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");
> }
>
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