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:
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?
2012/9/6 Justin Holewinski <email@example.com>:
During app compilation we have a crash in NVPTX backend:
LLVM ERROR: Cannot select: 0x732b270: i64 = ExternalSymbol’__powisf2’
As I understand LLVM tries to lower the following call
%28 = call ptx_device float @llvm.powi.f32(float 2.000000e+00, i32 %8)
to device intrinsic. The table llvm/IntrinsicsNVVM.td does not contain
such intrinsic, however it should be builtin, according to
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
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).