Mesa (master): v3d: Use stvpmd for non-uniform offsets in GS

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Module: Mesa
Branch: master
Commit: 3b1c511b099e88d1b153de784e3bf7e951a6288f
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=3b1c511b099e88d1b153de784e3bf7e951a6288f

Author: Neil Roberts <nroberts at igalia.com>
Date:   Wed Jun 24 00:16:43 2020 +0200

v3d: Use stvpmd for non-uniform offsets in GS

The offset for the VPM write for storing outputs from the geometry
shader isn’t necessarily uniform across all the lanes. This can happen
if some of the lanes don’t emit some of the vertices. In that case the
offset for the subsequent vertices will be different in each lane. In
that case we need to use the stvpmd instruction instead of stvpmv
because it will scatter the values out.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3150

Reviewed-by: Iago Toral Quiroga <itoral at igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5621>

---

 src/broadcom/compiler/nir_to_vir.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/broadcom/compiler/nir_to_vir.c b/src/broadcom/compiler/nir_to_vir.c
index 1086e288a60..e7b8cf9cb3a 100644
--- a/src/broadcom/compiler/nir_to_vir.c
+++ b/src/broadcom/compiler/nir_to_vir.c
@@ -2027,7 +2027,18 @@ emit_store_output_gs(struct v3d_compile *c, nir_intrinsic_instr *instr)
                            V3D_QPU_PF_PUSHZ);
         }
 
-        vir_VPM_WRITE_indirect(c, ntq_get_src(c, instr->src[0], 0), offset);
+        struct qreg val = ntq_get_src(c, instr->src[0], 0);
+
+        /* The offset isn’t necessarily dynamically uniform for a geometry
+         * shader. This can happen if the shader sometimes doesn’t emit one of
+         * the vertices. In that case subsequent vertices will be written to
+         * different offsets in the VPM and we need to use the scatter write
+         * instruction to have a different offset for each lane.
+         */
+        if (nir_src_is_dynamically_uniform(instr->src[1]))
+                vir_VPM_WRITE_indirect(c, val, offset);
+        else
+                vir_STVPMD(c, offset, val);
 
         if (vir_in_nonuniform_control_flow(c)) {
                 struct qinst *last_inst =