i965/fs: Don't emit null MOVs in CSE.

We'd like to CSE some instructions, like CMP, that often have null
destinations. Instead of replacing them with MOVs to null, just don't
emit the MOV.

Reviewed-by: Paul Berry <stereotype441@gmail.com>

diff --git a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
index 0afc5f65f6e55ed7eef325b46697cf48ff29df34..3f59339f716aba8a0775465d6d8f902e6791ce88 100644 (file)
--- a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
@@ -150,7 +150,7 @@ fs_visitor::opt_cse_local(bblock_t *block, exec_list *aeb)
             * If we don't have a temporary already, make one.
             */
            bool no_existing_temp = entry->tmp.file == BAD_FILE;
-           if (no_existing_temp) {
+           if (no_existing_temp && !entry->generator->dst.is_null()) {
                int written = entry->generator->regs_written;
 
                fs_reg orig_dst = entry->generator->dst;
@@ -171,29 +171,37 @@ fs_visitor::opt_cse_local(bblock_t *block, exec_list *aeb)
            }
 
            /* dest <- temp */
-            int written = inst->regs_written;
-            assert(written == entry->generator->regs_written);
-            assert(inst->dst.type == entry->tmp.type);
-            fs_reg dst = inst->dst;
-            fs_reg tmp = entry->tmp;
-            fs_inst *copy = NULL;
-            for (int i = 0; i < written; i++) {
-               copy = MOV(dst, tmp);
-               copy->force_writemask_all = inst->force_writemask_all;
-               inst->insert_before(copy);
-
-               dst.reg_offset++;
-               tmp.reg_offset++;
+            if (!inst->dst.is_null()) {
+               int written = inst->regs_written;
+               assert(written == entry->generator->regs_written);
+               assert(inst->dst.type == entry->tmp.type);
+               fs_reg dst = inst->dst;
+               fs_reg tmp = entry->tmp;
+               fs_inst *copy = NULL;
+               for (int i = 0; i < written; i++) {
+                  copy = MOV(dst, tmp);
+                  copy->force_writemask_all = inst->force_writemask_all;
+                  inst->insert_before(copy);
+
+                  dst.reg_offset++;
+                  tmp.reg_offset++;
+               }
             }
+
+            /* Set our iterator so that next time through the loop inst->next
+             * will get the instruction in the basic block after the one we've
+             * removed.
+             */
+            fs_inst *prev = (fs_inst *)inst->prev;
+
             inst->remove();
 
            /* Appending an instruction may have changed our bblock end. */
            if (inst == block->end) {
-              block->end = copy;
+              block->end = prev;
            }
 
-           /* Continue iteration with copy->next */
-           inst = copy;
+            inst = prev;
         }
       }