gimplify: Clear TREE_READONLY on automatic vars being stored into [PR104529]

The following testcase regressed when SRA started punting on stores to
TREE_READONLY vars.  We document that:
"In a VAR_DECL, PARM_DECL or FIELD_DECL, or any kind of ..._REF node,
nonzero means it may not be the lhs of an assignment."
so the SRA change looks desirable.  On the other side, at least in this
testcase the TREE_READONLY is set there intentionally from the
PR85873 fix, because gimplify_init_constructor itself uses TREE_READONLY
on the object to determine if it can perform promotion to static const
or not.

So, similarly to other spots in the gimplifier where we also clear
TREE_READONLY when we emit IL that stores into the object, this
does the same in gimplify_init_constructor, but in the way so that
the TREE_READONLY test for the promotion to static const keeps working
and doesn't change anything for notify_temp_creation mode, which doesn't
emit any IL, just tests if it would need a temporary or not.

This keeps PR85873 testcase working as before and fixes this regression.

2022-03-04  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/104529
	* gimplify.cc (gimplify_init_constructor): Clear TREE_READONLY
	on automatic objects which will be runtime initialized.

	* g++.dg/tree-ssa/pr104529.C: New test.

2 files changed, 31 insertions, 0 deletions

diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
index be8f0d3..c6ba0c3 100644
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -5120,6 +5120,12 @@ gimplify_init_constructor (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
 	  {
 	    if (notify_temp_creation)
 	      return GS_OK;
+
+	    /* The var will be initialized and so appear on lhs of
+	       assignment, it can't be TREE_READONLY anymore.  */
+	    if (VAR_P (object))
+	      TREE_READONLY (object) = 0;
+
 	    is_empty_ctor = true;
 	    break;
 	  }
@@ -5171,6 +5177,11 @@ gimplify_init_constructor (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
 	    break;
 	  }
 
+	/* The var will be initialized and so appear on lhs of
+	   assignment, it can't be TREE_READONLY anymore.  */
+	if (VAR_P (object) && !notify_temp_creation)
+	  TREE_READONLY (object) = 0;
+
 	/* If there are "lots" of initialized elements, even discounting
 	   those that are not address constants (and thus *must* be
 	   computed at runtime), then partition the constructor into
diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr104529.C b/gcc/testsuite/g++.dg/tree-ssa/pr104529.C
new file mode 100644
index 0000000..3cb3853
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/pr104529.C
@@ -0,0 +1,20 @@
+// PR middle-end/104529
+// { dg-do compile { target c++11 } }
+// { dg-options "-O2 -fdump-tree-optimized" }
+// { dg-final { scan-tree-dump-not "MEM\[^\n\r]*MEM" "optimized" } }
+
+#include <cstddef>
+#include <vector>
+
+struct S {
+  unsigned int a;
+  std::vector<unsigned char> b;
+  std::vector<unsigned char> c;
+};
+
+std::size_t
+foo ()
+{
+  S test[] = { { 48, { 255, 0, 0, 0, 0, 0 } } };
+  return sizeof (test);
+}