[Mesa-dev] [PATCH 2/2] i965: Remove redundant discard jumps.

[Kenneth Graunke]

With the previous optimization in place, some shaders wind up with
multiple discard jumps in a row, or jumps directly to the next
instruction.  We can remove those.

total instructions in shared programs: 5122144 -> 5121604 (-0.01%)
instructions in affected programs:     11928 -> 11388 (-4.53%)

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/drivers/dri/i965/brw_fs.cpp | 40 ++++++++++++++++++++++++++++++++++++
 src/mesa/drivers/dri/i965/brw_fs.h   |  1 +
 2 files changed, 41 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index 6d80603..8cace49 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -2177,6 +2177,45 @@ fs_visitor::opt_conditional_discard()
    return progress;
 }
 
+/**
+ * Remove redundant or useless discard jumps.
+ *
+ * For example, we can eliminate jumps in the following sequence:
+ *
+ * discard-jump       (redundant with the next jump)
+ * discard-jump       (useless; jumps to the next instruction)
+ * placeholder-halt
+ */
+bool
+fs_visitor::opt_redundant_discard_jumps()
+{
+   bool progress = false;
+
+   fs_inst *placeholder_halt = NULL;
+   foreach_in_list_reverse(fs_inst, inst, &instructions) {
+      if (inst->opcode == FS_OPCODE_PLACEHOLDER_HALT) {
+         placeholder_halt = inst;
+         break;
+      }
+   }
+
+   if (!placeholder_halt)
+      return false;
+
+   /* Delete any HALTs immediately before the placeholder halt. */
+   for (fs_inst *prev = (fs_inst *) placeholder_halt->prev;
+        prev->opcode == FS_OPCODE_DISCARD_JUMP;
+        prev = (fs_inst *) placeholder_halt->prev) {
+      prev->remove();
+      progress = true;
+   }
+
+   if (progress)
+      invalidate_live_intervals();
+
+   return progress;
+}
+
 bool
 fs_visitor::compute_to_mrf()
 {
@@ -3298,6 +3337,7 @@ fs_visitor::run()
          OPT(dead_control_flow_eliminate, this);
          OPT(opt_register_renaming);
          OPT(opt_conditional_discard);
+         OPT(opt_redundant_discard_jumps);
          OPT(opt_saturate_propagation);
          OPT(register_coalesce);
          OPT(compute_to_mrf);
diff --git a/src/mesa/drivers/dri/i965/brw_fs.h b/src/mesa/drivers/dri/i965/brw_fs.h
index 1660ad7..1de8254 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.h
+++ b/src/mesa/drivers/dri/i965/brw_fs.h
@@ -333,6 +333,7 @@ public:
    void calculate_register_pressure();
    bool opt_algebraic();
    bool opt_conditional_discard();
+   bool opt_redundant_discard_jumps();
    bool opt_cse();
    bool opt_cse_local(bblock_t *block);
    bool opt_copy_propagate();
-- 
2.0.2