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=6e20d190dccccbce33750cbdbe4a95f95bf8d87f'>root</a>/<a href='/guix/log/tests?id=6e20d190dccccbce33750cbdbe4a95f95bf8d87f'>tests</a>/<a href='/guix/log/tests/nar.scm?id=6e20d190dccccbce33750cbdbe4a95f95bf8d87f'>nar.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/tests/nar.scm?id=6e20d190dccccbce33750cbdbe4a95f95bf8d87f&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-12-15 17:32:09 +0100'>2020-12-15</span></td><td><a href='/guix/commit/tests/nar.scm?id=2718c29c3fb9f9de2ec897248ad49ae11ca39b7a'>nar: Deduplicate files right as they are restored.</a><span class='msg-avail'>...<span class='msg-tooltip'>This avoids having to traverse and re-read the files that we have just
restored, thereby reducing I/O.

* guix/serialization.scm (dump-file): New procedure.
(restore-file): Add #:dump-file parameter and honor it.
* guix/store/deduplication.scm (tee, dump-file/deduplicate): New
procedures.
* guix/nar.scm (restore-one-item): Pass #:dump-file to 'restore-file'.
(finalize-store-file): Pass #:deduplicate? #f to 'register-items'.
* tests/nar.scm &lt;top level&gt;: Call 'setenv' to set "NIX_STORE".
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-12-15 17:32:09 +0100'>2020-12-15</span></td><td><a href='/guix/commit/tests/nar.scm?id=ed7d02f7c198970ce3fe94bcee47592963326446'>serialization: 'restore-file' sets canonical timestamp and permissions.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/serialization.scm (restore-file): Set the permissions and mtime
of FILE.
* guix/nar.scm (finalize-store-file): Pass #:reset-timestamps? #f to
'register-items'.
* tests/nar.scm (rm-rf): Add 'chmod' calls to ensure files are writable.
("write-file + restore-file with symlinks"): Ensure every file in OUTPUT
passes 'canonical-file?'.
* tests/guix-archive.sh: Run "chmod -R +w" before "rm -rf".
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-12-15 17:32:09 +0100'>2020-12-15</span></td><td><a href='/guix/commit/tests/nar.scm?id=465d2cb286170933577de045e6e6dad7205bfe10'>serialization: 'fold-archive' notifies about directory processing completion.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/serialization.scm (fold-archive): Call PROC with a
'directory-complete tag when done with a directory.
(restore-file): Handle it.
* guix/scripts/archive.scm (list-contents): Likewise.
* guix/scripts/challenge.scm (archive-contents): Likewise.
* tests/nar.scm ("write-file-tree + fold-archive"): Adjust accordingly.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-12-12 17:56:57 +0100'>2019-12-12</span></td><td><a href='/guix/commit/tests/nar.scm?id=12c1afcdbdc984c760d00932bce64288b385bbc9'>serialization: Add 'fold-archive'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/serialization.scm (read-contents): Remove.
(read-file-type, fold-archive): New procedures.
(restore-file): Rewrite in terms of 'fold-archive'.
* tests/nar.scm ("write-file-tree + fold-archive")
("write-file-tree + fold-archive, flat file"): New tests.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-01-18 17:51:34 +0100'>2019-01-18</span></td><td><a href='/guix/commit/tests/nar.scm?id=9fe3f11398e858f1d06120bd046cab506efc86dc'>serialization: 'restore-file' errors out upon non-convertible file names.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://bugs.gnu.org/33603&gt;.
Reported by Maxim Cournoyer &lt;maxim.cournoyer@gmail.com&gt;.

* guix/serialization.scm (port-conversion-strategy): New variable.
(restore-file): Parameterize it.
* tests/nar.scm ("restore-file with non-UTF8 locale"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-13 14:59:46 +0100'>2018-11-13</span></td><td><a href='/guix/commit/tests/nar.scm?id=f5a2724ae453f4a4b55ff848f4ad7e30efb6eef8'>deduplication: Restore directory mtime and permissions after deduplication.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://bugs.gnu.org/33361&gt;.

* guix/store/deduplication.scm (replace-with-link): Call 'set-file-time'
and 'chmod' after 'rename-file'.
* tests/nar.scm ("restore-file-set with directories (signed, valid)"):
New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-13 14:59:46 +0100'>2018-11-13</span></td><td><a href='/guix/commit/tests/nar.scm?id=8390869811f56f5b2ff947efb9d48bcf219a0444'>tests: Check file canonicalization for 'restore-file-set'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/tests.scm (canonical-file?): New procedure.
* tests/nar.scm ("restore-file-set (signed, valid)"): Check that every
item of FILES matches 'canonical-file?'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-13 14:59:45 +0100'>2018-11-13</span></td><td><a href='/guix/commit/tests/nar.scm?id=3ed56ad09b1d2be15e98d195b56886ec14899518'>tests: Remove check for a feature that appeared in Guile 2.0.10.</a><span class='msg-avail'>...<span class='msg-tooltip'>* tests/nar.scm: Remove test for 'open-sha256-input-port'.
</span></span></td><td>Ludovic Courtès</td></tr>