[Mesa-dev] [PATCH 3/4] i965/fs: Optimize (gl_FrontFacing ? x : y) where x and y are ±1.0.

[Ian Romanick]

On 01/09/2015 07:59 PM, Matt Turner wrote:
> total instructions in shared programs: 5886767 -> 5881642 (-0.09%)
> instructions in affected programs:     532199 -> 527074 (-0.96%)
> GAINED:                                9
> LOST:                                  6

I'll did out the rest of the data later, but... I had a more general
implementation of this in my bool-optimizations branch.  There a bunch
of places that do things like

    if (bool_condition)
        x = some_constant;
    else
        x = other_constant;

{0, 1} and {-1, 1} were the most common ones that I observed.

Now... gl_FrontFacing is special because we do special things to
generate it.  It seems like we could take advantage of that and not
actually generate gl_FrontFacing.  Probably what your patch does... I'll
look closer after my LCA talk (in 90 minutes!).

There were some really "fun" shaders that did things like

    x = (gl_FrontFacing) ? 1.0 : 0.0;

    ...

    y = (x != 0) ? 1.0 : -1.0;

> ---
>  src/mesa/drivers/dri/i965/brw_fs.h           |  1 +
>  src/mesa/drivers/dri/i965/brw_fs_visitor.cpp | 94 ++++++++++++++++++++++++++++
>  2 files changed, 95 insertions(+)
> 
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.h b/src/mesa/drivers/dri/i965/brw_fs.h
> index 06575a5..24e1619 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.h
> +++ b/src/mesa/drivers/dri/i965/brw_fs.h
> @@ -528,6 +528,7 @@ public:
>     bool try_emit_saturate(ir_expression *ir);
>     bool try_emit_line(ir_expression *ir);
>     bool try_emit_mad(ir_expression *ir);
> +   bool try_opt_frontfacing_ternary(ir_if *ir);
>     void try_replace_with_sel();
>     bool opt_peephole_sel();
>     bool opt_peephole_predicated_break();
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> index bd9345e..1026af4 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> @@ -2684,6 +2684,97 @@ fs_visitor::emit_if_gen6(ir_if *ir)
>     emit(IF(this->result, fs_reg(0), BRW_CONDITIONAL_NZ));
>  }
>  
> +bool
> +fs_visitor::try_opt_frontfacing_ternary(ir_if *ir)
> +{
> +   ir_dereference_variable *deref = ir->condition->as_dereference_variable();
> +   if (!deref || strcmp(deref->var->name, "gl_FrontFacing") != 0)
> +      return false;
> +
> +   if (ir->then_instructions.length() != 1 ||
> +       ir->else_instructions.length() != 1)
> +      return false;
> +
> +   ir_assignment *then_assign =
> +         ((ir_instruction *)ir->then_instructions.head)->as_assignment();
> +   ir_assignment *else_assign =
> +         ((ir_instruction *)ir->else_instructions.head)->as_assignment();
> +
> +   if (!then_assign || then_assign->condition ||
> +       !else_assign || else_assign->condition ||
> +       then_assign->write_mask != else_assign->write_mask ||
> +       !then_assign->lhs->equals(else_assign->lhs))
> +      return false;
> +
> +   ir_constant *then_rhs = then_assign->rhs->as_constant();
> +   ir_constant *else_rhs = else_assign->rhs->as_constant();
> +
> +   if (!then_rhs || !else_rhs)
> +      return false;
> +
> +   if ((then_rhs->is_one() || then_rhs->is_negative_one()) &&
> +       (else_rhs->is_one() || else_rhs->is_negative_one())) {
> +      assert(then_rhs->is_one() == else_rhs->is_negative_one());
> +      assert(else_rhs->is_one() == then_rhs->is_negative_one());
> +
> +      then_assign->lhs->accept(this);
> +      fs_reg dst = this->result;
> +      dst.type = BRW_REGISTER_TYPE_D;
> +      fs_reg tmp = fs_reg(this, glsl_type::int_type);
> +
> +      if (brw->gen >= 6) {
> +         /* Bit 15 of g0.0 is 0 if the polygon is front facing. */
> +         fs_reg g0 = fs_reg(retype(brw_vec1_grf(0, 0), BRW_REGISTER_TYPE_W));
> +
> +         /* For (gl_FrontFacing ? 1.0 : -1.0), emit:
> +          *
> +          *    or(8)  tmp.1<2>W  g0.0<0,1,0>W  0x00003f80W
> +          *    and(8) dst<1>D    tmp<8,8,1>D   0xbf800000D
> +          *
> +          * and negate g0.0<0,1,0>W for (gl_FrontFacing ? -1.0 : 1.0).
> +          */
> +
> +         if (then_rhs->is_negative_one()) {
> +            assert(else_rhs->is_one());
> +            g0.negate = true;
> +         }
> +
> +         tmp.type = BRW_REGISTER_TYPE_W;
> +         tmp.subreg_offset = 2;
> +         tmp.stride = 2;
> +
> +         fs_inst *or_inst = emit(OR(tmp, g0, fs_reg(0x3f80)));
> +         or_inst->src[1].type = BRW_REGISTER_TYPE_UW;
> +
> +         tmp.type = BRW_REGISTER_TYPE_D;
> +         tmp.subreg_offset = 0;
> +         tmp.stride = 1;
> +      } else {
> +         /* Bit 31 of g1.6 is 0 if the polygon is front facing. */
> +         fs_reg g1_6 = fs_reg(retype(brw_vec1_grf(1, 6), BRW_REGISTER_TYPE_D));
> +
> +         /* For (gl_FrontFacing ? 1.0 : -1.0), emit:
> +          *
> +          *    or(8)  tmp<1>D  g1.6<0,1,0>D  0x3f800000D
> +          *    and(8) dst<1>D  tmp<8,8,1>D   0xbf800000D
> +          *
> +          * and negate g1.6<0,1,0>D for (gl_FrontFacing ? -1.0 : 1.0).
> +          */
> +
> +         if (then_rhs->is_negative_one()) {
> +            assert(else_rhs->is_one());
> +            g1_6.negate = true;
> +         }
> +
> +         emit(OR(tmp, g1_6, fs_reg(0x3f800000)));
> +      }
> +      emit(AND(dst, tmp, fs_reg(0xbf800000)));
> +      return true;
> +   }
> +
> +   return false;
> +}
> +
>  /**
>   * Try to replace IF/MOV/ELSE/MOV/ENDIF with SEL.
>   *
> @@ -2772,6 +2863,9 @@ fs_visitor::try_replace_with_sel()
>  void
>  fs_visitor::visit(ir_if *ir)
>  {
> +   if (try_opt_frontfacing_ternary(ir))
> +      return;
> +
>     if (brw->gen < 6) {
>        no16("Can't support (non-uniform) control flow on SIMD16\n");
>     }
>