[Mesa-dev] [PATCH 4/4] nir: add ARB_shader_ballot and ARB_shader_group_vote instructions

Tue Jun 6 04:52:29 UTC 2017

On Mon, Jun 5, 2017 at 6:37 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:

> I pushed a v2 at
> https://cgit.freedesktop.org/~cwabbott0/mesa/log/?h=nir-divergence-v2.
> I'm not sure if I like this version better, though. I'll have to think
> about it. In the meantime, feel free to take a look.
>

I've taken a skim through the branch and I agree that I'm not sure either.
Here's a few thoughts in no particular order:

 1) Other than the fact that it's a pile of churn, it doesn't seem to make
too much difference whether dFdx and dFdy are ALU or intrinsics

 2) Convergent instructions are, in a lot of ways, easier to deal with than
plain cross-thread ones.  Convergent ops can always be moved up the
dominance tree or down into uniform control-flow.  Regular cross-thread
instructions can't be moved across any non-uniform control-flow.

 3) dFdx and dFdy are weird because they're convergent so it's clear they
are special but not clear they should be intrinsics instead of ALU

 4) I like the nir_instr_is_convergent() and nir_instr_is_cross_thread()
helpers

 5) non-convergent cross-thread instructions should definitely be
intrinsics.

 6) I think the shader ballot stuff is all non-convergent cross-thread as
are some of the more advanced subgroup operations (see HLSL shader model
6.0).

