i965/fs: Compact the virtual GRF arrays.

During code generation, we create tons of temporary variables, many of
which get immediately killed and are never used.  Later optimization and
analysis passes, such as compute_live_intervals, loop over all the
virtual GRFs.  By compacting them, we can save a lot of overhead.

Reduces compilation time in L4D2's largest fragment shader from 10.2
seconds to 5.2 seconds (50%).  Drops compute_live_variables() from
10-12% of another game's startup time to 8%.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -1102,6 +1102,64 @@ fs_visitor::split_virtual_grfs()
    this->live_intervals_valid = false;
 }
 
+/**
+ * Remove unused virtual GRFs and compact the virtual_grf_* arrays.
+ *
+ * During code generation, we create tons of temporary variables, many of
+ * which get immediately killed and are never used again.  Yet, in later
+ * optimization and analysis passes, such as compute_live_intervals, we need
+ * to loop over all the virtual GRFs.  Compacting them can save a lot of
+ * overhead.
+ */
+void
+fs_visitor::compact_virtual_grfs()
+{
+   /* Mark which virtual GRFs are used, and count how many. */
+   int remap_table[this->virtual_grf_count];
+   memset(remap_table, -1, sizeof(remap_table));
+
+   foreach_list(node, &this->instructions) {
+      const fs_inst *inst = (const fs_inst *) node;
+
+      if (inst->dst.file == GRF)
+         remap_table[inst->dst.reg] = 0;
+
+      for (int i = 0; i < 3; i++) {
+         if (inst->src[i].file == GRF)
+            remap_table[inst->src[i].reg] = 0;
+      }
+   }
+
+   /* Compact the GRF arrays. */
+   int new_index = 0;
+   for (int i = 0; i < this->virtual_grf_count; i++) {
+      if (remap_table[i] != -1) {
+         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];
+         }
+         ++new_index;
+      }
+   }
+
+   this->virtual_grf_count = new_index;
+
+   /* Patch all the instructions to use the newly renumbered registers */
+   foreach_list(node, &this->instructions) {
+      fs_inst *inst = (fs_inst *) node;
+
+      if (inst->dst.file == GRF)
+         inst->dst.reg = remap_table[inst->dst.reg];
+
+      for (int i = 0; i < 3; i++) {
+         if (inst->src[i].file == GRF)
+            inst->src[i].reg = remap_table[inst->src[i].reg];
+      }
+   }
+}
+
 bool
 fs_visitor::remove_dead_constants()
 {
@@ -1860,6 +1918,8 @@ fs_visitor::run()
       do {
 	 progress = false;
 
+         compact_virtual_grfs();
+
 	 progress = remove_duplicate_mrf_writes() || progress;
 
 	 progress = opt_algebraic() || progress;

src/mesa/drivers/dri/i965/brw_fs.h

@@ -240,6 +240,7 @@ public:
    int choose_spill_reg(struct ra_graph *g);
    void spill_reg(int spill_reg);
    void split_virtual_grfs();
+   void compact_virtual_grfs();
    void setup_pull_constants();
    void calculate_live_intervals();
    bool opt_algebraic();