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=36becc57d265b1010375acd784badecd7fe48745'>root</a>/<a href='/guix/log/gnu?id=36becc57d265b1010375acd784badecd7fe48745'>gnu</a>/<a href='/guix/log/gnu/services?id=36becc57d265b1010375acd784badecd7fe48745'>services</a>/<a href='/guix/log/gnu/services/authentication.scm?id=36becc57d265b1010375acd784badecd7fe48745'>authentication.scm</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/gnu/services/authentication.scm?id=36becc57d265b1010375acd784badecd7fe48745&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-03-10 18:01:47 +0100'>2021-03-10</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=520bac7ed00a949a0391ad680de65a1498105c2b'>services: Prevent following symlinks during activation.</a><span class='msg-avail'>...<span class='msg-tooltip'>This addresses a potential security issue, where a compromised
service could trick the activation code in changing the permissions,
owner and group of arbitrary files.  However, this patch is
currently only a partial fix, due to a TOCTTOU (time-of-check to
time-of-use) race, which can be fixed once guile has bindings
to openat and friends.

Fixes: &lt;https://lists.gnu.org/archive/html/guix-devel/2021-01/msg00388.html&gt;

* gnu/build/activation.scm: new procedure 'mkdir-p/perms'.
* gnu/services/authentication.scm
  (%nslcd-activation, nslcd-service-type): use new procedure.
* gnu/services/cups.scm (%cups-activation): likewise.
* gnu/services/dbus.scm (dbus-activation): likewise.
* gnu/services/dns.scm (knot-activation): likewise.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Maxime Devos</td></tr>
<tr><td><span title='2019-04-24 22:21:35 +0200'>2019-04-24</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=9374cbd1fb29395a5e849e95e8a249e5f9f944c0'>services: fprintd: Provide polkit policy.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/authentication.scm (fprintd-service-type)[extensions]: Add
polkit-service-type.
</span></span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2019-04-21 10:40:19 +0200'>2019-04-21</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=dc01978288318a8155e2e2657be9b278a754ebac'>services: fprintd: Use define-configuration.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/authentication.scm: Use define-configuration in fprintd.
</span></span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2019-04-17 08:51:11 +0200'>2019-04-17</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=0682f084635cdc289aabafc4b2583639e00a28b3'>services: fprintd: Fix service.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/authentication.scm (&lt;fprintd-configuration&gt;)[ntp]: Rename to...
[fprintd]: ...this.
(fprintd-dbus-service): New procedure.
(fprintd-service-type): Use it.
[default-value]: Add value.
</span></span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2019-03-20 20:31:15 +0100'>2019-03-20</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=c16423f143919916a5273761d7ed29bd49f14519'>services: Add nslcd-service-type.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/authentication.scm (nslcd-service-type, nslcd-configuration,
%nslcd-accounts): New variables.
(uglify-field-name, value-&gt;string, serialize-field, serialize-list,
ssl-option?, tls-reqcert-option?, deref-option?,
comma-separated-list-of-strings?, serialize-ignore-users-option, log-option?,
serialize-log-option, valid-map?, scope-option?, serialize-scope-option,
map-entry?, list-of-map-entries?, filter-entry?, list-of-filter-entries?,
serialize-filter-entry, serialize-list-of-filter-entries, serialize-map-entry,
serialize-list-of-map-entries, nslcd-config-file, nslcd-etc-service,
nslcd-shepherd-service, pam-ldap-pam-services, pam-ldap-pam-service,
generate-nslcd-documentation): New procedures.
* gnu/tests/ldap.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* doc/guix.texi (LDAP Services): Document it.
</span></span></td><td>Ricardo Wurmus</td></tr>