [test_param_0]; 
atom.add.noftz.f16 %rs1, [%rd1], 0x3C00;

And this fails verification with this error:

Arguments mismatch for instruction 'atom'

I lack the knowledge about PTX to know what is wrong with that. Will try to figure it out.

[akuegel]

Ok, seems that ptx makes a difference between floating point constants and integer constants. And we are generating a integer constant, I guess due to using Int16Register. @Artem-B in case you can give me a pointer where I can make sure that we are using a floating point constant here, that would be great :)

[akuegel]

Ok, I found something in NVPTXISelDagToDag.cpp, seems like there is no hex representation for f16 constants, and it is replaced by loading from a f16 register. Is this the right place where to look further?

[Artem-B]

Huh. It appears that the instruction does not accept immetiate arguments for f16 variants, though it does accept them for f32. This looks like a bug in ptxas.
https://godbolt.org/z/9Gv1McM4M

Normally f16 instruction variants accept plain hex immediate values.

Looks like we'll need to disable insttruction variant with an immediate argument and force passing it via a register.

[akuegel]

Tried to disable the instruction variant with an immediate argument, and that seems to work:

#85197