[Mesa-dev] [PATCH 4/6] i965/fs: Simplify interference scan in register coalescing.

[Matt Turner]

We were starting at the beginning of the instruction list, rather than
with the MOV instruction itself. This allows us to coalesce after
control flow.

Excluding the shaders from an unreleased title, the shader-db results:

total instructions in shared programs: 1603791 -> 1594215 (-0.60%)
instructions in affected programs:     678772 -> 669196 (-1.41%)
GAINED:                                5
LOST:                                  0
---
 .../drivers/dri/i965/brw_fs_register_coalesce.cpp  | 22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp b/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp
index 5836fb7..58f6ca7 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp
@@ -73,7 +73,7 @@ is_coalesce_candidate(const fs_inst *inst, const int *virtual_grf_sizes)
 
 static bool
 can_coalesce_vars(brw::fs_live_variables *live_intervals,
-                  const exec_list *instructions, const fs_inst *inst,
+                  const exec_list *instructions, const fs_inst *inst, int ip,
                   int var_to, int var_from)
 {
    if (!live_intervals->vars_interfere(var_from, var_to))
@@ -92,21 +92,15 @@ can_coalesce_vars(brw::fs_live_variables *live_intervals,
    if (live_intervals->end[var_to] > live_intervals->end[var_from])
       return false;
 
-   int scan_ip = -1;
-
-   foreach_list(n, instructions) {
-      fs_inst *scan_inst = (fs_inst *)n;
-      scan_ip++;
+   assert(ip >= live_intervals->start[var_to]);
 
+   fs_inst *scan_inst;
+   for (scan_inst = (fs_inst *)inst->next;
+        scan_inst->next != NULL && ip <= live_intervals->end[var_to];
+        scan_inst = (fs_inst *)scan_inst->next, ip++) {
       if (scan_inst->is_control_flow())
          return false;
 
-      if (scan_ip <= live_intervals->start[var_to])
-         continue;
-
-      if (scan_ip > live_intervals->end[var_to])
-         break;
-
       if (scan_inst->dst.equals(inst->dst) ||
           scan_inst->dst.equals(inst->src[0]))
          return false;
@@ -129,9 +123,11 @@ fs_visitor::register_coalesce()
    fs_inst *mov[MAX_SAMPLER_MESSAGE_SIZE];
    int var_to[MAX_SAMPLER_MESSAGE_SIZE];
    int var_from[MAX_SAMPLER_MESSAGE_SIZE];
+   int ip = -1;
 
    foreach_list(node, &this->instructions) {
       fs_inst *inst = (fs_inst *)node;
+      ip++;
 
       if (!is_coalesce_candidate(inst, virtual_grf_sizes))
          continue;
@@ -170,7 +166,7 @@ fs_visitor::register_coalesce()
          var_to[i] = live_intervals->var_from_vgrf[reg_to] + reg_to_offset[i];
          var_from[i] = live_intervals->var_from_vgrf[reg_from] + i;
 
-         if (!can_coalesce_vars(live_intervals, &instructions, inst,
+         if (!can_coalesce_vars(live_intervals, &instructions, inst, ip,
                                 var_to[i], var_from[i])) {
             can_coalesce = false;
             reg_from = -1;
-- 
1.8.3.2