Hi All,
Thank you very much for all the great information. This is awesome!
To circle back on Craig's questions.
I did notice LLVM 11 behave very differently.
** Per: What does "incorrect math operations" mean?
The half is passed to the function as a float. The function does operations with other half numbers. On Windows when we don't get the float to half conversation the input is always truncated to 0.0.
** Per: "Do you have a more complete IR file for Windows that I can take a look at?"
I can get you our IR if you want, but I think it is more convoluted than required. I was working on a unit test and I think all one needs to see the anomaly is:
define void @foo(i8, i8, i8, i8, half) {
; CHECK-I686: callq __gnu_f2h_ieee
%6 = alloca half
store half %4, half* %6, align 1
ret void
}
x86_64-pc-windows gives:
push rax
.seh_stackalloc 8
.seh_endprologue
movss xmm0, dword ptr [rsp + 48] # xmm0 = mem[0],zero,zero,zero
movss dword ptr [rsp + 4], xmm0 # 4-byte Spill
pop rax
ret
.seh_handlerdata
.text
.seh_endproc
What I find extremely interesting is the behavior seems has something to do with the stack? For dropping the inputs by one then even Windows will generate the conversion.
define void @foo(i8, i8, i8, half) {
; CHECK-I686: callq __gnu_f2h_ieee
%5 = alloca half
store half %3, half* %5, align 1
ret void
}
x86_64-pc-windows gives:
sub rsp, 40
.seh_stackalloc 40
.seh_endprologue
movabs rax, offset __gnu_f2h_ieee
movaps xmm0, xmm3
call rax
mov word ptr [rsp + 38], ax
add rsp, 40
ret
.seh_handlerdata
.text
.seh_endproc
** If interested, here is a dissection of our real asm.
For both Windows and Linux our IR calls c2_foo() with a half(2):
...
call void @c2_foo(i8* %S_6, [21 x i8*]* %ptr_gvar_instance_7, %emlrtStack* %c2_b_st_, [18 x float]* @15, half 0xH4000, [18 x i8]* %t10)
They both register this in c2_foo as:
...
%c2_in2_ = alloca half
store half %c2_in2, half* %c2_in2_, align 1
When we compile them, they both send 0x40000000 to c2_foo (a single).
The Linux c2_foo() asm addresses this with a float2half conversion:
...
mov qword ptr [rsp + 448], rdi
mov qword ptr [rsp + 440], rsi
mov qword ptr [rsp + 432], rdx
mov qword ptr [rsp + 424], rcx
movabs rcx, offset __gnu_f2h_ieee # <---Convert Here
mov qword ptr [rsp + 336], r8 # 8-byte Spill
call rcx
mov word ptr [rsp + 422], ax
mov rcx, qword ptr [rsp + 336] # 8-byte Reload
mov qword ptr [rsp + 408], rcx
mov qword ptr [rsp + 392], 0
mov qword ptr [rsp + 384], 0
mov qword ptr [rsp + 376], 0
mov qword ptr [rsp + 368], 0
mov rdx, qword ptr [rsp + 432]
mov qword ptr [rsp + 360], rdx
mov rdx, qword ptr [rsp + 432]
mov rdx, qword ptr [rdx + 8]
mov qword ptr [rsp + 352], rdx
mov rdx, qword ptr [rsp + 440]
mov rdx, qword ptr [rdx + 56]
mov qword ptr [rsp + 344], rdx
mov dword ptr [rsp + 400], 0
jmp .LBB9_9
The Windows c2_foo() asm is missing this conversion but treats the value as if it has been converted.
...
mov rax, qword ptr [rsp + 424]
movss xmm0, dword ptr [rsp + 416] # xmm0 = mem[0],zero,zero,zero # <-- moves the data like it wants to convert but never does
mov qword ptr [rsp + 344], rcx
mov qword ptr [rsp + 336], rdx
mov qword ptr [rsp + 328], r8
mov qword ptr [rsp + 320], r9
mov qword ptr [rsp + 304], 0
mov qword ptr [rsp + 296], 0
mov qword ptr [rsp + 288], 0
mov qword ptr [rsp + 280], 0
mov rcx, qword ptr [rsp + 328]
mov qword ptr [rsp + 272], rcx
mov rcx, qword ptr [rsp + 328]
mov rcx, qword ptr [rcx + 8]
mov qword ptr [rsp + 264], rcx
mov rcx, qword ptr [rsp + 336]
mov rcx, qword ptr [rcx + 56]
mov qword ptr [rsp + 256], rcx
mov dword ptr [rsp + 312], 0
mov qword ptr [rsp + 248], rax # 8-byte Spill
movss dword ptr
________________________________
From: Wang, Pengfei <pengfei.wang at intel.com>
Sent: Friday, March 5, 2021 7:30 AM
To: Sjoerd Meijer <Sjoerd.Meijer at arm.com>; Jason Hafer <jhafer at mathworks.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: RE: 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<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<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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