This causes a O(N^2) behavior in the number of feedback'd variables,
which I suspect is undesirable. Elsewhere we use arrays and/or maps --
since the size of the xfb offset has a max value, an array-based
solution may work well.
On Fri, Feb 1, 2019 at 1:08 PM Andres Gomez <ago...@igalia.com> wrote:
>
> From page 76 (page 80 of the PDF) of the GLSL 4.60 v.5 spec:
>
> " No aliasing in output buffers is allowed: It is a compile-time or
> link-time error to specify variables with overlapping transform
> feedback offsets."
>
> Currently, this is expected to fail, but it succeeds:
>
> "
>
> ...
>
> layout (xfb_offset = 0) out vec2 a;
> layout (xfb_offset = 0) out vec4 b;
>
> ...
>
> "
>
> Cc: Timothy Arceri <tarc...@itsqueeze.com>
> Signed-off-by: Andres Gomez <ago...@igalia.com>
> ---
> src/compiler/glsl/link_varyings.cpp | 23 +++++++++++++++++++++++
> 1 file changed, 23 insertions(+)
>
> diff --git a/src/compiler/glsl/link_varyings.cpp
> b/src/compiler/glsl/link_varyings.cpp
> index ab66ceb0d00..28e47f15733 100644
> --- a/src/compiler/glsl/link_varyings.cpp
> +++ b/src/compiler/glsl/link_varyings.cpp
> @@ -1173,6 +1173,29 @@ tfeedback_decl::store(struct gl_context *ctx, struct
> gl_shader_program *prog,
> unsigned location = this->location;
> unsigned location_frac = this->location_frac;
> unsigned num_components = this->num_components();
> +
> + /* From the OpenGL 4.60.5 spec, section 4.4.2. Output Layout
> Qualifiers,
> + * Page 76, (Transform Feedback Layout Qualifiers):
> + *
> + * "No aliasing in output buffers is allowed: It is a compile-time or
> + * link-time error to specify variables with overlapping transform
> + * feedback offsets."
> + */
> + for (unsigned i = 0; i < info->NumOutputs; i++) {
> + const struct gl_transform_feedback_output &output =
> info->Outputs[i];
> +
> + if (output.OutputBuffer != buffer)
> + continue;
> +
> + if ((output.DstOffset < xfb_offset + num_components) &&
> + (output.DstOffset + output.NumComponents > xfb_offset)) {
> + linker_error(prog,
> + "variable '%s', xfb_offset (%d) is causing
> aliasing.",
> + this->orig_name, xfb_offset * 4);
> + return false;
> + }
> + }
> +
> while (num_components > 0) {
> unsigned output_size = MIN2(num_components, 4 - location_frac);
> assert((info->NumOutputs == 0 && max_outputs == 0) ||
> --
> 2.20.1
>
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