Mesa (master): i965/vec4_live_variables: Do liveness analysis bottom-to-top

[Jason Ekstrand]

Module: Mesa
Branch: master
Commit: 316206ee9ea06419c9a2ea6fe48d66a0b805319d
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=316206ee9ea06419c9a2ea6fe48d66a0b805319d

Author: Jason Ekstrand <jason.ekstrand at intel.com>
Date:   Thu Jun 25 08:08:27 2015 -0700

i965/vec4_live_variables: Do liveness analysis bottom-to-top

>From Muchnick's Advanced Compiler Design and Implementation:

"To determine which variables are live at each point in a flowgraph, we
perform a backward data-flow analysis"

Previously, we were walking the blocks forwards and updating the livein and
then the liveout.  However, the livein calculation depends on the liveout
and the liveout depends on the successor blocks.  The net result is that it
takes one full iteration to go from liveout to livein and then another
full iteration to propagate to the predecessors.  This works out to an
O(n^2) computation where n is the number of blocks.  If we run things in
the other order, it's O(nl) where l is the maximum loop depth which is
practically bounded by 3.

In b2c6ba0c4b21391dc35018e1c8c4f7f7d8952bea, we made this same change in
the FS backend to great effect.  Might as well keep it consistent and make
the same change for vec4.  Also, this took the time to run the test:

ES31-CTS.arrays_of_arrays.InteractionFunctionCalls1

from 6:49.62 to 3:31.40 on Timothy Arceri's machine.

Reviewed-by: Matt Turner <mattst88 at gmail.com>

---

 .../drivers/dri/i965/brw_vec4_live_variables.cpp   |   38 ++++++++++----------
 1 file changed, 19 insertions(+), 19 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp b/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp
index 95b9d90..29b4a53 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp
@@ -133,27 +133,9 @@ vec4_live_variables::compute_live_variables()
    while (cont) {
       cont = false;
 
-      foreach_block (block, cfg) {
+      foreach_block_reverse (block, cfg) {
          struct block_data *bd = &block_data[block->num];
 
-	 /* Update livein */
-	 for (int i = 0; i < bitset_words; i++) {
-            BITSET_WORD new_livein = (bd->use[i] |
-                                      (bd->liveout[i] &
-                                       ~bd->def[i]));
-            if (new_livein & ~bd->livein[i]) {
-               bd->livein[i] |= new_livein;
-               cont = true;
-	    }
-	 }
-         BITSET_WORD new_livein = (bd->flag_use[0] |
-                                   (bd->flag_liveout[0] &
-                                    ~bd->flag_def[0]));
-         if (new_livein & ~bd->flag_livein[0]) {
-            bd->flag_livein[0] |= new_livein;
-            cont = true;
-         }
-
 	 /* Update liveout */
 	 foreach_list_typed(bblock_link, child_link, link, &block->children) {
             struct block_data *child_bd = &block_data[child_link->block->num];
@@ -173,6 +155,24 @@ vec4_live_variables::compute_live_variables()
                cont = true;
             }
 	 }
+
+         /* Update livein */
+         for (int i = 0; i < bitset_words; i++) {
+            BITSET_WORD new_livein = (bd->use[i] |
+                                      (bd->liveout[i] &
+                                       ~bd->def[i]));
+            if (new_livein & ~bd->livein[i]) {
+               bd->livein[i] |= new_livein;
+               cont = true;
+            }
+         }
+         BITSET_WORD new_livein = (bd->flag_use[0] |
+                                   (bd->flag_liveout[0] &
+                                    ~bd->flag_def[0]));
+         if (new_livein & ~bd->flag_livein[0]) {
+            bd->flag_livein[0] |= new_livein;
+            cont = true;
+         }
       }
    }
 }