On 10/07/13 08:17, Marc Glisse wrote:
Hello,
this patch adds an attribute to let the compiler know that a function
never returns NULL. I saw some ECF_* flags, but the attribute seems
sufficient. I considered using nonnull(0), but then it would have been
confusing that the version of nonnull without arguments applies only to
parameters and not the return value.
2013-10-08 Marc Glisse <marc.gli...@inria.fr>
PR tree-optimization/20318
gcc/c-family/
* c-common.c (handle_returns_nonnull_attribute): New function.
(c_common_attribute_table): Add returns_nonnull.
gcc/
* doc/extend.texi (returns_nonnull): New function attribute.
* fold-const.c (tree_expr_nonzero_warnv_p): Look for returns_nonnull
attribute.
* tree-vrp.c (gimple_stmt_nonzero_warnv_p): Likewise.
(stmt_interesting_for_vrp): Accept all GIMPLE_CALL.
gcc/testsuite/
* c-c++-common/pr20318.c: New file.
* gcc.dg/tree-ssa/pr20318.c: New file.
--
Marc Glisse
p12
Index: c-family/c-common.c
===================================================================
--- c-family/c-common.c (revision 203241)
+++ c-family/c-common.c (working copy)
@@ -740,20 +741,22 @@ const struct attribute_spec c_common_att
{ "*tm regparm", 0, 0, false, true, true,
ignore_attribute, false },
{ "no_split_stack", 0, 0, true, false, false,
handle_no_split_stack_attribute, false },
/* For internal use (marking of builtins and runtime functions) only.
The name contains space to prevent its usage in source code. */
{ "fn spec", 1, 1, false, true, true,
handle_fnspec_attribute, false },
{ "warn_unused", 0, 0, false, false, false,
handle_warn_unused_attribute, false },
+ { "returns_nonnull", 0, 0, false, true, true,
+ handle_returns_nonnull_attribute, false },
{ NULL, 0, 0, false, false, false, NULL, false }
};
I'm going to assume this is correct -- it looks sane, but I've never
really done much with the attribute tables.
+
+/* Handle a "returns_nonnull" attribute; arguments as in
+ struct attribute_spec.handler. */
+
+static tree
+handle_returns_nonnull_attribute (tree *node, tree, tree, int,
+ bool *no_add_attrs)
+{
+ // Even without a prototype we still have a return type we can check.
+ if (TREE_CODE (TREE_TYPE (*node)) != POINTER_TYPE)
+ {
+ error ("returns_nonnull attribute on a function not returning a
pointer");
+ *no_add_attrs = true;
+ }
+ return NULL_TREE;
+}
Glad to see you checked this and have a test for it.
Not required for approval, but an "extra credit" -- a warning if a NULL
value flows into a return statement in a function with this marking.
Similarly, not required for approval, but it'd be real cool if we could
back-propagate the non-null return value attribute. ie, any value
flowing into the return statement of one of these functions can be
assumed to be non-zero, which may help eliminate more null pointer
checks in the decorated function. I guess ultimately we'd have to see
if noting this actually helps any real code.
Also not required for approval, but adding returns_nonnull markups to
appropriate functions in gcc itself.
Index: testsuite/gcc.dg/tree-ssa/pr20318.c
===================================================================
--- testsuite/gcc.dg/tree-ssa/pr20318.c (revision 0)
+++ testsuite/gcc.dg/tree-ssa/pr20318.c (working copy)
@@ -0,0 +1,19 @@
+/* { dg-do compile { target { ! keeps_null_pointer_checks } } } */
+/* { dg-options "-O2 -fdump-tree-original -fdump-tree-vrp1" } */
+
+extern int* f(int) __attribute__((returns_nonnull));
+extern void eliminate ();
+void g () {
+ if (f (2) == 0)
+ eliminate ();
+}
+void h () {
+ int *p = f (2);
+ if (p == 0)
+ eliminate ();
+}
+
+/* { dg-final { scan-tree-dump-times "== 0" 1 "original" } } */
+/* { dg-final { scan-tree-dump-times "Folding predicate\[^\\n\]*to 0" 1 "vrp1"
} } */
+/* { dg-final { cleanup-tree-dump "original" } } */
+/* { dg-final { cleanup-tree-dump "vrp1" } } */
Presumably g() is testing the fold-const.c and h() tests the tree-vrp
changes, right?
This is OK for the trunk, please install.
Jeff
