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

[Wang, Pengfei via llvm-dev]

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 <Sjoerd.Meijer at arm.com>
Sent: Friday, March 5, 2021 5:49 PM
To: Jason Hafer <jhafer at mathworks.com>; Wang, Pengfei <pengfei.wang at intel.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
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<https://gitlab.com/x86-psABIs/x86-64-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 at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> on behalf of Wang, Pengfei via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Sent: 05 March 2021 06:28
To: Jason Hafer <jhafer at mathworks.com<mailto:jhafer at mathworks.com>>
Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at 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<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<mailto: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<mailto:llvm-dev at lists.llvm.org>
Cc: Jason Hafer <jhafer at mathworks.com<mailto: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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