sra: Fix bug in grp_write propagation (PR 97009)

SRA represents parts of aggregates which are arrays accessed with
unknown index as "unscalarizable regions."  When there are two such
regions one within another and the outer is only read whereas the
inner is written to, SRA fails to propagate that write information
across assignments.  This means that a second aggregate can contain
data while SRA thinks it does not and the pass can wrongly eliminate
big chunks of assignment from that second aggregate into a third
aggregate, which is what happens in PR 97009.

Fixed by checking all children of unscalariable accesses for the
grp_write flag.

gcc/ChangeLog:

2021-03-31  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/97009
	* tree-sra.c (access_or_its_child_written): New function.
	(propagate_subaccesses_from_rhs): Use it instead of a simple grp_write
	test.

gcc/testsuite/ChangeLog:

2021-03-31  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/97009
	* gcc.dg/tree-ssa/pr97009.c: New test.

2 files changed, 80 insertions, 1 deletions

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr97009.c b/gcc/testsuite/gcc.dg/tree-ssa/pr97009.c
new file mode 100644
index 0000000..741dbc2
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr97009.c
@@ -0,0 +1,66 @@
+/* { dg-do run } */
+/* { dg-options "-O1" } */
+
+static int __attribute__((noipa))
+get_5 (void)
+{
+  return 5;
+}
+
+static int __attribute__((noipa))
+verify_5 (int v)
+{
+  if (v != 5)
+    __builtin_abort ();
+}
+
+struct T
+{
+  int w;
+  int a[4];
+};
+
+struct S
+{
+  int v;
+  int x;
+  struct T t[2];
+  char alotofstuff[128];
+};
+
+volatile int vol;
+
+void __attribute__((noipa))
+consume_t (struct T t)
+{
+  vol = t.a[0];
+}
+
+int __attribute__((noipa))
+foo (int l1, int l2)
+{
+  struct S s1, s2, s3;
+  int i, j;
+
+  s1.v = get_5 ();
+  for (i = 0; i < l1; i++)
+    {
+      for (j = 0; j < l2; j++)
+	s1.t[i].a[j] = get_5 ();
+      consume_t(s1.t[i]);
+    }
+
+  s2 = s1;
+
+  s3 = s2;
+  for (i = 0; i < l1; i++)
+    for (j = 0; j < l2; j++)
+      verify_5 (s3.t[i].a[j]);
+}
+
+int
+main (int argc, char *argv[])
+{
+  foo (2, 4);
+  return 0;
+}
diff --git a/gcc/tree-sra.c b/gcc/tree-sra.c
index d177f1b..8dfc923 100644
--- a/gcc/tree-sra.c
+++ b/gcc/tree-sra.c
@@ -2723,6 +2723,19 @@ budget_for_propagation_access (tree decl)
   return true;
 }
 
+/* Return true if ACC or any of its subaccesses has grp_child set.  */
+
+static bool
+access_or_its_child_written (struct access *acc)
+{
+  if (acc->grp_write)
+    return true;
+  for (struct access *sub = acc->first_child; sub; sub = sub->next_sibling)
+    if (access_or_its_child_written (sub))
+      return true;
+  return false;
+}
+
 /* Propagate subaccesses and grp_write flags of RACC across an assignment link
    to LACC.  Enqueue sub-accesses as necessary so that the write flag is
    propagated transitively.  Return true if anything changed.  Additionally, if
@@ -2836,7 +2849,7 @@ propagate_subaccesses_from_rhs (struct access *lacc, struct access *racc)
       if (rchild->grp_unscalarizable_region
 	  || !budget_for_propagation_access (lacc->base))
 	{
-	  if (rchild->grp_write && !lacc->grp_write)
+	  if (!lacc->grp_write && access_or_its_child_written (rchild))
 	    {
 	      ret = true;
 	      subtree_mark_written_and_rhs_enqueue (lacc);