Sjoerd Meijer via llvm-dev
2021-Mar-05 09:49 UTC
[llvm-dev] 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> on behalf of Wang,
Pengfei via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 05 March 2021 06:28
To: Jason Hafer <jhafer at mathworks.com>
Cc: llvm-dev <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> 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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Wang, Pengfei via llvm-dev
2021-Mar-05 12:30 UTC
[llvm-dev] Is it legal to pass a half by value on x86_64?
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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