vc4: Allow dead code elimination of unused varyings.

total instructions in shared programs: 39022 -> 37341 (-4.31%)
instructions in affected programs:     26979 -> 25298 (-6.23%)
total uniforms in shared programs: 11242 -> 10523 (-6.40%)
uniforms in affected programs:     5836 -> 5117 (-12.32%)

1 files changed, 13 insertions, 2 deletions

diff --git a/src/gallium/drivers/vc4/vc4_qir.c b/src/gallium/drivers/vc4/vc4_qir.c
index 9c7c15e4924..a7a4d96f758 100644
--- a/src/gallium/drivers/vc4/vc4_qir.c
+++ b/src/gallium/drivers/vc4/vc4_qir.c
@@ -122,12 +122,23 @@ qir_get_op_nsrc(enum qop qop)
                 abort();
 }
 
+/**
+ * Returns whether the instruction has any side effects that must be
+ * preserved.
+ */
 bool
-qir_has_side_effects(struct qinst *inst)
+qir_has_side_effects(struct vc4_compile *c, struct qinst *inst)
 {
+        /* We can dead-code eliminate varyings, because we only tell the VS
+         * about the live ones at the end.  But we have to preserve the
+         * point/line coordinates reads, because they're generated by
+         * fixed-function hardware.
+         */
         for (int i = 0; i < qir_get_op_nsrc(inst->op); i++) {
-                if (inst->src[i].file == QFILE_VARY)
+                if (inst->src[i].file == QFILE_VARY &&
+                    c->input_semantics[inst->src[i].index].semantic == 0xff) {
                         return true;
+                }
         }
 
         return qir_op_info[inst->op].has_side_effects;