On Thu, 2018-02-08 at 15:56 -0800, Francisco Jerez wrote: > Jan Vesely <jan.ves...@rutgers.edu> writes: > > > On Thu, 2018-02-08 at 23:16 +0100, Pierre Moreau wrote: > > > (Moving the conversation to its own thread.) > > > > > > > target agnostic libclc is rather difficult to do. CLC includes 3 levels > > > > of precision on float (fp32) operands; full, half, native. The > > > > implementation of each depends on capabilites of specific device (e.g. > > > > vega(VI+?) can do 1 ULP log2/exp2 in hw, other targets need sw > > > > implementation to meet CLC requirement of 3ulp). Any conversion backend > > > > would thus need to implement sw versions of math builtins for targets > > > > that can't perform the op in HW. > > > > > > My initial thought for the target agnostic libclc, was to just provide > > > some > > > (fake?) implementations of OpenCL built-in functions to make clang happy > > > and > > > let me compile kernels using “get_global_id()”, as well as include headers > > > defining OpenCL specific types like “float4” or others. If there is > > > another > > > (better?) way to achieve this, I am all ears. (There is probably one, as > > > I had > > > no issues when using the Khronos LLVM/clang fork rather than Tomeu’s > > > out-of-tree module, the former having also some bits and pieces in clang.) > > > > I don't think you need libclc for this. workitem IDs are > > platform/device specific, and iiuc SPIR-V builtins should handle it in > > an abstract way [0]. any conversion consuming SPIR-V needs to replace > > those with device/platform specific way of obtaining the information. > > you can also use clang's clc header to provide data types [1]. > > > > > > [0] https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#B > > uiltIn > > [1] https://github.com/llvm-mirror/clang/blob/master/lib/Headers/opencl > > -c.h > > > > > > > > > Extending the current libclc to provide target specific SPIR-V binaries > > > > in addition to/in place of LLVM IR is rather straightforward. Adding > > > > additional targets it's more work since it relies on clang to support > > > > those targets. > > > > > > I’m curious how those target specific SPIR-V binaries would look like. I > > > can > > > imagine how some functions like “OpSign” could be implemented using other > > > SPIR-V functions, but how would you handle something like > > > “get_local_id()”? If > > > you define it as the built-in “LocalInvocationId” and don’t supply an > > > implementation of it, then you lose the target specificness. On the other > > > hand, > > > if you want to keep it device-specific, how would you express that in > > > SPIR-V? > > > > getting IDs is not a problem. SPIR-V should provide builtins for that. > > > > The problem I had in mind is when SPIR-V binary calls e.g. exp2(). You > > can either assume that the op needs CLC precision (3 ulp), or device > > native precision. > > That's up to the SPIR-V extended instruction set specification to define > what precision the exp2 built-in is supposed to have. > > > SPIR-V binary can also call exp2(fp64), which does not have an > > equivalent GPU instruction. > > Then it should probably be lowered by the SPIR-V front-end, right?
I'm not sure what you mean by "spir-v frontend". If it's the tool that generates SPIR-V, then no, not really. My understanding is that those are run prior to application distribution, and therefore have no information about the target HW. So if a program imports "CLC.std.11" extended instruction set to get access CLC builtin functions. What would a generic SPIR-V libclc provide? > > > It's easier to translate these to libclc function calls (combined with > > the right library implementation of the exp2 builtin), than try to > > generate exp2 algorithm when converting to NIR (or anything else > > really). > > > > But the SPIR-V front-end will need to lower that in terms of > instructions supported by the back-end anyway in order to be able to > handle general SPIR-V shaders as input, right? So why re-implement the > lowering for those operations in libclc in a way that's only going to be > useful for the OpenCL C language but not for other APIs? > > > The current libclc mostly assumes that LLVM ops are done in device > > native precision, and provides sw implementation of operations that > > don't have conformant device instruction. > > But I don't think there is any disadvantage from having a libclc > implementation that doesn't make any precision assumptions beyond what > is stated in the SPIR-V spec. In fact that would have the IMO more > desirable advantage that you could re-use one and the same libclc > implementation for *all* back-ends that want SPIR-V as input. Sure, a compiler-rt library would be more useful (usable by multiple languages). However, unlike target specific libclc, it's not available atm. Jan > > > This obviates the need for compiler-rt library. And alternative > > approach is to assume that the ops provide full precision and use > > target intrinsics for native precision. it's still target specific if > > a library call uses the former or the latter. > > > > regards, > > Jan > > > > > > > > Regards, > > > Pierre > > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/mesa-dev -- Jan Vesely <jan.ves...@rutgers.edu>
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