[Mesa-dev] [PATCH] ra: Disable round-robin strategy for optimistically colorable nodes.

[Francisco Jerez]

The round-robin allocation strategy is expected to decrease the amount
of false dependencies created by the register allocator and give the
post-RA scheduling pass more freedom to move instructions around.  On
the other hand it has the disadvantage of increasing fragmentation and
decreasing the number of equally-colored nearby nodes, what increases
the likelihood of failure in presence of optimistically colorable
nodes.

This patch disables the round-robin strategy for optimistically
colorable nodes.  These typically arise in situations of high register
pressure or for registers with large live intervals, in both cases the
task of the instruction scheduler shouldn't be constrained excessively
by the dense packing of those nodes, and a spill (or on Intel hardware
a fall-back to SIMD8 mode) is invariably worse than a slightly less
optimal scheduling.

Shader-db results on the i965 driver:

total instructions in shared programs: 5488539 -> 5488489 (-0.00%)
instructions in affected programs:     1121 -> 1071 (-4.46%)
helped:                                1
HURT:                                  0
GAINED:                                49
LOST:                                  5
---
 src/util/register_allocate.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/src/util/register_allocate.c b/src/util/register_allocate.c
index af7a20c..d63d8eb 100644
--- a/src/util/register_allocate.c
+++ b/src/util/register_allocate.c
@@ -168,6 +168,12 @@ struct ra_graph {
 
    unsigned int *stack;
    unsigned int stack_count;
+
+   /**
+    * Tracks the start of the set of optimistically-colored registers in the
+    * stack.
+    */
+   unsigned int stack_optimistic_start;
 };
 
 /**
@@ -454,6 +460,7 @@ static void
 ra_simplify(struct ra_graph *g)
 {
    bool progress = true;
+   unsigned int stack_optimistic_start = ~0;
    int i;
 
    while (progress) {
@@ -483,12 +490,16 @@ ra_simplify(struct ra_graph *g)
 
       if (!progress && best_optimistic_node != ~0U) {
 	 decrement_q(g, best_optimistic_node);
+         stack_optimistic_start =
+            MIN2(stack_optimistic_start, g->stack_count);
 	 g->stack[g->stack_count] = best_optimistic_node;
 	 g->stack_count++;
 	 g->nodes[best_optimistic_node].in_stack = true;
 	 progress = true;
       }
    }
+
+   g->stack_optimistic_start = stack_optimistic_start;
 }
 
 /**
@@ -542,7 +553,16 @@ ra_select(struct ra_graph *g)
       g->nodes[n].reg = r;
       g->stack_count--;
 
-      if (g->regs->round_robin)
+      /* Rotate the starting point except for optimistically colorable nodes.
+       * The likelihood that we will succeed at allocating optimistically
+       * colorable nodes is highly dependent on the way that the previous
+       * nodes popped off the stack are laid out.  The round-robin strategy
+       * increases the fragmentation of the register file and decreases the
+       * number of nearby nodes assigned to the same color, what increases the
+       * likelihood of spilling with respect to the dense packing strategy.
+       */
+      if (g->regs->round_robin &&
+          g->stack_count <= g->stack_optimistic_start)
          start_search_reg = r + 1;
    }
 
-- 
2.1.3