(original) (raw)

I guess it’s designed for language portability. You can use this type across different platforms. Nevertheless, I’m not a FE expert, so I cannot think out other intentions.

The \_Float16 is a primitive type in the latest x86 ABI, but there’s no X86 target that supports it yet. So you cannot use it on X86 by now. I think that’s the difference from \_\_fp16 and why should use it.

We also have some discussion here. https://reviews.llvm.org/D97318

Thanks

Pengfei

From: Sjoerd Meijer
Sent: Friday, March 5, 2021 5:49 PM
To: Jason Hafer ; Wang, Pengfei
Cc: llvm-dev
Subject: Re: Is it legal to pass a half by value on x86\_64?

\_\_fp16 is a pure storage format. You cannot pass it by value, because only ABI permissive types can be passed by value while \_\_fp16 is not one of them.

Yep. Any specific reason to use a pure storage format? The native type is \_Float16 and would give some benefits, but this is not yet supported on x86, see also:

https://clang.llvm.org/docs/LanguageExtensions.html#half-precision-floating-point

Cheers,
Sjoerd.

From: llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Wang, Pengfei via llvm-dev <llvm-dev@lists.llvm.org>
Sent: 05 March 2021 06:28
To: Jason Hafer <jhafer@mathworks.com>
Cc: llvm-dev <llvm-dev@lists.llvm.org>
Subject: Re: \[llvm-dev\] Is it legal to pass a half by value on x86\_64?

Hi Jason,

\_\_fp16 is a pure storage format. You cannot pass it by value, because only 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@lists.llvm.org> On Behalf Of Jason Hafer via llvm-dev
Sent: Friday, March 5, 2021 10:46 AM
To: llvm-dev@lists.llvm.org
Cc: Jason Hafer <jhafer@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,