That's all for now,

--Jason

> On Mon, Jun 5, 2017 at 2:43 PM, Jason Ekstrand <jason at jlekstrand.net>
> wrote:
> > On Mon, Jun 5, 2017 at 1:50 PM, Connor Abbott <cwabbott0 at gmail.com>
> wrote:
> >>
> >> On Mon, Jun 5, 2017 at 1:37 PM, Jason Ekstrand <jason at jlekstrand.net>
> >> wrote:
> >> > I'm not sure how I feel about having these as ALU operations.  ALU
> >> > operations are generally pure functions (with the exception
> derivative)
> >> > that
> >> > can be re-ordered at will.  I don't really like breaking that.  In
> fact,
> >> > I'd
> >> > almost be inclined to make derivatives intrinsics and just
> special-case
> >> > them
> >> > in constant folding.  Thoughts?
> >>
> >> I wasn't too sure about this either. It is a little weird to make
> >> these ALU instructions. I followed the rule here that if something can
> >> be constant-folded, it should be an ALU instruction, but I guess you
> >> can argue that it's just a coincidence that these can be
> >> constant-folded anyways.
> >
> >
> > Yeah.  As subgroup ops get more complicated, I think a log of the
> subgroup
> > operations can be constant-folded after a fashion but the rules get weird
> > fast.
> >
> >>
> >> I guess the main downside is that it would be
> >> impossible to make nir_algebraic patterns with these, although I can't
> >> think of too many simple pattern-matching type things you'd want to do
> >> on these instructions anyways.
> >
> >
> > Yeah.  My gut also tells me that shaders which are "advanced" enough to
> use
> > subgroup features probably don't need (or it can't be done) the massive
> > reductions we do for D3D9-generated shaders.
> >
> >>
> >> Maybe something like not(any(not(foo)))
> >> -> all(foo) and vice-versa?
> >>
> >> >
> >> > On Mon, Jun 5, 2017 at 12:22 PM, Connor Abbott <cwabbott0 at gmail.com>
> >> > wrote:
> >> >>
> >> >> Signed-off-by: Connor Abbott <cwabbott0 at gmail.com>
> >> >> ---
> >> >>  src/compiler/nir/nir_intrinsics.h | 14 ++++++++++++++
> >> >>  src/compiler/nir/nir_opcodes.py   | 18 ++++++++++++++++--
> >> >>  2 files changed, 30 insertions(+), 2 deletions(-)
> >> >>
> >> >> diff --git a/src/compiler/nir/nir_intrinsics.h
> >> >> b/src/compiler/nir/nir_intrinsics.h
> >> >> index 21e7d90..157df7f 100644
> >> >> --- a/src/compiler/nir/nir_intrinsics.h
> >> >> +++ b/src/compiler/nir/nir_intrinsics.h
> >> >> @@ -330,6 +330,20 @@ SYSTEM_VALUE(channel_num, 1, 0, xx, xx, xx)
> >> >>  SYSTEM_VALUE(alpha_ref_float, 1, 0, xx, xx, xx)
> >> >>  SYSTEM_VALUE(layer_id, 1, 0, xx, xx, xx)
> >> >>  SYSTEM_VALUE(view_index, 1, 0, xx, xx, xx)
> >> >> +SYSTEM_VALUE(subgroup_invocation, 1, 0, xx, xx, xx)
> >> >> +
> >> >> +
> >> >> +/* ARB_shader_ballot instructions */
> >> >> +
> >> >> +SYSTEM_VALUE(subgroup_eq_mask, 1, 0, xx, xx, xx)
> >> >> +SYSTEM_VALUE(subgroup_ge_mask, 1, 0, xx, xx, xx)
> >> >> +SYSTEM_VALUE(subgroup_gt_mask, 1, 0, xx, xx, xx)
> >> >> +SYSTEM_VALUE(subgroup_le_mask, 1, 0, xx, xx, xx)
> >> >> +SYSTEM_VALUE(subgroup_lt_mask, 1, 0, xx, xx, xx)
> >> >> +
> >> >> +INTRINSIC(ballot, 1, ARR(0), true, 0, 0, 0, xx, xx, xx,
> >> >> +          NIR_INTRINSIC_CAN_ELIMINATE | NIR_INTRINSIC_CAN_REORDER |
> >> >> +          NIR_INTRINSIC_CROSS_THREAD)
> >> >>
> >> >>  /* Blend constant color values.  Float values are clamped. */
> >> >>  SYSTEM_VALUE(blend_const_color_r_float, 1, 0, xx, xx, xx)
> >> >> diff --git a/src/compiler/nir/nir_opcodes.py
> >> >> b/src/compiler/nir/nir_opcodes.py
> >> >> index be3ab6d..05a80b2 100644
> >> >> --- a/src/compiler/nir/nir_opcodes.py
> >> >> +++ b/src/compiler/nir/nir_opcodes.py
> >> >> @@ -120,8 +120,10 @@ def opcode(name, output_size, output_type,
> >> >> input_sizes, input_types,
> >> >>                            input_types, convergent, cross_thread,
> >> >>                            algebraic_properties, const_expr)
> >> >>
> >> >> -def unop_convert(name, out_type, in_type, const_expr):
> >> >> -   opcode(name, 0, out_type, [0], [in_type], "", const_expr)
> >> >> +def unop_convert(name, out_type, in_type, const_expr,
> >> >> cross_thread=False,
> >> >> +                 convergent=False):
> >> >> +   opcode(name, 0, out_type, [0], [in_type], "", const_expr,
> >> >> convergent,
> >> >> +          cross_thread)
> >> >>
> >> >>  def unop(name, ty, const_expr, convergent=False,
> cross_thread=False):
> >> >>     opcode(name, 0, ty, [0], [ty], "", const_expr, convergent,
> >> >> cross_thread)
> >> >> @@ -355,6 +357,18 @@ for i in xrange(1, 5):
> >> >>     for j in xrange(1, 5):
> >> >>        unop_horiz("fnoise{0}_{1}".format(i, j), i, tfloat, j,
> tfloat,
> >> >> "0.0f")
> >> >>
> >> >> +# ARB_shader_ballot instructions
> >> >> +
> >> >> +opcode("read_invocation", 0, tuint, [0, 1], [tuint, tuint32], "",
> >> >> "src0",
> >> >> +        cross_thread=True)
> >> >> +unop("read_first_invocation", tuint, "src0", cross_thread=True)
> >> >> +
> >> >> +# ARB_shader_group_vote instructions
> >> >> +
> >> >> +unop("any_invocations", tbool, "src0", cross_thread=True)
> >> >> +unop("all_invocations", tbool, "src0", cross_thread=True)
> >> >> +unop("all_invocations_equal", tbool, "true", cross_thread=True)
> >> >> +
> >> >>  def binop_convert(name, out_type, in_type, alg_props, const_expr):
> >> >>     opcode(name, 0, out_type, [0, 0], [in_type, in_type], alg_props,
> >> >> const_expr)
> >> >>
> >> >> --
> >> >> 2.9.3
> >> >>
> >> >> _______________________________________________
> >> >> mesa-dev mailing list
> >> >> mesa-dev at lists.freedesktop.org
> >> >> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
> >> >
> >> >
> >
> >
>
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