lto: Add an entry for cold attribute to lto_gnu_attributes

PR 118125 is a performance regression stemming from the fact that we
lose the cold attribute of our __builtin_unreachable.  The attribute
is simply and silently dropped on the floor by decl_attributes (in
attribs.cc) in the process of building decls for builtins because it
cannot look it up in the gnu attribute name space by
lookup_scoped_attribute_spec.  For that not to happen it must be in
lto_gnu_attributes and this patch adds it there.

In comment 13 of the bug Andrew identified other attributes which are
in builtin-attrs.def but missing in lto_gnu_attributes but apart from
cold it seems that they are either not used in builtins.def or are
used in DEF_LIB_BUILTIN which I guess might be less critical?
Eventually I decided to go for the most simple of patches and only add
things if they are requested.  For the same reason I also did not add
any checking to the attribute "handle" callback or any exclusion check.
They seem to be mostly relevant before LTO FE kicks in to me, but
again, I'm happy to add any if they seem to be useful.

Since Ian fixed PR 118746, the same issue has also been fixed in the
Go front-end and so I have added a simple checking assert to the
redirect_to_unreachable function to make sure it has the intended
effect.

gcc/ChangeLog:

2025-02-03  Martin Jambor  <mjambor@suse.cz>

	PR lto/118125
	* ipa-fnsummary.cc (redirect_to_unreachable): Add checking assert
	that the builtin_unreachable decl has attribute cold.

gcc/lto/ChangeLog:

2025-02-03  Martin Jambor  <mjambor@suse.cz>

	PR lto/118125
	* lto-lang.cc (lto_gnu_attributes): Add an entry for cold attribute.
	(handle_cold_attribute): New function.

Author: jamborm

gcc/ipa-fnsummary.cc

@@ -255,6 +255,9 @@ redirect_to_unreachable (struct cgraph_edge *e)
   struct cgraph_node *target
     = cgraph_node::get_create (builtin_decl_unreachable ());
 
+  gcc_checking_assert (lookup_attribute ("cold",
+					 DECL_ATTRIBUTES (target->decl)));
+
   if (e->speculative)
     e = cgraph_edge::resolve_speculation (e, target->decl);
   else if (!e->callee)

gcc/lto/lto-lang.cc

@@ -60,6 +60,7 @@ static tree ignore_attribute (tree *, tree, tree, int, bool *);
 static tree handle_format_attribute (tree *, tree, tree, int, bool *);
 static tree handle_fnspec_attribute (tree *, tree, tree, int, bool *);
 static tree handle_format_arg_attribute (tree *, tree, tree, int, bool *);
+static tree handle_cold_attribute (tree *, tree, tree, int, bool *);
 
 /* Helper to define attribute exclusions.  */
 #define ATTR_EXCL(name, function, type, variable)	\
@@ -128,6 +129,8 @@ static const attribute_spec lto_gnu_attributes[] =
 			      handle_sentinel_attribute, NULL },
   { "type generic",           0, 0, false, true, true, false,
 			      handle_type_generic_attribute, NULL },
+  { "cold",		      0, 0, false,  false, false, false,
+			      handle_cold_attribute, NULL },
   { "fn spec",	 	      1, 1, false, true, true, false,
 			      handle_fnspec_attribute, NULL },
   { "transaction_pure",	      0, 0, false, true, true, false,
@@ -598,6 +601,16 @@ handle_fnspec_attribute (tree *node ATTRIBUTE_UNUSED, tree ARG_UNUSED (name),
   return NULL_TREE;
 }
 
+/* Handle a "cold" attribute; arguments as in
+   struct attribute_spec.handler.  */
+
+static tree
+handle_cold_attribute (tree *, tree, tree, int, bool *)
+{
+  /* Nothing to be done here.  */
+  return NULL_TREE;
+}
+
 /* Cribbed from c-common.cc.  */
 
 static void