tree-sra: Fix union handling in build_reconstructed_reference

As the testcase in PR 105860 shows, the code that tries to re-use the
handled_component chains in SRA can be horribly confused by unions,
where it thinks it has found a compatible structure under which it can
chain the references, but in fact it found the type it was looking
for elsewhere in a union and generated a write to a completely wrong
part of an aggregate.

I don't remember whether the plan was to support unions at all in
build_reconstructed_reference but it can work, to an extent, if we
make sure that we start the search only outside the outermost union,
which is what the patch does (and the extra testcase verifies).

Additionally, this commit also contains sqashed in it a backport of
b984b84cbe4bf026edef2ba37685f3958a1dc1cf which fixes the testcase
gcc.dg/tree-ssa/alias-access-path-13.c for many 32-bit targets.

gcc/ChangeLog:

2022-07-01  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/105860
	* tree-sra.cc (build_reconstructed_reference): Start expr
	traversal only just below the outermost union.

gcc/testsuite/ChangeLog:

2022-07-01  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/105860
	* gcc.dg/tree-ssa/alias-access-path-13.c: New test.
	* gcc.dg/tree-ssa/pr105860.c: Likewise.

(cherry picked from commit b110e5283e368b5377e04766e4ff82cd52634208)

2 files changed, 99 insertions, 0 deletions

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/alias-access-path-13.c b/gcc/testsuite/gcc.dg/tree-ssa/alias-access-path-13.c
new file mode 100644
index 00000000000..87a94f5bf31
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/alias-access-path-13.c
@@ -0,0 +1,36 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-fre1" } */
+
+struct inn
+{
+  int val;
+};
+
+struct biggerstruct
+{
+  int a, b;
+};
+
+union foo
+{
+  struct inn inn;
+  struct biggerstruct baz;
+} *fooptr;
+
+struct bar
+{
+  union foo foo;
+  int val2;
+} *barptr;
+
+int
+test ()
+{
+  union foo foo;
+  foo.inn.val = 0;
+  barptr->val2 = 123;
+  *fooptr = foo;
+  return barptr->val2;
+}
+
+/* { dg-final { scan-tree-dump-times "return 123" 1 "fre1"} } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr105860.c b/gcc/testsuite/gcc.dg/tree-ssa/pr105860.c
new file mode 100644
index 00000000000..77bcb4a6739
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr105860.c
@@ -0,0 +1,63 @@
+/* { dg-do run } */
+/* { dg-options "-O1" } */
+
+struct S1  {
+        unsigned int _0;
+        unsigned int _1;
+} ;
+struct S2  {
+        struct S1 _s1;
+        unsigned long _x2;
+} ;
+
+struct ufld_type1  {
+        unsigned int _u1t;
+        struct S2 _s2;
+} ;
+
+struct ufld_type2  {
+        unsigned int _u2t;
+        struct S1 _s1;
+} ;
+struct parm_type {
+        union {
+                struct ufld_type1 var_1;
+                struct ufld_type2 var_2;
+        } U;
+};
+
+struct parm_type  bad_function( struct parm_type arg0 )
+{
+        struct parm_type rv;
+        struct S2 var4;
+        switch( arg0.U.var_2._u2t ) {
+        case 4294967041:
+                var4._s1 = arg0.U.var_1._s2._s1;
+                rv.U.var_1._u1t = 4294967041;
+                rv.U.var_1._s2 = var4;
+                break;
+        case 4294967043:
+                rv.U.var_2._u2t = 4294967043;
+                rv.U.var_2._s1 = arg0.U.var_2._s1;
+                break;
+        default:
+                break;
+        }
+        return rv;
+}
+
+int main() {
+        struct parm_type val;
+        struct parm_type out;
+        val.U.var_2._u2t = 4294967043;
+        val.U.var_2._s1._0 = 0x01010101;
+        val.U.var_2._s1._1 = 0x02020202;
+        out = bad_function(val);
+	if (val.U.var_2._u2t != 4294967043)
+	  __builtin_abort ();
+        if (out.U.var_2._s1._0 != 0x01010101)
+	  __builtin_abort ();
+        if (val.U.var_2._s1._1 != 0x02020202 )
+	  __builtin_abort ();
+	return 0;
+}