Mesa (main): pan/bi: Do not cull post-RA staging writes

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Module: Mesa
Branch: main
Commit: e5423bb1295b1cb254cd94408a5e933ea1221610
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=e5423bb1295b1cb254cd94408a5e933ea1221610

Author: Alyssa Rosenzweig <alyssa at collabora.com>
Date:   Thu Dec 23 12:14:41 2021 -0500

pan/bi: Do not cull post-RA staging writes

Bifrost post-RA dead code elimination can cull the destinations of
regular ALU instructions, by weakening from a register write to a
temporary write. However, there is no way to suppress staging writes, so
culling the destinations will result in invalid code generation.

Fixes a regression in
dEQP-GLES3.functional.shaders.switch.switch_in_for_loop_static_vertex
with scoreboarding. The root cause there is the backend dead code
elimination not being sufficiently aggressive in the presence of control
flow. Usually this does not matter, since the backend optimizations are
intended to be local with global optimizations happening in NIR.
Unfortunately, our implementation of IDVS hits this hard. That will need
to be optimized (probably by specializing IDVS shaders in NIR instead of
the backend). In the mean time, let's fix the actual bug affecting
scoreboarding.

No shader-db changes.

Signed-off-by: Alyssa Rosenzweig <alyssa at collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14298>

---

 src/panfrost/bifrost/bi_opt_dce.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/panfrost/bifrost/bi_opt_dce.c b/src/panfrost/bifrost/bi_opt_dce.c
index d1d35920961..5fbb2cf0f01 100644
--- a/src/panfrost/bifrost/bi_opt_dce.c
+++ b/src/panfrost/bifrost/bi_opt_dce.c
@@ -171,6 +171,7 @@ bi_opt_dce_post_ra(bi_context *ctx)
                                 unsigned reg = ins->dest[d].value;
                                 uint64_t mask = (BITFIELD64_MASK(nr) << reg);
                                 bool cullable = (ins->op != BI_OPCODE_BLEND);
+                                cullable &= !bi_opcode_props[ins->op].sr_write;
 
                                 if (!(live & mask) && cullable)
                                         ins->dest[d] = bi_null();