[Mesa-dev] [PATCH] glsl: use bitfield_insert instead of and + shift + or for packing

[Matt Turner]

On Thu, Aug 20, 2015 at 6:00 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> It is fairly tricky to detect the proper conditions for using bitfield
> insert, but easy to just use it up front. This removes a lot of
> instructions on nvc0 when invoking the packing builtins.
>
> Signed-off-by: Ilia Mirkin <imirkin at alum.mit.edu>
> ---
>
> Not sure if all backends will benefit from this, but nvc0 definitely
> does, since it has a "insert bitfield" instruction which handles
> everything all-in-one (the combined offset/bits argument takes the
> position of the third source, which requires no extra logic when
> they're both immediate).
>
> It would be just as easy to use the 3-arg bfi op (no masking necessary
> since we know that we start with a clean value), but gallium doesn't
> support it. (Not for the least of reasons is that nvc0 doesn't support
> it and I'm the one that added BFI to TGSI.)
>
> Unpacking could use BFE, really the snorm logic would greatly benefit
> from using an IBFE (thus avoiding the shifts back and forth),
> but... later.
>
>  src/glsl/ir_optimization.h                 |  4 +++-
>  src/glsl/lower_packing_builtins.cpp        | 24 +++++++++++++++++++++++-
>  src/mesa/state_tracker/st_glsl_to_tgsi.cpp |  3 +++
>  3 files changed, 29 insertions(+), 2 deletions(-)
>
> diff --git a/src/glsl/ir_optimization.h b/src/glsl/ir_optimization.h
> index eef107e..b955874 100644
> --- a/src/glsl/ir_optimization.h
> +++ b/src/glsl/ir_optimization.h
> @@ -66,7 +66,9 @@ enum lower_packing_builtins_op {
>     LOWER_UNPACK_SNORM_4x8               = 0x0200,
>
>     LOWER_PACK_UNORM_4x8                 = 0x0400,
> -   LOWER_UNPACK_UNORM_4x8               = 0x0800
> +   LOWER_UNPACK_UNORM_4x8               = 0x0800,
> +
> +   LOWER_PACK_USE_BFI                   = 0x1000,
>  };
>
>  bool do_common_optimization(exec_list *ir, bool linked,
> diff --git a/src/glsl/lower_packing_builtins.cpp b/src/glsl/lower_packing_builtins.cpp
> index a6fb8a8..be17e1d 100644
> --- a/src/glsl/lower_packing_builtins.cpp
> +++ b/src/glsl/lower_packing_builtins.cpp
> @@ -225,6 +225,14 @@ private:
>                                            "tmp_pack_uvec2_to_uint");
>        factory.emit(assign(u, uvec2_rval));
>
> +      if (op_mask & LOWER_PACK_USE_BFI) {
> +         return bitfield_insert(
> +            bit_and(swizzle_x(u), constant(0xffffu)),
> +            swizzle_y(u),
> +            constant(16),
> +            constant(16));

Indent bitfield_insert's arguments to align with its "(" (and probably
put bit_and on the same line)

> +      }
> +
>        /* return (u.y << 16) | (u.x & 0xffff); */
>        return bit_or(lshift(swizzle_y(u), constant(16u)),
>                      bit_and(swizzle_x(u), constant(0xffffu)));
> @@ -242,9 +250,23 @@ private:
>     {
>        assert(uvec4_rval->type == glsl_type::uvec4_type);
>
> -      /* uvec4 u = UVEC4_RVAL; */
>        ir_variable *u = factory.make_temp(glsl_type::uvec4_type,
>                                            "tmp_pack_uvec4_to_uint");
> +
> +      if (op_mask & LOWER_PACK_USE_BFI) {
> +         /* uvec4 u = UVEC4_RVAL; */
> +         factory.emit(assign(u, uvec4_rval));
> +
> +         return bitfield_insert(
> +            bitfield_insert(
> +               bitfield_insert(
> +                  bit_and(swizzle_x(u), constant(0xffu)),
> +                  swizzle_y(u), constant(8), constant(8)),
> +               swizzle_z(u), constant(16), constant(8)),
> +            swizzle_w(u), constant(24), constant(8));

I don't know if there's a nice way to indent such a long expression,
but I'd indent the lines after the return three spaces more than the
first bitfield_insert.

> +      }
> +
> +      /* uvec4 u = UVEC4_RVAL & 0xff */
>        factory.emit(assign(u, bit_and(uvec4_rval, constant(0xffu))));
>
>        /* return (u.w << 24) | (u.z << 16) | (u.y << 8) | u.x; */
> diff --git a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
> index eb47685..d3c7238 100644
> --- a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
> +++ b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
> @@ -5995,6 +5995,9 @@ st_link_shader(struct gl_context *ctx, struct gl_shader_program *prog)
>                                 LOWER_PACK_HALF_2x16 |
>                                 LOWER_UNPACK_HALF_2x16;
>
> +         if (ctx->Extensions.ARB_gpu_shader5)
> +            lower_inst |= LOWER_PACK_USE_BFI;
> +
>           lower_packing_builtins(ir, lower_inst);
>        }