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))
ix/refs/?id=92fba6c72acb2dd5f67e5bb7b0fc6d70e2a4a9ee'>refs</a><a class='active' href='/guix/log/gnu/system/mapped-devices.scm'>log</a><a href='/guix/tree/gnu/system/mapped-devices.scm?id=92fba6c72acb2dd5f67e5bb7b0fc6d70e2a4a9ee'>tree</a><a href='/guix/commit/gnu/system/mapped-devices.scm?id=92fba6c72acb2dd5f67e5bb7b0fc6d70e2a4a9ee'>commit</a><a href='/guix/diff/gnu/system/mapped-devices.scm?id=92fba6c72acb2dd5f67e5bb7b0fc6d70e2a4a9ee'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/system/mapped-devices.scm'>
<input type='hidden' name='id' value='92fba6c72acb2dd5f67e5bb7b0fc6d70e2a4a9ee'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=92fba6c72acb2dd5f67e5bb7b0fc6d70e2a4a9ee'>root</a>/<a href='/guix/log/gnu?id=92fba6c72acb2dd5f67e5bb7b0fc6d70e2a4a9ee'>gnu</a>/<a href='/guix/log/gnu/system?id=92fba6c72acb2dd5f67e5bb7b0fc6d70e2a4a9ee'>system</a>/<a href='/guix/log/gnu/system/mapped-devices.scm?id=92fba6c72acb2dd5f67e5bb7b0fc6d70e2a4a9ee'>mapped-devices.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/system/mapped-devices.scm?id=92fba6c72acb2dd5f67e5bb7b0fc6d70e2a4a9ee&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2024-05-27 19:30:22 +0200'>2024-05-27</span></td><td><a href='/guix/commit/gnu/system/mapped-devices.scm?id=ed5bf620ea6366d8468e5b7dad9874c6e0e8c661'>mapped-devices: lvm: Add missing module import.</a><span class='msg-avail'>...<span class='msg-tooltip'>Tested with (locally fixed) lvm-separate-home-os system test.

* gnu/system/mapped-devices.scm (lvm-device-mapping): Add SRFI-1 to
modules.

Change-Id: I8c155b47c29004bdc59057391dfba94ce33bdbc2
</span></span></td><td>Lars-Dominik Braun</td></tr>
<tr><td><span title='2024-04-08 00:17:57 +0200'>2024-04-08</span></td><td><a href='/guix/commit/gnu/system/mapped-devices.scm?id=49f82fca4130ffcfb16aa0cf89750ab56fb99ad7'>mapped-devices: luks: Specify modules needed at the top-level.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://issues.guix.gnu.org/70266&gt;.

* gnu/system/mapped-devices.scm (luks-device-mapping)[modules]: New
field.
(open-luks-device): Remove non-top-level ‘use-modules’ form.
* gnu/system/linux-initrd.scm (raw-initrd): Remove modules that were
added specifically for ‘luks-device-mapping’.

Change-Id: I4253c3dd5e3cbcee41ec84fd57227abd428d1bd6
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2024-04-08 00:13:25 +0200'>2024-04-08</span></td><td><a href='/guix/commit/gnu/system/mapped-devices.scm?id=6062339156ecc42cac59af0d34e22fcc7da3aa4a'>mapped-devices: &lt;mapped-device-type&gt; can specify modules to import.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/system/mapped-devices.scm (&lt;mapped-device-type&gt;)[modules]: New
field.
(device-mapping-service-type): Honor it.
* gnu/system/linux-initrd.scm (raw-initrd): Likewise.

Change-Id: Icc702cb6f281741975e33203f87fbc1ffa9856da
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2024-01-14 23:00:03 +0100'>2024-01-14</span></td><td><a href='/guix/commit/gnu/system/mapped-devices.scm?id=d082312ef7adfea69c79d30ef947817b39832161'>mapped-devices: Allow unlocking by a key file.</a><span class='msg-avail'>...<span class='msg-tooltip'>Requiring the user to input their password in order to unlock a device is not
always reasonable, so having an option to unlock the device using a key file
is a nice quality of life change.

* gnu/system/mapped-devices.scm (open-luks-device): Add #:key-file argument.
(luks-device-mapping-with-options): New procedure.
* doc/guix.texi (Mapped Devices): Describe the new procedure.

Change-Id: I1de4e045f8c2c11f9a94f1656e839c785b0c11c4
Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Tomas Volf</td></tr>