[LLVMdev] PTX builtin functions.

[Justin Holewinski]

On Mon, Nov 21, 2011 at 7:01 AM, Alberto Magni <alberto.magni86 at gmail.com>wrote:

> Hi Justin,
>
> attached you find the patch for the integer max instruction.
> The multiclass PTX_INTRINSIC_INT3 in file PTXIntrinsicInstrInfo.td
> is almost an exact copy of  PTX_INT3 in PTXInstrInfo.td, maybe
> a modification of this class can be defined in a separate file.
>

I'm copying llvmdev.  We should keep discussions like this on the list for
the benefit of others.

We can probably factor out a generic description, or even just use the
PTX_INT3 multiclass directly.  The PTXIntrinsicInstrInfo.td file is
included by PTXInstrInfo.td, so anything defined in PTXInstrInfo.td is
available in PTXIntrinsicInstrInfo.td.


>
> Do you agree with this approach ?
> Also, do you think that a class like PTX_INTRINSIC_INT3_SIGNED
> (a clone of PTX_INT3_SIGNED) is required ?
>

Yes, I believe we should split these into signed and unsigned variants.
 The results of max/min operations can definitely be different depending on
whether the operands are signed or unsigned.  Since this information is not
encoded in LLVM types, we may want to create two versions for each integer
type; something like:

i32 @llvm.ptx.max.signed.i32(i32, i32)
i32 @llvm.ptx.max.unsigned.i32(i32, i32)

Otherwise, the patch looks good.


>
> Thanks,
>
> Alberto
>
> On Wed, Nov 16, 2011 at 5:44 PM, Alberto Magni
> <alberto.magni86 at gmail.com> wrote:
> > On Wed, Nov 16, 2011 at 2:17 PM, Justin Holewinski
> > <justin.holewinski at gmail.com> wrote:
> >> On Wed, Nov 16, 2011 at 9:16 AM, Justin Holewinski
> >> <justin.holewinski at gmail.com> wrote:
> >>>
> >>> On Wed, Nov 16, 2011 at 8:05 AM, Alberto Magni <
> alberto.magni86 at gmail.com>
> >>> wrote:
> >>>>
> >>>> Dear Justin,
> >>>>
> >>>> I am trying to add the support for some OpenCL builtin functions to
> >>>> the PTX backend.
> >>>> The attached file represent the first stub of a patch for the fmax
> >>>> builtin function.
> >>>
> >>> First off, thanks for helping to improve the PTX back-end!
> >>> There are really two main issues here.  First, OpenCL built-in
> functions
> >>> do not belong in the PTX back-end.  These will be implemented in the
> libclc
> >>> library (http://www.pcc.me.uk/~peter/libclc).  The back-end will only
> >>> implement PTX intrinsics, which may be used by the OpenCL built-in
> functions
> >>> in libclc.  However, this particular function (max) corresponds to a
> PTX
> >>> instruction, so it makes sense to implement it as an intrinsic in the
> >>> back-end.
> >>> Second, intrinsic functions require a bit more work.  You're off to a
> >>> great start, but intrinsics are implemented a bit differently.  It
> looks
> >>> like LLVM does not have a max intrinsic, so we'll need to create one.
>  Have
> >>> a look at include/llvm/IntrinsicsPTX.td.  This file defines the
> PTX-specific
> >>> intrinsics.  You can add an intrinsic for max here, and then implement
> a
> >>> pattern-match in the PTXInstrInfo.td file.  There is no need to create
> a new
> >>> SDNode type for intrinsics, unless they require some special handling
> in the
> >>> C++ code, which I do not see being the case here.
> >>
> >> Sorry, there's a typo here.  The intrinsic pattern matching goes in
> >> PTXInstrinsicInstrInfo.td.
> >>
> >
> > Thank you for the pointers I will let you know when I have the first
> patch.
> >
> >>>
> >>> When you define a new intrinsic, use the following template as a name:
> >>> int_ptx_max.  This will define the LLVM intrinsic as @llvm.ptx.max().
> >>>  Please follow the same convention when naming the __builtin_*
> function.
> >>>
> >>>>
> >>>> The test case I am trying is the following:
> >>>>
> >>>> define ptx_device float @f(float %x, float %y) {
> >>>> entry:
> >>>>  %z = call float @fmax(float %x, float %y)
> >>>>  ret float %z
> >>>> }
> >>>>
> >>>> declare float @fmax(float, float)
> >>>>
> >>>> But at the moment llc crashes saying that "calls are not supported",
> >>>> this does not
> >>>> happens with llvm builtins like llvm.sqrt.f32
> >>>
> >>> Which version of LLVM are you using?  Calls to PTX device functions
> have
> >>> been implemented for a little while now, so I'm surprised to see that
> error.
> >>>  Perhaps it's because the fmax function is not defined as ptx_device.
> >>>
> >
> > This is the testcase that I am using to verify I the max builtin
> > function I am impementing
> > is actually recognised. I took inspiration from the llvm-intrinsic.ll
> test case.
> > The command I am using to compile is:
> >
> > llc -march=ptx32 -mattr=+ptx22 fmax.ll
> >
> > The option -mattr does not seem to have any effect.
> > I tried also with the ptx_device qualifier with the same outcome.
> > I am using llvm from the svn repository.
> >
> > Bye,
> >
> > Alberto
> >
> >>>>
> >>>> Can you please give me a hint on what I am missing, or some general
> >>>> advice on how
> >>>> to add builtin functions.
> >>>>
> >>>> Thank you in advance,
> >>>>
> >>>> Alberto.
> >>>>
> >>>> _______________________________________________
> >>>> LLVM Developers mailing list
> >>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >>>>
> >>>
> >>>
> >>>
> >>> --
> >>>
> >>> Thanks,
> >>> Justin Holewinski
> >>
> >>
> >>
> >> --
> >>
> >> Thanks,
> >> Justin Holewinski
> >>
>



-- 

Thanks,

Justin Holewinski
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