[Mesa-dev] [PATCH 02/10] glsl: do second pass of const propagation in loops

[Caio Marcelo de Oliveira Filho]

When handling loops in constant propagation, implement the "FINISHME"
comment like copy propagation: perform a first pass to find values
that can't be propagated, then perform a second pass with the ACP
containing still valid values.

Certain values are killed because the loop may run more than one
iteration, so we can't copy propagate them as they would be invalid in
the later iterations.
---
 .../glsl/opt_constant_propagation.cpp         | 29 +++++++++++++++----
 1 file changed, 23 insertions(+), 6 deletions(-)

diff --git a/src/compiler/glsl/opt_constant_propagation.cpp b/src/compiler/glsl/opt_constant_propagation.cpp
index 25db3622aba..1b29fa6e967 100644
--- a/src/compiler/glsl/opt_constant_propagation.cpp
+++ b/src/compiler/glsl/opt_constant_propagation.cpp
@@ -123,6 +123,7 @@ public:
    void constant_propagation(ir_rvalue **rvalue);
    void kill(ir_variable *ir, unsigned write_mask);
    void handle_if_block(exec_list *instructions, hash_table *kills, bool *killed_all);
+   void handle_loop(class ir_loop *, bool keep_acp);
    void handle_rvalue(ir_rvalue **rvalue);
 
    /** List of acp_entry: The available constants to propagate */
@@ -410,22 +411,24 @@ ir_constant_propagation_visitor::visit_enter(ir_if *ir)
    return visit_continue_with_parent;
 }
 
-ir_visitor_status
-ir_constant_propagation_visitor::visit_enter(ir_loop *ir)
+void
+ir_constant_propagation_visitor::handle_loop(ir_loop *ir, bool keep_acp)
 {
    exec_list *orig_acp = this->acp;
    hash_table *orig_kills = this->kills;
    bool orig_killed_all = this->killed_all;
 
-   /* FINISHME: For now, the initial acp for loops is totally empty.
-    * We could go through once, then go through again with the acp
-    * cloned minus the killed entries after the first run through.
-    */
    this->acp = new(mem_ctx) exec_list;
    this->kills = _mesa_hash_table_create(mem_ctx, _mesa_hash_pointer,
                                          _mesa_key_pointer_equal);
    this->killed_all = false;
 
+   if (keep_acp) {
+      foreach_in_list(acp_entry, a, orig_acp) {
+         this->acp->push_tail(new(this->lin_ctx) acp_entry(a));
+      }
+   }
+
    visit_list_elements(this, &ir->body_instructions);
 
    if (this->killed_all) {
@@ -442,6 +445,20 @@ ir_constant_propagation_visitor::visit_enter(ir_loop *ir)
       kill_entry *k = (kill_entry *) htk->data;
       kill(k->var, k->write_mask);
    }
+}
+
+ir_visitor_status
+ir_constant_propagation_visitor::visit_enter(ir_loop *ir)
+{
+   /* Make a conservative first pass over the loop with an empty ACP set.
+    * This also removes any killed entries from the original ACP set.
+    */
+   handle_loop(ir, false);
+
+   /* Then, run it again with the real ACP set, minus any killed entries.
+    * This takes care of propagating values from before the loop into it.
+    */
+   handle_loop(ir, true);
 
    /* already descended into the children. */
    return visit_continue_with_parent;
-- 
2.17.1