Jan Vesely <jan.ves...@rutgers.edu> writes: > On Thu, 2018-02-15 at 20:36 -0800, Francisco Jerez wrote: >> Jan Vesely <jan.ves...@rutgers.edu> writes: >> >> > 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. >> >> No, I meant the SPIR-V front-end of the driver (or whatever translation >> pass in control of the driver is translating machine-agnostic SPIR-V >> into some other more hardware-specific representation of the program). > > OK. my question still stands. How does generic SPIR-V based libclc > help the process? >
That I can think of now, it would remove the need for maintaining any target-specific knowledge in libclc, for plumbing target-specific information in order to select the right libclc flavour at link time, and it would allow solving common problems in a place where there is a chance that the solution could be shared among different drivers and APIs (e.g. the exp2(fp64) lowering example you mentioned earlier is not exclusively useful to CL). > Jan > >> >> > 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> > > -- > Jan Vesely <jan.ves...@rutgers.edu>
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