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

[Emil Velikov]

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.

> 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

Cheers,
Emil