Fix a regression in GCC 10/11/12 where some union structures
could get miscompiled when optimizations are enabled:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105860

Taken from upstream:

  https://gcc.gnu.org/g:16afe2e2862f3dd93c711d7f8d436dee23c6c34d

diff --git a/gcc/tree-sra.c b/gcc/tree-sra.c
index 09d951a261b..420329f63f6 100644
--- a/gcc/tree-sra.c
+++ b/gcc/tree-sra.c
@@ -1647,7 +1647,18 @@ build_ref_for_offset (location_t loc, tree base, poly_int64 offset,
 static tree
 build_reconstructed_reference (location_t, tree base, struct access *model)
 {
-  tree expr = model->expr, prev_expr = NULL;
+  tree expr = model->expr;
+  /* We have to make sure to start just below the outermost union.  */
+  tree start_expr = expr;
+  while (handled_component_p (expr))
+    {
+      if (TREE_CODE (TREE_TYPE (TREE_OPERAND (expr, 0))) == UNION_TYPE)
+	start_expr = expr;
+      expr = TREE_OPERAND (expr, 0);
+    }
+
+  expr = start_expr;
+  tree prev_expr = NULL_TREE;
   while (!types_compatible_p (TREE_TYPE (expr), TREE_TYPE (base)))
     {
       if (!handled_component_p (expr))
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<div class='path'>path: <a href='/guix/log/?id=87ac76d4ba8eb8318f287b83e90e516ddccaf709'>root</a>/<a href='/guix/log/tests?id=87ac76d4ba8eb8318f287b83e90e516ddccaf709'>tests</a>/<a href='/guix/log/tests/ed25519bis.sec?id=87ac76d4ba8eb8318f287b83e90e516ddccaf709'>ed25519bis.sec</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/ed25519bis.sec?id=87ac76d4ba8eb8318f287b83e90e516ddccaf709&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-06-05 22:54:06 +0200'>2020-06-05</span></td><td><a href='/guix/commit/tests/ed25519bis.sec?id=c83eedba2361c3831d3efb5ad63d5077d636fb28'>git-authenticate: Add tests.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/tests/git.scm (call-with-environment-variables)
(with-environment-variables): Remove.
* guix/tests/git.scm (populate-git-repository): Add clauses for signed
commits and signed merges.
* guix/tests/gnupg.scm: New file.
* tests/git-authenticate.scm: New file.
* tests/ed25519bis.key, tests/ed25519bis.sec: New files.
* Makefile.am (dist_noinst_DATA): Add 'guix/tests/gnupg.scm'.
(SCM_TESTS): Add 'tests/git-authenticate.scm'.
(EXTRA_DIST): Add tests/ed25519bis.{key,sec}.
</span></span></td><td>Ludovic Courtès</td></tr>