i965/fs: Make virtual grf live intervals actually cover their used range.

Previously, we would sometimes not consider a write to a register to
extend the end of the interval, nor would we consider a read before a
write to extend the start.  This made for a bunch of complicated logic
related to how to treat the results when dead code might be present.
Instead, just extend the interval and fix dead code elimination to know
how to remove it.

Interestingly, this actually results in a tiny bit more optimization:
total instructions in shared programs: 1391220 -> 1390799 (-0.03%)
instructions in affected programs:     14037 -> 13616 (-3.00%)

v2: Fix a theoretical problem with the simd16 workaround if dst == src,
    where we would revert the bump of the live range.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> (v1)

1 files changed, 11 insertions, 10 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index 778a69e7091..baaa25c1347 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -1456,8 +1456,8 @@ fs_visitor::compact_virtual_grfs()
          remap_table[i] = new_index;
          virtual_grf_sizes[new_index] = virtual_grf_sizes[i];
          if (live_intervals_valid) {
-            virtual_grf_use[new_index] = virtual_grf_use[i];
-            virtual_grf_def[new_index] = virtual_grf_def[i];
+            virtual_grf_start[new_index] = virtual_grf_start[i];
+            virtual_grf_end[new_index] = virtual_grf_end[i];
          }
          ++new_index;
       }
@@ -1771,10 +1771,8 @@ fs_visitor::opt_algebraic()
 }
 
 /**
- * Must be called after calculate_live_intervales() to remove unused
- * writes to registers -- register allocation will fail otherwise
- * because something deffed but not used won't be considered to
- * interfere with other regs.
+ * Removes any instructions writing a VGRF where that VGRF is not used by any
+ * later instruction.
  */
 bool
 fs_visitor::dead_code_eliminate()
@@ -1787,9 +1785,12 @@ fs_visitor::dead_code_eliminate()
    foreach_list_safe(node, &this->instructions) {
       fs_inst *inst = (fs_inst *)node;
 
-      if (inst->dst.file == GRF && this->virtual_grf_use[inst->dst.reg] <= pc) {
-	 inst->remove();
-	 progress = true;
+      if (inst->dst.file == GRF) {
+         assert(this->virtual_grf_end[inst->dst.reg] >= pc);
+         if (this->virtual_grf_end[inst->dst.reg] == pc) {
+            inst->remove();
+            progress = true;
+         }
       }
 
       pc++;
@@ -2201,7 +2202,7 @@ fs_visitor::compute_to_mrf()
       /* Can't compute-to-MRF this GRF if someone else was going to
        * read it later.
        */
-      if (this->virtual_grf_use[inst->src[0].reg] > ip)
+      if (this->virtual_grf_end[inst->src[0].reg] > ip)
 	 continue;
 
       /* Found a move of a GRF to a MRF.  Let's see if we can go