[Mesa-dev] [PATCH 17/20] i965: Preserve CFG when deleting dead control flow.

[Matt Turner]

This pass deletes an IF/ELSE/ENDIF or IF/ENDIF sequence, or the ELSE in
an ELSE/ENDIF sequence.

In the typical case (where IF and ENDIF) aren't the only instructions in
their basic blocks, we can simply remove the instructions (implicitly
deleting the block containing only the ELSE), and attempt to merge
blocks B0 and B2 together.

B0: ...
    (+f0) if(8)
B1: else(8)
B2: endif(8)
    ...

If the IF or ENDIF instructions are the only instructions in their
respective basic blocks (which are deleted by the removal of the
instructions), we'll want to instead merge the next blocks.

Both B0 and B2 are possibly removed by the removal of if & endif.
Same situation for if/endif. E.g., in the following example we'd remove
blocks B1 and B2, and then attempt to combine B0 and B3.

B0: ...
B1: (+f0) if(8)
B2: endif(8)
B3: ...
---
 .../drivers/dri/i965/brw_dead_control_flow.cpp     | 44 ++++++++++++++++++----
 1 file changed, 36 insertions(+), 8 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp b/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp
index e6ace5c..56dc79e 100644
--- a/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp
+++ b/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp
@@ -42,7 +42,8 @@ dead_control_flow_eliminate(backend_visitor *v)
 
    v->calculate_cfg();
 
-   foreach_block (block, v->cfg) {
+   foreach_block_safe (block, v->cfg) {
+      bblock_t *if_block = NULL, *else_block = NULL, *endif_block = block;
       bool found = false;
 
       /* ENDIF instructions, by definition, can only be found at the start of
@@ -56,6 +57,7 @@ dead_control_flow_eliminate(backend_visitor *v)
       backend_instruction *prev_inst = (backend_instruction *) endif_inst->prev;
       if (prev_inst->opcode == BRW_OPCODE_ELSE) {
          else_inst = prev_inst;
+         else_block = (bblock_t *)endif_block->link.prev;
          found = true;
 
          prev_inst = (backend_instruction *) prev_inst->prev;
@@ -63,6 +65,8 @@ dead_control_flow_eliminate(backend_visitor *v)
 
       if (prev_inst->opcode == BRW_OPCODE_IF) {
          if_inst = prev_inst;
+         if_block = else_block != NULL ? (bblock_t *)else_block->link.prev
+                                       : (bblock_t *)endif_block->link.prev;
          found = true;
       } else {
          /* Don't remove the ENDIF if we didn't find a dead IF. */
@@ -70,18 +74,42 @@ dead_control_flow_eliminate(backend_visitor *v)
       }
 
       if (found) {
-         if (if_inst)
-            if_inst->remove();
-         if (else_inst)
-            else_inst->remove();
-         if (endif_inst)
-            endif_inst->remove();
+         bblock_t *earlier_block = NULL, *later_block = NULL;
+
+         if (if_inst) {
+            if (if_block->start_ip == if_block->end_ip) {
+               earlier_block = (bblock_t *)if_block->link.prev;
+            } else {
+               earlier_block = if_block;
+            }
+            if_inst->remove(if_block);
+         }
+
+         if (else_inst) {
+            else_block->if_block->else_block = NULL;
+            else_inst->remove(else_block);
+         }
+
+         if (endif_inst) {
+            if (endif_block->start_ip == endif_block->end_ip) {
+               later_block = (bblock_t *)endif_block->link.next;
+            } else {
+               later_block = endif_block;
+            }
+            endif_inst->remove(endif_block);
+         }
+
+         assert((earlier_block == NULL) == (later_block == NULL));
+         if (earlier_block && earlier_block->can_combine_with(later_block)) {
+            earlier_block->combine_with(later_block);
+         }
+
          progress = true;
       }
    }
 
    if (progress)
-      v->invalidate_live_intervals();
+      v->invalidate_live_intervals(false);
 
    return progress;
 }
-- 
1.8.5.5