llvm dev - Mar 2021 - Is it legal to pass a half by value on x86_64?

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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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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Hi Jason,

__fp16 is a pure storage format. You cannot pass it by value, because only
ABI<https://gitlab.com/x86-psABIs/x86-64-ABI> permissive types can be
passed by value while __fp16 is not one of them.


  *   if "define void @foo(i8, i8, i8, i8, half) " is even legal to
use
half as a target independent type is legal for LLVM. It's not legal for
unsupported target like X86. The behavior depends on how we lowering it. But I
don't know why there's differences between Linux and Windows. Maybe
because "__gnu_f2h_ieee" is a Linux only function?

Thanks
Pengfei

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Jason
Hafer via llvm-dev
Sent: Friday, March 5, 2021 10:46 AM
To: llvm-dev at lists.llvm.org
Cc: Jason Hafer <jhafer at mathworks.com>
Subject: [llvm-dev] Is it legal to pass a half by value on x86_64?

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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