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

[Connor Abbott]

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.

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."

> +
>  /** A conditional discard, with a single boolean source. */
>  INTRINSIC(discard_if, 1, ARR(1), false, 0, 0, 0, 0)
>
> --
> 2.5.0
>
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