[Mesa-dev] [PATCH] glsl: xfb_stride applies to buffers, not block members

[Timothy Arceri]

On 8 July 2017 12:19:48 am GMT+08:00, "Juan A. Suarez Romero" <jasuarez at igalia.com> wrote:
>When we have an interface block like:
>
>layout (xfb_buffer = 0, xfb_offset = 0) out Block {
>                             vec4 var1;
>    layout (xfb_stride = 48) vec4 var2;
>                             vec4 var3;
>};
>
>According to ARB_enhanced_layouts spec:
>
>   "The *xfb_stride* qualifier specifies how many bytes are consumed by
>    each captured vertex.  It applies to the transform feedback buffer
>    for that declaration, whether it is inherited or explicitly
>    declared. It can be applied to variables, blocks, block members, or
>    just the qualifier out. [ ...] While *xfb_stride* can be declared
>    multiple times for the same buffer, it is a compile-time or
>    link-time error to have different values specified for the stride
>    for the same buffer."
>
>This means xfb_stride actually applies to the buffer, and not to the
>individual components.

Right. As I understand it, it applies to the buffer but can be set via a qualifier on a block member as the spec quote says above. 

>
>In the above example, it means that var2 consumes 16 bytes, and var3 is
>at offset 32.
>
>This has been confirmed also by John Kessenich, the main contact for
>the
>ARB_enhanced_layouts specs, 

What exactly did he confirm? The example you give above or just that the stride applies to the buffer. 

Are you saying that the qualifier should be ignored besides throwing link/compile errors for block members? 

My assumption was it should just apply to the whole block if it was set for any member. 

I can't look at the CTS until next week. 

and also because this commit fixes:
>
>GL45.enhanced_layouts.xfb_block_member_stride
>
>This commit is in practice a revert of 598790e8564 (glsl: apply
>xfb_stride to implicit offsets for ifc block members).
>---
> src/compiler/glsl/ast_to_hir.cpp | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
>diff --git a/src/compiler/glsl/ast_to_hir.cpp
>b/src/compiler/glsl/ast_to_hir.cpp
>index c338ad7..3968657 100644
>--- a/src/compiler/glsl/ast_to_hir.cpp
>+++ b/src/compiler/glsl/ast_to_hir.cpp
>@@ -7372,14 +7372,13 @@
>ast_process_struct_or_iface_block_members(exec_list *instructions,
>                                          qual->offset, &xfb_offset)) {
>                fields[i].offset = xfb_offset;
>                block_xfb_offset = fields[i].offset +
>-                  MAX2(xfb_stride, (int) (4 *
>field_type->component_slots()));
>+                  4 * field_type->component_slots();
>             }
>          } else {
>             if (layout && layout->flags.q.explicit_xfb_offset) {
>                unsigned align = field_type->is_64bit() ? 8 : 4;
>                fields[i].offset = glsl_align(block_xfb_offset, align);
>-               block_xfb_offset +=
>-                  MAX2(xfb_stride, (int) (4 *
>field_type->component_slots()));
>+               block_xfb_offset += 4 * field_type->component_slots();
>             }
>          }
> 
>-- 
>2.9.4
>
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