Clarify a comment.

[libfirm] / ir / be / ia32 / ia32_address_mode.c

diff --git a/ir/be/ia32/ia32_address_mode.c b/ir/be/ia32/ia32_address_mode.c
index 02aa05c..d6e4359 100644 (file)
--- a/ir/be/ia32/ia32_address_mode.c
+++ b/ir/be/ia32/ia32_address_mode.c
@@ -45,6 +45,7 @@
 /* gas/ld don't support negative symconsts :-( */
 #undef SUPPORT_NEGATIVE_SYMCONSTS
 
+static be_lv_t  *lv;
 static bitset_t *non_address_mode_nodes;
 
 /**
@@ -350,8 +351,9 @@ void ia32_create_address_mode(ia32_address_t *addr, ir_node *node, int force)
        }
 
        /* starting point Add, Sub or Shl, FrameAddr */
-       if(is_Shl(node)) { /* we don't want to eat add x, x as shl here, so only
-                             test for real Shl instructions */
+       if(is_Shl(node)) {
+               /* We don't want to eat add x, x as shl here, so only test for real Shl
+                * instructions, because we want the former as Lea x, x, not Shl x, 1 */
                if(eat_shl(addr, node))
                        return;
        } else if(is_immediate(addr, node, 0)) {
@@ -437,9 +439,11 @@ static void mark_non_address_nodes(ir_node *node, void *env)
        ir_node *ptr;
        ir_node *mem;
        ir_node *val;
+       ir_node *block;
        ir_node *left;
        ir_node *right;
        ir_mode *mode;
+       const ir_edge_t *edge;
        (void) env;
 
        mode = get_irn_mode(node);
@@ -463,23 +467,45 @@ static void mark_non_address_nodes(ir_node *node, void *env)
                bitset_set(non_address_mode_nodes, get_irn_idx(mem));
                break;
 
+       case iro_Shl:
        case iro_Add:
-               left  = get_Add_left(node);
-               right = get_Add_right(node);
-#if 0
-               /* if we can do source address mode then we will never fold the add
-                * into address mode */
-               if((is_immediate_simple(right)) ||
-                        (!ia32_use_source_address_mode(get_nodes_block(node), left, right)
-                    && !ia32_use_source_address_mode(get_nodes_block(node), right, left)))
-               {
-                   break;
+               /* only 1 user: AM folding is always beneficial */
+               if(get_irn_n_edges(node) <= 1)
+                       break;
+
+               /* for adds and shls with multiple users we use this heuristic:
+                * we do not fold them into address mode if their operands don't live
+                * out of the block, because in this case we will reduce register
+                * pressure. Otherwise we fold them in aggressively in the hope, that
+                * the node itself doesn't exist anymore and we were able to save the
+                * register for the result */
+               block = get_nodes_block(node);
+               left  = get_binop_left(node);
+               right = get_binop_right(node);
+
+               /* live end: we won't save a register by AM folding */
+               if(be_is_live_end(lv, block, left) || be_is_live_end(lv, block, right))
+                       return;
+
+               /* if multiple nodes in this block use left/right values, then we
+                * can't really decide wether the values will die after node.
+                * We use aggressive mode then, since it's usually just multiple address
+                * calculations. */
+               foreach_out_edge(left, edge) {
+                       ir_node *user = get_edge_src_irn(edge);
+                       if(user != node && get_nodes_block(user) == block)
+                               return;
+               }
+               foreach_out_edge(right, edge) {
+                       ir_node *user = get_edge_src_irn(edge);
+                       if(user != node && get_nodes_block(user) == block)
+                               return;
                }
+
+               /* noone-else in this block is using left/right so we'll reduce register
+                * pressure if we don't fold the node */
                bitset_set(non_address_mode_nodes, get_irn_idx(node));
-#else
                break;
-#endif
-               /* fallthrough */
 
        default:
                arity = get_irn_arity(node);
@@ -492,8 +518,11 @@ static void mark_non_address_nodes(ir_node *node, void *env)
        }
 }
 
-void ia32_calculate_non_address_mode_nodes(ir_graph *irg)
+void ia32_calculate_non_address_mode_nodes(be_irg_t *birg)
 {
+       ir_graph *irg = be_get_birg_irg(birg);
+
+       lv                     = be_assure_liveness(birg);
        non_address_mode_nodes = bitset_malloc(get_irg_last_idx(irg));
 
        irg_walk_graph(irg, NULL, mark_non_address_nodes, NULL);