[cfe-dev] [RFC] Unified offloading option for CUDA/HIP/OpenMP

Wed Mar 10 10:53:22 PST 2021

On 3/10/21 12:37 PM, Artem Belevich wrote:
> On Wed, Mar 10, 2021 at 7:38 AM Doerfert, Johannes <jdoerfert at anl.gov>
> wrote:
>
>> On 3/8/21 6:59 PM, Artem Belevich wrote:
>>> On Mon, Mar 8, 2021 at 11:23 AM Liu, Yaxun (Sam) <Yaxun.Liu at amd.com>
>> wrote:
>>>> [AMD Public Use]
>>>>
>>>>
>>>>
>>>> The amdgpu xnack and sramecc need to be part of GPU arch name the same
>> way
>>>> as for --offload-arch, e.g.
>>>>
>>>>
>>>>
>>>> --offload=amdgcn-gfx906:xnack+,amdgcn-gfx906:xnack-
>>>>
>>>>
>>>>
>>>> They behave like GPU arch.
>>>>
>>>>
>>>>
>>> It's just that it's rather unwieldy to use in practice. It's not a
>>> showstopper, but perhaps now may be a convenient point to consider the
>>> naming scheme for AMDGPU sub-compilations again.
>>>
>>> It should be easy enough to add useful or commonly used names/aliases.
>>>
>>> E.g. `--offload=nvidia-ampere` would be equivalent to
>>> `--offload=sm_80,sm_86`.
>>> Or `--offload=amd-navi33` ->
>> `--offload=gfx3011:+something:-something_else`
>>> Even for CUDA and NVIDIA GPUs that've been around for a pretty long time,
>>> I'm still getting the questions from the users -- "I've got this
>>> GTX/RTX-whatever video card and can't figure out how to compile for it.
>>> What are those compute_XY and sm_YZ and which ones should I use?"
>>> I can only imagine trying to explain to someone : "You need to use
>>> gfx-XYZ<colon><dash>xnack<colon><plus>sram-ecc.... Oh, you must have
>>> mistyped that, let's try it again."
>>>
>>> Perhaps we need to split offloading machinery further.
>>>
>>> The --offloat=target still serves the double purpose of creating a
>>> sub-compilation *and* specifying the target details, providing the
>> initial
>>> set of parameters for the given target. It also prevents creation for
>>> multiple subcompilations for targets with minor differences which may be
>>> one of the reasons that led to AMDGPU's encoding various features in the
>>> target name.
>>>
>>> What if we were to modify the scheme a bit in a way that allows better
>>> handling of multiple variants of the same target.
>>> E.g.:
>>> --offload=gfx906 at A,gfx906 at B   -- creates two sub-compilations both
>>> targeting gfx906. Optional @suffix makes it possible to match them
>>> independently.
>>> -Xoffload=@A --set-features=xnack+,sram-ecc-
>>> -Xoffload=@B --set-features=xnack-,sram-ecc+
>>>
>>> Would something like this help with AMDGPU's feature handling?
>> Don't we need that also for NVIDIA?
>>
>> -offload=nvptx64 at A,nvptx64 at B -Xoffload=@A -march=sm_30 -Xoffload=@B
>> -march=sm_70
>>
> It could be phrased that way, but it's not, strictly speaking, necessary
> for NVPTX.
> Unlike AMDGPU it does not support targeting the same GPU in multiple
> sub-compilations.
> The question is -- what do we want '--offload' parameter to mean? It could
> range from 'just an arbitrary test string only to be used as the key for
> the -Xoffload matching' to 'specify all essential
> subcompilation parameters' which is what  --offload-arch does now. We may
> want to have both.

I should have been more specific. CUDA might not want to include
NVPTX of the code with different sm versions but we are expecting
this to be an OpenMP use case. Compile for old and new GPUs and
pick at runtime the newest the hardware supports.

> "Just a string" would be rather verbose to use in practice as the top-level
> mechanism, unless we also implement some sort of shortcut/alias system to
> allow specifying common configurations without having to type them all. One
> would be able to create their own subcompilation, if necessary, but for
> most use cases one of the standard ones would do.
>
> E.g. `--offload=sm_70` by itself should be sufficient to target sm_70 GPU
> with all the necessary options and would be what's used in vast majority of
> cases.
> `-Xoffload=sm70 -ffoo` would still work, if it would need to be tweaked.
> If I wanted a custom sub-compilation, I'd do `-offload=my_sm77
> -Xoffload=my_sm77 -mcpu=sm_77 -other -options`.
> Because my_sm77 is not a known configuration, it would start with the
> defaults and it would be up to the user to provide additional options to
> customize it.

