[Mesa-dev] [PATCH 4/6] nir: add shader_clock intrinsic

[Connor Abbott]

On Thu, Oct 8, 2015 at 6:54 AM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
> On 7 October 2015 at 23:50, Connor Abbott <cwabbott0 at gmail.com> wrote:
>> On Wed, Oct 7, 2015 at 4:48 PM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
>>> On 7 October 2015 at 18:04, Connor Abbott <cwabbott0 at gmail.com> wrote:
>>>> On Wed, Oct 7, 2015 at 7:51 AM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
>>>>> XXX: commit message, comment in nir_intrinsics.h
>>>>>
>>>>> Signed-off-by: Emil Velikov <emil.velikov at collabora.com>
>>>>> ---
>>>>>  src/glsl/nir/glsl_to_nir.cpp  | 6 ++++++
>>>>>  src/glsl/nir/nir_intrinsics.h | 2 ++
>>>>>  2 files changed, 8 insertions(+)
>>>>>
>>>>> diff --git a/src/glsl/nir/glsl_to_nir.cpp b/src/glsl/nir/glsl_to_nir.cpp
>>>>> index efaa73e..231bdbf 100644
>>>>> --- a/src/glsl/nir/glsl_to_nir.cpp
>>>>> +++ b/src/glsl/nir/glsl_to_nir.cpp
>>>>> @@ -698,6 +698,8 @@ nir_visitor::visit(ir_call *ir)
>>>>>           op = nir_intrinsic_ssbo_atomic_exchange;
>>>>>        } else if (strcmp(ir->callee_name(), "__intrinsic_ssbo_atomic_comp_swap_internal") == 0) {
>>>>>           op = nir_intrinsic_ssbo_atomic_comp_swap;
>>>>> +      } else if (strcmp(ir->callee_name(), "__intrinsic_shader_clock") == 0) {
>>>>> +         op = nir_intrinsic_shader_clock;
>>>>>        } else {
>>>>>           unreachable("not reached");
>>>>>        }
>>>>> @@ -802,6 +804,10 @@ nir_visitor::visit(ir_call *ir)
>>>>>        case nir_intrinsic_memory_barrier:
>>>>>           nir_instr_insert_after_cf_list(this->cf_node_list, &instr->instr);
>>>>>           break;
>>>>> +      case nir_intrinsic_shader_clock:
>>>>> +         nir_ssa_dest_init(&instr->instr, &instr->dest, 1, NULL);
>>>>> +         nir_instr_insert_after_cf_list(this->cf_node_list, &instr->instr);
>>>>> +         break;
>>>>>        case nir_intrinsic_store_ssbo: {
>>>>>           exec_node *param = ir->actual_parameters.get_head();
>>>>>           ir_rvalue *block = ((ir_instruction *)param)->as_rvalue();
>>>>> diff --git a/src/glsl/nir/nir_intrinsics.h b/src/glsl/nir/nir_intrinsics.h
>>>>> index 263d8c1..4b32215 100644
>>>>> --- a/src/glsl/nir/nir_intrinsics.h
>>>>> +++ b/src/glsl/nir/nir_intrinsics.h
>>>>> @@ -83,6 +83,8 @@ BARRIER(discard)
>>>>>   */
>>>>>  BARRIER(memory_barrier)
>>>>>
>>>>> +INTRINSIC(shader_clock, 0, ARR(), true, 1, 1, 0, 0 /* flags ? */)
>>>>
>>>> This should have NIR_INTRINSIC_CAN_DELETE, since if the result is
>>>> unused we can safely delete it (i.e. it has no side effects), but we
>>>> can't safely reorder it.
>>>>
>>> Thanks. Will do.
>>>
>>>> Side note: NIR's current model, as well as any more flexible memory
>>>> model we might adopt in the future, assumes that intrinsics which are
>>>> marked as reorderable, as well as ALU operations which are implicitly
>>>> reorderable, can be freely reordered with respect to *any* other
>>>> operation, even one that's explicitly not reorderable. So, for
>>>> example, if you do:
>>>>
>>>> ... = clock();
>>>> a = b + c;
>>>> ... = clock();
>>>>
>>>> then there are no guarantees that the addition won't get moved outside
>>>> the clock() calls. Currently, this will only happen if the addition
>>>> becomes involved in some algebraic optimization or CSE, but in the
>>>> future with passes like GCM that move code around indiscriminately
>>>> it's going to be much more of a problem. I don't think we could really
>>>> solve this problem in a useful and general way without making the rest
>>>> of NIR significantly more complicated and slower, which I definitely
>>>> don't want. I think the best answer is to say "really these tools are
>>>> unreliable and meant mainly for driver developers and people who know
>>>> what they're doing, and if you use them you have to be prepared to
>>>> look at the assembly source and see if it matches what you expected."
>>>>
>>> I haven't looked at the optimisations closely and I assumed that all
>>> intrinsics act as motion barriers. Seems like I was mistaking.
>>> Can we call it a "where sub-group is implementation dependent" and be
>>> done with it ;-)
>>
>> Or even "The units of time are not defined and need not be constant"
>> -- I guess "return 0;" would be a legal implementation ;).
>>
> Bikeshedding aside - the spec is quite clear about the motion barrier
> part. Personally I'm fine either way - leave it as is or look closer
> at NIR. Just let know how you feel on the topic.

It's my opinion, and Jason's as well, that implementing a "code motion
barrier" as the spec describes is practically impossible in NIR or
really any decent SSA-based IR. So we just can't follow that part of
the spec.

>
>> But really the issue isn't with spec lawyering, it's with people
>> potentially using it without knowing the caveats about the underlying
>> compiler stack and how it might not always do what they think it does.
>>
> Replace "compiler stack" with "object X" and you can apply it to
> everything in life :-P

True :) although in this case, it's perhaps a bit more serious since
it's against the "spirit" of the spec. But again, there's nothing we
can really do about it.

>
> Cheers,
> Emil