[Mesa-dev] [PATCH] glsl: fix loop analysis of loop terminators

[Timothy Arceri]

This code incorrectly assumed that loop terminators will always be
at the start of the loop. Fortunately we *seem* to avoid any bugs
because the unrolling code loops over and correctly handles the
terminators.

However the incorrect analysis can result in loops not being
unrolled at all. For example the current code would unroll:

  int j = 0;
  do {
     if (j > 5)
        break;

     ... do stuff ...

     j++;
  } while (j < 4);

But would fail to unroll the following as no iteration limit was
calculated because it failed to find the terminator:

  int j = 0;
  do {
     ... do stuff ...

     j++;
  } while (j < 4);

Also we would fail to unroll the following as we ended up
calculating the iteration limit as 6 rather than 4. The unroll
code then assumed we had 3 terminators rather the 2 as it
wasn't able to determine that "if (j > 5)" was redundant.

  int j = 0;
  do {
     if (j > 5)
        break;

     ... do stuff ...

     if (bool(i))
        break;

     j++;
  } while (j < 4);
---
 src/compiler/glsl/loop_analysis.cpp | 2 --
 1 file changed, 2 deletions(-)

diff --git a/src/compiler/glsl/loop_analysis.cpp b/src/compiler/glsl/loop_analysis.cpp
index b9bae43536..253a405dfb 100644
--- a/src/compiler/glsl/loop_analysis.cpp
+++ b/src/compiler/glsl/loop_analysis.cpp
@@ -290,22 +290,20 @@ loop_analysis::visit_leave(ir_loop *ir)
    foreach_in_list(ir_instruction, node, &ir->body_instructions) {
       /* Skip over declarations at the start of a loop.
        */
       if (node->as_variable())
 	 continue;
 
       ir_if *if_stmt = ((ir_instruction *) node)->as_if();
 
       if ((if_stmt != NULL) && is_loop_terminator(if_stmt))
 	 ls->insert(if_stmt);
-      else
-	 break;
    }
 
 
    foreach_in_list_safe(loop_variable, lv, &ls->variables) {
       /* Move variables that are already marked as being loop constant to
        * a separate list.  These trivially don't need to be tested.
        */
       if (lv->is_loop_constant()) {
 	 lv->remove();
 	 ls->constants.push_tail(lv);
-- 
2.13.5