[Freedreno] [PATCH] ir3_compiler/nir: fix imageSize() for buffer-backed images

[Ilia Mirkin]

On Tue, Oct 23, 2018 at 3:03 PM Eduardo Lima Mitev <elima at igalia.com> wrote:
>
> GL_EXT_texture_buffer introduced texture buffers, which can be used
> in shaders through a new type imageBuffer.
>
> Because how image access is implemented in freedreno, calling
> imageSize on an imageBuffer returns the size in bytes instead of texels,
> which is incorrect.
>
> This patch adds a division of imageSize result by the bytes-per-pixel
> of the image format, when image is buffer-backed.
>
> Fixes all tests under
> dEQP-GLES31.functional.image_load_store.buffer.image_size.*
>
> v2: Pre-compute and submit the log2 of the image format's bpp as shader
>     constant instead of emitting the LOG2 instruction in code. (Rob Clark)
> ---
>  .../drivers/freedreno/ir3/ir3_compiler_nir.c  | 23 +++++++++++++++++++
>  .../drivers/freedreno/ir3/ir3_shader.c        | 10 ++++++++
>  2 files changed, 33 insertions(+)
>
> diff --git a/src/gallium/drivers/freedreno/ir3/ir3_compiler_nir.c b/src/gallium/drivers/freedreno/ir3/ir3_compiler_nir.c
> index 197196383b0..7a3c8a8579c 100644
> --- a/src/gallium/drivers/freedreno/ir3/ir3_compiler_nir.c
> +++ b/src/gallium/drivers/freedreno/ir3/ir3_compiler_nir.c
> @@ -2035,6 +2035,29 @@ emit_intrinsic_image_size(struct ir3_context *ctx, nir_intrinsic_instr *intr,
>
>         split_dest(b, tmp, sam, 0, 4);
>
> +       /* get_size instruction returns size in bytes instead of texels
> +        * for imageBuffer, so we need to divide it by the pixel size
> +        * of the image format.
> +        *
> +        * TODO: This is at least true on a5xx. Check other gens.
> +        */
> +       enum glsl_sampler_dim dim =
> +               glsl_get_sampler_dim(glsl_without_array(var->type));
> +       if (dim == GLSL_SAMPLER_DIM_BUF) {
> +               /* Since all the possible values the divisor can take are
> +                * power-of-two (4, 8, or 16), the division is implemented
> +                * as a shift-right.
> +                * During shader setup, the log2 of the image format's
> +                * bytes-per-pixel should have been emitted in 2nd slot of
> +                * image_dims. See ir3_shader::emit_image_dims().
> +                */
> +               unsigned cb = regid(ctx->so->constbase.image_dims, 0) +
> +                       ctx->so->const_layout.image_dims.off[var->data.driver_location];
> +               struct ir3_instruction *aux = create_uniform(ctx, cb + 1);
> +
> +               tmp[0] = ir3_SHR_B(b, tmp[0], 0, aux, 0);
> +       }
> +
>         for (unsigned i = 0; i < ncoords; i++)
>                 dst[i] = tmp[i];
>
> diff --git a/src/gallium/drivers/freedreno/ir3/ir3_shader.c b/src/gallium/drivers/freedreno/ir3/ir3_shader.c
> index 9bf0a7f999c..de59e5888c6 100644
> --- a/src/gallium/drivers/freedreno/ir3/ir3_shader.c
> +++ b/src/gallium/drivers/freedreno/ir3/ir3_shader.c
> @@ -699,6 +699,16 @@ emit_image_dims(struct fd_context *ctx, const struct ir3_shader_variant *v,
>                                 } else {
>                                         dims[off + 2] = rsc->slices[lvl].size0;
>                                 }
> +                       } else {
> +                               /* For buffer-backed images, the log2 of the format's
> +                                * bytes-per-pixel is placed on the 2nd slot. This is useful
> +                                * when emitting image_size instructions, for which we need
> +                                * to divide by bpp for image buffers. Since the bpp
> +                                * can only be power-of-two, the division is implemented
> +                                * as a SHR, and for that it is handy to have the log2 of
> +                                * bpp as a constant.
> +                                */
> +                               dims[off + 1] = log (dims[off + 0]) / log (2);

This is not the log function you are looking for. You're looking for
ilog2 or ffs.

  -ilia