I like the idea. What if we say `--offload=sm_70` will populate
the defaults for NVIDIA sm70 and people can add custom options
through `-Xoffload`, e.g., `-Xoffload=sm_70 -foobar`. I don't mind
the custom name pipelines but unsure if we need them. If so, maybe
we append them to a identifier that defines the target, e.g.,
`--offload=sm_70 at A` or `--offload=nvptx64 at B`. `-Xoffload` should
always match the `--offload` name regardless if it is a known default
(or a custom name).

Does that make sense or am I too confused by now?

~ Johannes

>
> --Artem
>
>
>> WDYT?
>>
>> ~ JOhannes
>>
>>
>>> --Artem
>>>
>>>
>>>> Sam
>>>>
>>>>
>>>>
>>>> *From:* Artem Belevich <tra at google.com>
>>>> *Sent:* Monday, March 8, 2021 2:01 PM
>>>> *To:* Liu, Yaxun (Sam) <Yaxun.Liu at amd.com>
>>>> *Cc:* Doerfert, Johannes <jdoerfert at anl.gov>; Ben Boeckel <
>>>> ben.boeckel at kitware.com>; Lieberman, Ron <Ron.Lieberman at amd.com>;
>>>> a.bataev at hotmail.com; Chan, SiuChi <siuchi.chan at amd.com>; Searles,
>> Mark <
>>>> Mark.Searles at amd.com>; cfe-dev (cfe-dev at lists.llvm.org) <
>>>> cfe-dev at lists.llvm.org>; jeffrey.sandoval at hpe.com; Jon Chesterfield <
>>>> jonathanchesterfield at gmail.com>; Rodgers, Gregory <
>> Gregory.Rodgers at amd.com
>>>> *Subject:* Re: [cfe-dev] [RFC] Unified offloading option for
>>>> CUDA/HIP/OpenMP
>>>>
>>>>
>>>>
>>>> [CAUTION: External Email]
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Sat, Mar 6, 2021 at 7:13 AM Liu, Yaxun (Sam) <Yaxun.Liu at amd.com>
>> wrote:
>>>> [AMD Public Use]
>>>>
>>>> We need to different target triples since it may not always be possible
>> to
>>>> infer target triple by cpu name. So I guess it would be like:
>>>>
>>>> "--offload=amdgcn-gfx906,amdgcn-gfx1010"
>>>> "--Xoffload=amdgcn-gfx* options common to all AMD GPUs"
>>>> "--Xoffload=amdgcn-gfx906 -mcpu=gfx906 --fsomething-specific-to-gfx906"
>>>>
>>>>
>>>>
>>>> SGTM.
>>>>
>>>> Do you expect the AMDGPU's features (+xnack, -ecc, etc) to be part of
>> the
>>>> offload target ? Or would they be specified via -Xoffload arguments?
>>>>
>>>>
>>>>
>>>> --Artem
>>>>
>>>>
>>>>
>>>>
>>>> Sam
>>>>
>>>> -----Original Message-----
>>>> From: Doerfert, Johannes <jdoerfert at anl.gov>
>>>> Sent: Friday, March 5, 2021 1:25 PM
>>>> To: Artem Belevich <tra at google.com>; Liu, Yaxun (Sam) <
>> Yaxun.Liu at amd.com>
>>>> Cc: Ben Boeckel <ben.boeckel at kitware.com>; Lieberman, Ron <
>>>> Ron.Lieberman at amd.com>; a.bataev at hotmail.com; Chan, SiuChi <
>>>> siuchi.chan at amd.com>; Searles, Mark <Mark.Searles at amd.com>; cfe-dev (
>>>> cfe-dev at lists.llvm.org) <cfe-dev at lists.llvm.org>;
>> jeffrey.sandoval at hpe.com;
>>>> Jon Chesterfield <jonathanchesterfield at gmail.com>; Rodgers, Gregory <
>>>> Gregory.Rodgers at amd.com>
>>>> Subject: Re: [cfe-dev] [RFC] Unified offloading option for
>> CUDA/HIP/OpenMP
>>>> [CAUTION: External Email]
>>>>
>>>> On 3/4/21 3:05 PM, Artem Belevich wrote:
>>>>> On Thu, Mar 4, 2021 at 10:34 AM Liu, Yaxun (Sam) <Yaxun.Liu at amd.com>
>>>> wrote:
>>>>>> [AMD Public Use]
>>>>>>
>>>>>> There is another aspect we need to consider: how to modify the
>>>>>> -target option by additional options?
>>>>>>
>>>>>> For the existing --offload-arch option, we could use -Xarch_ to add
>>>>>> specific options for it.
>>>>>>
>>>>> `-Xarch_xxx` as implemented right now is a rather limiter hack. IIRC
>>>>> it only accepts options w/o arguments which limits its usability.
>>>>>
>>>>>
