* tree.h (phi_arg_d): Expand a comment in phi_arg_d.

From-SVN: r98813

gcc/ChangeLog

+2005-04-27  Kazu Hirata  <kazu@cs.umass.edu>
+
+	* tree.h (phi_arg_d): Expand a comment in phi_arg_d.
+
 2005-04-27  Joseph S. Myers  <joseph@codesourcery.com>
 
 	PR c/21213

gcc/tree.h

@@ -1424,7 +1424,9 @@ struct edge_def;
 
 struct phi_arg_d GTY(())
 {
-  struct ssa_imm_use_d imm_use;	/* imm_use MUST be first element in struct.  */
+  /* imm_use MUST be the first element in struct because we do some
+     pointer arithmetic with it.  See phi_arg_index_from_use.  */
+  struct ssa_imm_use_d imm_use;
   tree def;
   bool nonzero;
 };