i965/fs: Readd opt_drop_redundant_mov_to_flags().

This reverts commit b449366587b5f3f64c6fb45fe22c39e4bc8a4309.

I removed the pass thinking that it was now not useful, but that was not
true. I believe I ran shader-db on HSW and saw no results, but HSW does
not use the unlit centroid workaround code and as a result does not emit
redundant MOV_DISPATCH_TO_FLAGS instructions.

On IVB, the shader-db results are:

total instructions in shared programs: 6650806 -> 6646303 (-0.07%)
instructions in affected programs: 106893 -> 102390 (-4.21%)
helped: 793

total cycles in shared programs: 56195538 -> 56103720 (-0.16%)
cycles in affected programs: 873048 -> 781230 (-10.52%)
helped: 553
HURT: 209

On SNB, the shader-db results are:

total instructions in shared programs: 7173074 -> 7168541 (-0.06%)
instructions in affected programs: 119757 -> 115224 (-3.79%)
helped: 799

total cycles in shared programs: 98128032 -> 98072938 (-0.06%)
cycles in affected programs: 1437104 -> 1382010 (-3.83%)
helped: 454
HURT: 237

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

2 files changed, 41 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index fc72cb3..5d6a107 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -5186,6 +5186,44 @@ fs_visitor::calculate_register_pressure()
    }
 }
 
+/**
+ * Look for repeated FS_OPCODE_MOV_DISPATCH_TO_FLAGS and drop the later ones.
+ *
+ * The needs_unlit_centroid_workaround ends up producing one of these per
+ * channel of centroid input, so it's good to clean them up.
+ *
+ * An assumption here is that nothing ever modifies the dispatched pixels
+ * value that FS_OPCODE_MOV_DISPATCH_TO_FLAGS reads from, but the hardware
+ * dictates that anyway.
+ */
+bool
+fs_visitor::opt_drop_redundant_mov_to_flags()
+{
+   bool flag_mov_found[2] = {false};
+   bool progress = false;
+
+   /* Instructions removed by this pass can only be added if this were true */
+   if (!devinfo->needs_unlit_centroid_workaround)
+      return false;
+
+   foreach_block_and_inst_safe(block, fs_inst, inst, cfg) {
+      if (inst->is_control_flow()) {
+         memset(flag_mov_found, 0, sizeof(flag_mov_found));
+      } else if (inst->opcode == FS_OPCODE_MOV_DISPATCH_TO_FLAGS) {
+         if (!flag_mov_found[inst->flag_subreg]) {
+            flag_mov_found[inst->flag_subreg] = true;
+         } else {
+            inst->remove(block);
+            progress = true;
+         }
+      } else if (inst->writes_flag()) {
+         flag_mov_found[inst->flag_subreg] = false;
+      }
+   }
+
+   return progress;
+}
+
 void
 fs_visitor::optimize()
 {
@@ -5243,6 +5281,8 @@ fs_visitor::optimize()
    int iteration = 0;
    int pass_num = 0;
 
+   OPT(opt_drop_redundant_mov_to_flags);
+
    OPT(lower_simd_width);
    OPT(lower_logical_sends);
 
diff --git a/src/mesa/drivers/dri/i965/brw_fs.h b/src/mesa/drivers/dri/i965/brw_fs.h
index c7f20ca..bcd2e3e 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.h
+++ b/src/mesa/drivers/dri/i965/brw_fs.h
@@ -153,6 +153,7 @@ public:
    bool try_constant_propagate(fs_inst *inst, acp_entry *entry);
    bool opt_copy_propagate_local(void *mem_ctx, bblock_t *block,
                                  exec_list *acp);
+   bool opt_drop_redundant_mov_to_flags();
    bool opt_register_renaming();
    bool register_coalesce();
    bool compute_to_mrf();