>>>>>> Assuming we have an -offload="amdgcn -mcpu=gfx906" option, then we
>>>>>> want to add some options specific to it by an additional option, what
>>>>>> should we do?
>>>>>>
>>>>> I think we've been conflating telling the driver what to compile for
>>>>> and customizing individual sub-compilations.
>>>>>
>>>>> We could explicitly separate the two tasks. E.g.:
>>>>> `--[no-]offload=target1,target2,target3...`
>>>>> `--Xoffload=target_pattern target_options...`
>>>>>
>>>>> This way your example would be handled with:
>>>>> "--offload=gfx906,gfx1010"
>>>>> "--Xoffload=gfx* options common to all AMD GPUs"
>>>>> "--Xoffload=gfx906 -mcpu=gfx906 --fsomething-specific-to-gfx906"
>>>>>
>>>>> In the end `-Xarch_xxx` would become an alias for '-Xoffload=xxx'.
>>>> +1
>>>>
>>>>
>>>>> --Artem
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>> Thanks.
>>>>>>
>>>>>> Sam
>>>>>>
>>>>>> -----Original Message-----
>>>>>> From: Doerfert, Johannes <jdoerfert at anl.gov>
>>>>>> Sent: Thursday, February 11, 2021 12:59 PM
>>>>>> To: Artem Belevich <tra at google.com>; Liu, Yaxun (Sam)
>>>>>> <Yaxun.Liu at amd.com>
>>>>>> Cc: Ben Boeckel <ben.boeckel at kitware.com>; Lieberman, Ron <
>>>>>> Ron.Lieberman at amd.com>; a.bataev at hotmail.com; Chan, SiuChi <
>>>>>> siuchi.chan at amd.com>; Searles, Mark <Mark.Searles at amd.com>; cfe-dev (
>>>>>> cfe-dev at lists.llvm.org) <cfe-dev at lists.llvm.org>;
>>>>>> jeffrey.sandoval at hpe.com; Jon Chesterfield
>>>>>> <jonathanchesterfield at gmail.com>
>>>>>> Subject: Re: [cfe-dev] [RFC] Unified offloading option for
>>>>>> CUDA/HIP/OpenMP
>>>>>>
>>>>>> [CAUTION: External Email]
>>>>>>
>>>>>> I'm OK with either.
>>>>>>
>>>>>> On 2/11/21 11:42 AM, Artem Belevich wrote:
>>>>>>> On Thu, Feb 11, 2021 at 8:30 AM Liu, Yaxun (Sam) <Yaxun.Liu at amd.com>
>>>>>> wrote:
>>>>>>>> [AMD Public Use]
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Sorry for the delay.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Both Johannes’ and Artem’s proposals should satisfy the needs of
>>>> users:
>>>>>>>>
>>>>>>>> Option 1:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> `-offload=<offload-pattern> optA optB optC`.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Option 2:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> `-offload=<offload-pattern>,optA,optB,optC`.
>>>>>>>>
>>>>>>> I'm fine with #2. We're using something similar with our build tools
>>>>>>> and it works reasonably well.
>>>>>>> However, it does have one annoying corner case. There's no easy way
>>>>>>> to pass an option which has a comma in it. E.g. if I want to pass
>>>>>>> `-Wl,something,something`. Perhaps we could use sed-like approach
>>>>>>> and allow changing the separator. E.g. `s/a/b/` == `s at a@b@`.
>>>>>>>
>>>>>>> --Artem
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>> Compared to the old options, they are more concise and more
>> readable.
>>>>>>>>
>>>>>>>>
>>>>>>>> The main difference is the delimiter. To me option 2 is more
>>>>>>>> attractive since it does not need quotations for most cases.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Can we reach an agreement on option 2?
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Thanks.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Sam
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> *From:* Artem Belevich <tra at google.com>
>>>>>>>> *Sent:* Tuesday, December 15, 2020 2:13 PM
>>>>>>>> *To:* Ben Boeckel <ben.boeckel at kitware.com>
