i965: Eliminate dead code more aggressively.

If an instruction writes reg but nothing later uses it, then we don't
need to bother doing it.  Before, we were just killing code that was
never read after it was ever written.

This removes many interpolation instructions for attributes with only
a few comopnents used.  Improves nexuiz high-settings performance .46%
+/- .12% (n=3) on my Ironlake.

1 files changed, 4 insertions, 13 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index b12a4800410..ac795e0bda1 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -2801,26 +2801,17 @@ bool
 fs_visitor::dead_code_eliminate()
 {
    bool progress = false;
-   int num_vars = this->virtual_grf_next;
-   bool dead[num_vars];
-
-   for (int i = 0; i < num_vars; i++) {
-      dead[i] = this->virtual_grf_def[i] >= this->virtual_grf_use[i];
-
-      if (dead[i]) {
-	 /* Mark off its interval so it won't interfere with anything. */
-	 this->virtual_grf_def[i] = -1;
-	 this->virtual_grf_use[i] = -1;
-      }
-   }
+   int pc = 0;
 
    foreach_iter(exec_list_iterator, iter, this->instructions) {
       fs_inst *inst = (fs_inst *)iter.get();
 
-      if (inst->dst.file == GRF && dead[inst->dst.reg]) {
+      if (inst->dst.file == GRF && this->virtual_grf_use[inst->dst.reg] <= pc) {
 	 inst->remove();
 	 progress = true;
       }
+
+      pc++;
    }
 
    return progress;