[LLVMdev] [NVPTX] We need an LLVM CUDA math library, after all

[Dmitry Mikushin]

Hi Justin, gentlemen,

I'm afraid I have to escalate this issue at this point. Since it was
discussed for the first time last summer, it was sufficient for us for a
while to have lowering of math calls into intrinsics disabled at DragonEgg
level, and link them against CUDA math functions at LLVM IR level. Now I
can say: this is not sufficient any longer, and we need NVPTX backend to
deal with GPU math.

> There also is no standard libm for PTX.

Yes, that's right, but there is an interesting idea to codegen CUDA math
headers into LLVM IR and link it with user module at IR level. This method
gives a perfect degree of flexibility with respect to high-level languages:
the user no longer needs to deal with headers and can have math right in
the IR, regardless the language it was lowered from. I can confirm this
method works for us very well with C and Fortran, but in order to make
accurate replacements of unsupported intrinsics calls, it needs to become
aware of NVPTX backend capabilities in the form of:

bool NVPTXTargetMachine::
isIntrinsicSupported(Function& intrinsic) and
string NVPTXTargetMachine::whichMathCallReplacesIntrinsic(Function&
intrinsic)

> I would prefer not to lower such things in the back-end since different
compilers may want to implement such functions differently based on speed
vs. accuracy trade-offs.

Who are those different compilers? We are LLVM, the complete compiler
stack, which should handle these things on its specific preference. Derived
compilers may certainly think different, and it's their own business to
change anything they want and never contribute back. We should not forget
there are a lot of derived projects that use LLVM directly, like KernelGen
or many of those embedded DSLs recently started flourishing. Their
completeness and future relies on LLVM. For these reasons, I would strongly
prefer LLVM/NVPTX should supply a reference GPU math implementation and
invite you and everyone else to form a joint roadmap to deliver it.

Before we started, IANAL, but something tells me there could be a licensing
issue about releasing the LLVM IR emitted from CUDA headers.
Could you please check this with NVIDIA?

Many thanks,
- D.

2012/9/6 Justin Holewinski <justin.holewinski at gmail.com>:
> On 09/06/2012 10:02 AM, Dmitry N. Mikushin wrote:
>>
>> Dear all,
>>
>> During app compilation we have a crash in NVPTX backend:
>>
>> LLVM ERROR: Cannot select: 0x732b270: i64 = ExternalSymbol'__powisf2'
>> [ID=18]
>>
>> As I understand LLVM tries to lower the following call
>>
>> %28 = call ptx_device float @llvm.powi.f32(float 2.000000e+00, i32 %8)
>> nounwind readonly
>>
>> to device intrinsic. The table llvm/IntrinsicsNVVM.td does not contain
>> such intrinsic, however it should be builtin, according to
>> cuda/include/math_functions.h
>
>
> It actually gets lowered into an external function call.
>
>
>>
>> Is my understanding correct, and we need simply add the corresponding
>> definition to llvm/IntrinsicsNVVM.td ? How to do that, what are the
>> rules?
>
>
> PTX does not have an instruction (or simple series of instructions) that
> implements pow, so this will not be handled.  I would prefer not to lower
> such things in the back-end since different compilers may want to
implement
> such functions differently based on speed vs. accuracy trade-offs.
>
> There also is no standard libm for PTX.  It is up to the higher-level
> compiler to link against a run-time library that provides functions like
pow
> (see include/math_functions.h in a CUDA distribution).
>
>>
>> Thanks,
>> - D.
>> _______________________________________________
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>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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>
>
> --
> Thanks,
>
> Justin Holewinski
>
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