[PATCH] ipa/105676 - pure attribute suggestion for const function

[Richard Biener <rguenther@suse.de>]

When a function is declared const (even though it technically
accesses memory), ipa-modref discovering pureness shouldn't end
up suggesting that attribute.  The following thus exempts
'const' functions from ipa_make_function_pure handling.

Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.

	PR ipa/105676
	* ipa-pure-const.cc (ipa_make_function_pure): Skip also
	for functions already being const.

	* gcc.dg/pr105676.c: New testcase.
---
 gcc/ipa-pure-const.cc           |  5 +++--
 gcc/testsuite/gcc.dg/pr105676.c | 14 ++++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/pr105676.c

diff --git a/gcc/ipa-pure-const.cc b/gcc/ipa-pure-const.cc
index 572a6da274f..0b748eee0ee 100644
--- a/gcc/ipa-pure-const.cc
+++ b/gcc/ipa-pure-const.cc
@@ -1526,8 +1526,9 @@ ipa_make_function_pure (struct cgraph_node *node, bool looping, bool local)
 {
   bool cdtor = false;
 
-  if (DECL_PURE_P (node->decl)
-      && (looping || !DECL_LOOPING_CONST_OR_PURE_P (node->decl)))
+  if (TREE_READONLY (node->decl)
+      || (DECL_PURE_P (node->decl)
+	  && (looping || !DECL_LOOPING_CONST_OR_PURE_P (node->decl))))
     return false;
   warn_function_pure (node->decl, !looping);
   if (local && skip_function_for_local_pure_const (node))
diff --git a/gcc/testsuite/gcc.dg/pr105676.c b/gcc/testsuite/gcc.dg/pr105676.c
new file mode 100644
index 00000000000..077fc18a17f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr105676.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -Wsuggest-attribute=pure" } */
+
+__attribute__((const))
+extern int do_expensive_calculation(void);
+
+__attribute__((const))
+int getval(void) /* { dg-bogus "candidate for attribute" } */
+{
+  static int cache = -1;
+  if (cache == -1)
+    cache = do_expensive_calculation();
+  return cache;
+}
-- 
2.35.3