>>>>>>>> *Cc:* Doerfert, Johannes <jdoerfert at anl.gov>; Liu, Yaxun (Sam) <
>>>>>>>> Yaxun.Liu at amd.com>; Lieberman, Ron <Ron.Lieberman at amd.com>;
>>>>>>>> a.bataev at hotmail.com; Chan, SiuChi <siuchi.chan at amd.com>; Searles,
>>>>>>>> Mark < Mark.Searles at amd.com>; cfe-dev (cfe-dev at lists.llvm.org) <
>>>>>>>> cfe-dev at lists.llvm.org>
>>>>>>>> *Subject:* Re: [cfe-dev] [RFC] Unified offloading option for
>>>>>>>> CUDA/HIP/OpenMP
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> [CAUTION: External Email]
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Tue, Dec 15, 2020 at 10:23 AM Ben Boeckel
>>>>>>>> <ben.boeckel at kitware.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>> On Mon, Dec 14, 2020 at 14:04:43 -0800, Artem Belevich via cfe-dev
>>>>>> wrote:
>>>>>>>>> It all may be an utter overkill, too. WDYT?
>>>>>>>> Note that tools such as ccache and sccache generally need to be
>>>>>>>> able to understand what's going on (I believe distcc and other
>>>>>>>> distributed compilation tools also generally need to know too), so
>>>>>>>> making it sensible enough for interpretation based on just the
>>>>>>>> flags to be possible should be considered.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> I think this is somewhat orthogonal to how we specify per-target
>>>>>> options.
>>>>>>>> Such a tool almost never knows about all possible compiler options
>>>>>>>> and has to pass through the unknown options as-is.  However, any
>>>>>>>> form
>>>>>> of 'nested'
>>>>>>>> options specified on the command line will have a chance to confuse
>>>>>>>> such tool. E.g. if I want to pass '-E' to some sub-tool for a
>>>>>>>> particular offload-target, ccache, not being aware that it's not a
>>>>>>>> top-level compilation option, may interpret it as an attempt to
>>>>>> preprocess the TU.
>>>>>>>> I wonder if it would make sense to just move all this per-target
>>>>>>>> option complexity into an external response file. As far as
>>>>>>>> existing tools are concerned, it would look like
>>>>>>>> `--offload-options=target-opts.file` without affecting tool's
>>>>>>>> general idea what this compilation is about to do, and the external
>>>>>>>> file would allow us to be as flexible as we need to be to specify
>>>>>>>> per-target
>>>>>> options. It could be just a flat list of pairs `-Xarch_...
>>>>>>>> optA`.  Or we could use YAML.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> That approach, however, has its own issues and would still need to
>>>>>>>> be optional. If it's the only way to specify offload options, that
>>>>>>>> will complicate other use cases as now they would have to deal with
>>>>>>>> temporary files.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Maybe a slightly modified variant of jdoefert@'s idea would work
>>>>>> better:
>>>>>>>>>>>        -offload="amd -march=gfx906 -fno-vectorize" -fopenmp
>>>>>>>> Implement it in a way similar to -Wl,optA,optB,optC and extend it
>>>>>>>> to match an offload scope glob/regex.
>>>>>>>>
>>>>>>>> E.g. `-offload=<offload-pattern>,optA,optB,optC`.
>>>>>>>>
>>>>>>>> As far as the external tools are concerned, it's just one option to
>>>>>>>> pass though. At the same time it should be flexible enough to apply
>>>>>>>> the options to subset of offload targets in a human-manageable way.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>>
>>>>>>>> --Artem Belevich
>>>>>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> --Artem Belevich
>>>>
>>