[Mesa-dev] [PATCH 12/15] i965: Add support for ir_triop_cond_sel.

[Paul Berry]

On 22 August 2013 16:08, Matt Turner <mattst88 at gmail.com> wrote:

> ---
>  src/mesa/drivers/dri/i965/brw_fs_channel_expressions.cpp | 1 +
>  src/mesa/drivers/dri/i965/brw_fs_visitor.cpp             | 6 ++++++
>  src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp           | 6 ++++++
>  3 files changed, 13 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_channel_expressions.cpp
> b/src/mesa/drivers/dri/i965/brw_fs_channel_expressions.cpp
> index 6ee6d01..34dbc90 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_channel_expressions.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_channel_expressions.cpp
> @@ -362,6 +362,7 @@
> ir_channel_expressions_visitor::visit_leave(ir_assignment *ir)
>
>     case ir_triop_fma:
>     case ir_triop_lrp:
> +   case ir_triop_cond_sel:
>     case ir_triop_bitfield_extract:
>        for (i = 0; i < vector_elements; i++) {
>          ir_rvalue *op0 = get_element(op_var[0], i);
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> index 4b54bee..27887d6 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> @@ -735,6 +735,12 @@ fs_visitor::visit(ir_expression *ir)
>     case ir_triop_lrp:
>        emit_lrp(this->result, op[0], op[1], op[2]);
>        break;
> +
> +   case ir_triop_cond_sel:
> +      emit(CMP(reg_null_d, op[0], fs_reg(0), BRW_CONDITIONAL_NZ));
> +      inst = emit(BRW_OPCODE_SEL, this->result, op[1], op[2]);
> +      inst->predicate = BRW_PREDICATE_NORMAL;
> +      break;
>     }
>  }
>

For the uses of ir_triop_cond_sel we have currently (the lowering passes in
patch 14), I believe this will generate efficient code.  But if we adapt
the mix() functions to use it, then there are probably going to be a lot of
uses like this:

x = mix(y, z, a < b);

Which will compile down to this silly assembly (please excuse the
pseudocode--I can't remember the assembly syntax exactly):

CMP.lt tmp a b
CMP.nz null tmp 0
SEL.f0 x y z

What if we modify the loop that calls ir->operands[operand]->accept(this)
on each operand (near the top of the visitor) so that it skips operand 0
when the expression is ir_triop_cond_sel.  Then, in the switch statement,
we can do something like this:

emit_bool_to_cond_code(ir->operands[0]);
inst = emit(BRW_OPCODE_SEL, this->result, op[1], op[2]);
inst->predicate = BRW_PREDICATE_NORMAL;

That should produce the assembly we want, which is:

CMP.lt null a b
SEL.fo x y z


>
>
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
> b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
> index fbdf73d..0615309 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
> @@ -1731,6 +1731,12 @@ vec4_visitor::visit(ir_expression *ir)
>        emit(LRP(result_dst, op[2], op[1], op[0]));
>        break;
>
> +   case ir_triop_cond_sel:
> +      emit(CMP(dst_null_d(), op[0], src_reg(0), BRW_CONDITIONAL_NZ));
> +      inst = emit(BRW_OPCODE_SEL, result_dst, op[1], op[2]);
> +      inst->predicate = BRW_PREDICATE_NORMAL;
> +      break;
> +
>

A nearly identical optimization ought to be possible here.


>     case ir_triop_bfi:
>        op[0] = fix_3src_operand(op[0]);
>        op[1] = fix_3src_operand(op[1]);
> --
> 1.8.3.2
>
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