Mesa (master): intel/fs: Don't use pixel_z for Gen4-5 source_depth_to_render_target

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Module: Mesa
Branch: master
Commit: 3f36e027d37e28af08b91019df4bf3cdf08bfa2f
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=3f36e027d37e28af08b91019df4bf3cdf08bfa2f

Author: Jason Ekstrand <jason at jlekstrand.net>
Date:   Mon May  3 13:59:30 2021 -0500

intel/fs: Don't use pixel_z for Gen4-5 source_depth_to_render_target

The source_depth_to_render_target flag can get set on old gen4-5 HW in a
few cases which are independent of the app writing gl_FragDepth.  It
should be safe to just use fetch_payload_reg in that case instead of
depending in interpolation setup.  This fixes a bug with certain very
simple shaders where we might end up not including the depth when we
should have.

While we're here, rework the logic around setting src_depth and add a
comment so it's more clear what's going on.

Fixes: 6d4070f3ddb5 "intel/compiler: add support for fragment coordinate..."
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10596>

---

 src/intel/compiler/brw_fs_visitor.cpp | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/src/intel/compiler/brw_fs_visitor.cpp b/src/intel/compiler/brw_fs_visitor.cpp
index 18148e97fe4..b58ec72e465 100644
--- a/src/intel/compiler/brw_fs_visitor.cpp
+++ b/src/intel/compiler/brw_fs_visitor.cpp
@@ -613,11 +613,18 @@ fs_visitor::emit_single_fb_write(const fs_builder &bld,
    const fs_reg dst_depth = fetch_payload_reg(bld, payload.dest_depth_reg);
    fs_reg src_depth, src_stencil;
 
-   if (source_depth_to_render_target) {
-      if (nir->info.outputs_written & BITFIELD64_BIT(FRAG_RESULT_DEPTH))
-         src_depth = frag_depth;
-      else
-         src_depth = this->pixel_z;
+   if (nir->info.outputs_written & BITFIELD64_BIT(FRAG_RESULT_DEPTH)) {
+      src_depth = frag_depth;
+   } else if (source_depth_to_render_target) {
+      /* If we got here, we're in one of those strange Gen4-5 cases where
+       * we're forced to pass the source depth, unmodified, to the FB write.
+       * In this case, we don't want to use pixel_z because we may not have
+       * set up interpolation.  It's also perfectly safe because it only
+       * happens on old hardware (no coarse interpolation) and this is
+       * explicitly the pass-through case.
+       */
+      assert(devinfo->ver <= 5);
+      src_depth = fetch_payload_reg(bld, payload.source_depth_reg);
    }
 
    if (nir->info.outputs_written & BITFIELD64_BIT(FRAG_RESULT_STENCIL))