Mesa (master): nir: do not try to merge xfb-outputs

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Module: Mesa
Branch: master
Commit: e838acf37d055ba3734d720804b72eb1ecf02577
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=e838acf37d055ba3734d720804b72eb1ecf02577

Author: Erik Faye-Lund <erik.faye-lund at collabora.com>
Date:   Thu Jun  4 12:38:08 2020 +0200

nir: do not try to merge xfb-outputs

It's tricky to merge XFB-outputs correctly, because we need there to not
be any overlaps when we get to `nir_gather_xfb_info_with_varyings` later
on. We currently trigger an assert there if we end up merging here.

So let's not even try. This is an optimization, and we can optimize this
in safe cases later if needed. For now, let's play it safe.

Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5329>

---

 src/compiler/nir/nir_lower_io_to_vector.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/compiler/nir/nir_lower_io_to_vector.c b/src/compiler/nir/nir_lower_io_to_vector.c
index f85e53ac758..b1ea871a402 100644
--- a/src/compiler/nir/nir_lower_io_to_vector.c
+++ b/src/compiler/nir/nir_lower_io_to_vector.c
@@ -135,6 +135,18 @@ variables_can_merge(const nir_shader *shader,
        a->data.index != b->data.index)
       return false;
 
+   /* It's tricky to merge XFB-outputs correctly, because we need there
+    * to not be any overlaps when we get to
+    * nir_gather_xfb_info_with_varyings later on. We'll end up
+    * triggering an assert there if we merge here.
+    */
+   if ((shader->info.stage == MESA_SHADER_VERTEX ||
+        shader->info.stage == MESA_SHADER_TESS_EVAL ||
+        shader->info.stage == MESA_SHADER_GEOMETRY) &&
+       a->data.mode == nir_var_shader_out &&
+       (a->data.explicit_xfb_buffer || b->data.explicit_xfb_buffer))
+      return false;
+
    return true;
 }