glsl2: Fix linear portion of index search for writes

If the array access is a read, read the first element of this
subregion unconditionally.  The remaining tests will possibly
overwrite this value with one of the other array elements.

This optimization cannot be done for writes because it will cause the
first element of the subregion to be written possibly *in addition* to
one of the other elements.

1 files changed, 15 insertions, 5 deletions

diff --git a/src/glsl/lower_variable_index_to_cond_assign.cpp b/src/glsl/lower_variable_index_to_cond_assign.cpp
index b313e9b460..cad43b427b 100644
--- a/src/glsl/lower_variable_index_to_cond_assign.cpp
+++ b/src/glsl/lower_variable_index_to_cond_assign.cpp
@@ -94,13 +94,23 @@ struct switch_generator
       if (begin == end)
          return;
 
-      /* do the first one unconditionally
-       * FINISHME: may not want this in some cases
+      /* If the array access is a read, read the first element of this subregion
+       * unconditionally.  The remaining tests will possibly overwrite this
+       * value with one of the other array elements.
+       *
+       * This optimization cannot be done for writes because it will cause the
+       * first element of the subregion to be written possibly *in addition* to
+       * one of the other elements.
        */
+      unsigned first;
+      if (!this->generator.is_write) {
+	 this->generator.generate(begin, 0, list);
+	 first = begin + 1;
+      } else {
+	 first = begin;
+      }
 
-      this->generator.generate(begin, 0, list);
-
-      for (unsigned i = begin + 1; i < end; i += 4) {
+      for (unsigned i = first; i < end; i += 4) {
          const unsigned comps = MIN2(condition_components, end - i);
 
          ir_rvalue *broadcast_index =