[PATCH] varpool: Restore GENERIC TREE_READONLY automatic var optimization [PR7260]

[Jakub Jelinek]

Hi!

In 4.8 and earlier we used to fold the following to 0 during GENERIC folding,
but we don't do that anymore because ctor_for_folding etc. has been turned into a
GIMPLE centric API, but as the testcase shows, it is invoked even during
GENERIC folding and there the automatic vars still should have meaningful
initializers.  I've verified that the C++ FE drops TREE_READONLY on
automatic vars with const qualified types if they require non-constant
(runtime) initialization.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2021-01-26  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/97260
	* varpool.c (ctor_for_folding): In GENERIC return DECL_INITIAL for
	TREE_READONLY non-TREE_SIDE_EFFECTS automatic variables.

	* gcc.dg/tree-ssa/pr97260.c: New test.

--- gcc/varpool.c.jj	2021-01-26 08:57:36.184290279 +0100
+++ gcc/varpool.c	2021-01-26 09:46:16.453619140 +0100
@@ -412,6 +412,12 @@ ctor_for_folding (tree decl)
   if (!TREE_STATIC (decl) && !DECL_EXTERNAL (decl))
     {
       gcc_assert (!TREE_PUBLIC (decl));
+      /* Unless this is called during FE folding.  */
+      if (!in_gimple_form
+	  && TREE_READONLY (decl)
+	  && !TREE_SIDE_EFFECTS (decl)
+	  && DECL_INITIAL (decl))
+	return DECL_INITIAL (decl);
       return error_mark_node;
     }
 
--- gcc/testsuite/gcc.dg/tree-ssa/pr97260.c.jj	2021-01-26 09:45:42.933003954 +0100
+++ gcc/testsuite/gcc.dg/tree-ssa/pr97260.c	2021-01-26 09:45:17.277298476 +0100
@@ -0,0 +1,11 @@
+/* PR tree-optimization/97260 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+/* { dg-final { scan-tree-dump "return 0;" "optimized" } } */
+
+int
+foo (void)
+{
+  const char a[] = "1234";
+  return __builtin_memcmp (a, "1234", 4);
+}

	Jakub