[Mesa-dev] Adding a SPIR-V target to libclc

Fri Feb 16 04:36:02 UTC 2018

Jan Vesely <jan.vesely at rutgers.edu> writes:

> On Thu, 2018-02-08 at 15:56 -0800, Francisco Jerez wrote:
>> Jan Vesely <jan.vesely at 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).

> 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
>> 
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>
> -- 
> Jan Vesely <jan.vesely at rutgers.edu>
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