core: add tree-node comments

As we've moved to 64-bit systems, the padding information has become
conditionally inaccurate.  I also hit cases where invalid tree codes
did not get flagged as invalid.

	gcc/
	* tree-core.h (tree_decl_with_vis, tree_function_decl):
	Note additional padding on 64-bits
	* tree.c (cache_integer_cst): Note why no caching of enum literals.
	(get_tree_code_name): Robustify error case.

2 files changed, 7 insertions, 1 deletions

diff --git a/gcc/tree-core.h b/gcc/tree-core.h
index 8c5a2e3..ba7f9ce 100644
--- a/gcc/tree-core.h
+++ b/gcc/tree-core.h
@@ -1826,6 +1826,7 @@ struct GTY(()) tree_decl_with_vis {
  /* Belong to FUNCTION_DECL exclusively.  */
  unsigned regdecl_flag : 1;
  /* 14 unused bits. */
+ /* 32 more unused on 64 bit HW. */
 };
 
 struct GTY(()) tree_var_decl {
@@ -1901,6 +1902,7 @@ struct GTY(()) tree_function_decl {
   unsigned replaceable_operator : 1;
 
   /* 11 bits left for future expansion.  */
+  /* 32 bits on 64-bit HW.  */
 };
 
 struct GTY(()) tree_translation_unit_decl {
diff --git a/gcc/tree.c b/gcc/tree.c
index 3d9968f..9102f8d 100644
--- a/gcc/tree.c
+++ b/gcc/tree.c
@@ -1771,6 +1771,8 @@ cache_integer_cst (tree t)
       break;
 
     case ENUMERAL_TYPE:
+      /* The slot used by TYPE_CACHED_VALUES is used for the enum
+	 members.  */
       break;
 
     default:
@@ -13254,7 +13256,9 @@ get_tree_code_name (enum tree_code code)
 {
   const char *invalid = "<invalid tree code>";
 
-  if (code >= MAX_TREE_CODES)
+  /* The tree_code enum promotes to signed, but we could be getting
+     invalid values, so force an unsigned comparison.  */
+  if (unsigned (code) >= MAX_TREE_CODES)
     {
       if (code == 0xa5a5)
 	return "ggc_freed";