[llvm-dev] Is it legal to pass a half by value on x86_64?

[Craig Topper via llvm-dev]

I'm not sure how robust the half support is on X86, clang should never
generate it. I believe in llvm 11 it changed to pass in the lower 16 bits
of an integer register instead of in a float register. What does "incorrect
math operations" mean? We're emulating half precision with floats and a
conversion function, I don't think it will always match exactly with what
native half would do.

Do you have a more complete IR file for Windows that I can take a look at?

~Craig


On Thu, Mar 4, 2021 at 9:45 PM Jason Hafer via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
> I am attempting to understand an anomaly I am seeing when dealing with
> half on Windows and could use some help.
>
> Using LLVM 8 or 10, if I have IR of the flavor below:
> define void @foo(i8, i8, i8, i8, half) {
>   %6 = alloca half
>   store half %4, half* %6, align 1
>   ...
>   ret void
> }
>
> Using x86_64-pc-linux, we convert the float passed in with __gnu_f2h_ieee.
> Using x86_64-pc-windows I do not get the conversion, so we end up with
> incorrect math operations.
>
> While investigating I noticed clang gave me the error below:
> error: parameters cannot have __fp16 type; did you forget * ?
> void foo(int dc1, int dc2,int dc3,int dc4, __fp16 in)
>
> So, this got me wondering if "define void @foo(i8, i8, i8, i8, half) " is
> even legal to use or if I should rather pass by ref?  I have yet to find
> documentation to convince me one way or the other.  Thus, I was hoping
> someone here might be able to shed some light on the issue.
>
> Thank you in advance!
>
> Cheers,
>
> JP
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