On Thu, Oct 8, 2015 at 6:54 AM, Emil Velikov <[email protected]> wrote: > On 7 October 2015 at 23:50, Connor Abbott <[email protected]> wrote: >> On Wed, Oct 7, 2015 at 4:48 PM, Emil Velikov <[email protected]> >> wrote: >>> On 7 October 2015 at 18:04, Connor Abbott <[email protected]> wrote: >>>> On Wed, Oct 7, 2015 at 7:51 AM, Emil Velikov <[email protected]> >>>> wrote: >>>>> XXX: commit message, comment in nir_intrinsics.h >>>>> >>>>> Signed-off-by: Emil Velikov <[email protected]> >>>>> --- >>>>> 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 _______________________________________________ mesa-dev mailing list [email protected] http://lists.freedesktop.org/mailman/listinfo/mesa-dev
