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))
guix/refs/?id=90ec03cae3866fc531da5b390b3ad47ccdc4514f'>refs</a><a class='active' href='/guix/log/tests/guix-git-authenticate.sh'>log</a><a href='/guix/tree/tests/guix-git-authenticate.sh?id=90ec03cae3866fc531da5b390b3ad47ccdc4514f'>tree</a><a href='/guix/commit/tests/guix-git-authenticate.sh?id=90ec03cae3866fc531da5b390b3ad47ccdc4514f'>commit</a><a href='/guix/diff/tests/guix-git-authenticate.sh?id=90ec03cae3866fc531da5b390b3ad47ccdc4514f'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/guix-git-authenticate.sh'>
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<div class='path'>path: <a href='/guix/log/?id=90ec03cae3866fc531da5b390b3ad47ccdc4514f'>root</a>/<a href='/guix/log/tests?id=90ec03cae3866fc531da5b390b3ad47ccdc4514f'>tests</a>/<a href='/guix/log/tests/guix-git-authenticate.sh?id=90ec03cae3866fc531da5b390b3ad47ccdc4514f'>guix-git-authenticate.sh</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/guix-git-authenticate.sh?id=90ec03cae3866fc531da5b390b3ad47ccdc4514f&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-02-14 11:23:08 +0100'>2022-02-14</span></td><td><a href='/guix/commit/tests/guix-git-authenticate.sh?id=ca87601dd97dd9d356409827802eb0f8a3a535f0'>git-authenticate: Ensure the target is a descendant of the introductory commit.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes a bug whereby authentication of a commit *not* descending from the
introductory commit could succeed, provided the commit verifies the
authorization invariant.

In the example below, A is a common ancestor of the introductory commit
I and of commit X.  Authentication of X would succeed, even though it is
not a descendant of I, as long as X is authorized according to the
'.guix-authorizations' in A:

   X   	 I
    \   /
      A

This is because, 'authenticate-repository' would not check whether X
descends from I, and the call (commit-difference X I) would return X.

In practice that only affects forks because it means that ancestors of
the introductory commit already contain a '.guix-authorizations' file.

* guix/git-authenticate.scm (authenticate-repository): Add call to
'commit-descendant?'.
* tests/channels.scm ("authenticate-channel, not a descendant of introductory commit"):
New test.
* tests/git-authenticate.scm ("authenticate-repository, target not a descendant of intro"):
New test.
* tests/guix-git-authenticate.sh: Expect earlier test to fail since
9549f0283a78fe36f2d4ff2a04ef8ad6b0c02604 is not a descendant of
$intro_commit.  Add new test targeting an ancestor of the introductory
commit, and another test targeting the v1.2.0 commit.
* doc/guix.texi (Specifying Channel Authorizations): Add a sentence.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-09-28 21:06:36 -0500'>2020-09-28</span></td><td><a href='/guix/commit/tests/guix-git-authenticate.sh?id=d8934360d2453a403b5433e71d09188e4ed23b57'>tests: Simplify shell exit status negation;</a><span class='msg-avail'>...<span class='msg-tooltip'>* tests/guix-archive.sh, tests/guix-build-branch.sh, tests/guix-build.sh,
tests/guix-daemon.sh, tests/guix-download.sh, tests/guix-environment.sh,
tests/guix-gc.sh, tests/guix-git-authenticate.sh, tests/guix-graph.sh,
tests/guix-hash.sh, tests/guix-lint.sh, tests/guix-pack-relocatable.sh,
tests/guix-pack.sh, tests/guix-package-aliases.sh, tests/guix-package-net.sh,
tests/guix-package.sh: Use the shell '!' keyword to negate command exit status
in place of 'if ...; then false; else true; fi'
</span></span></td><td>Eric Bavier</td></tr>
<tr><td><span title='2020-07-11 12:33:01 +0200'>2020-07-11</span></td><td><a href='/guix/commit/tests/guix-git-authenticate.sh?id=a98712785e0b042a290420fd74e5a4a5da4fc68f'>Add 'guix git authenticate'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/scripts/git.scm, guix/scripts/git/authenticate.scm,
tests/guix-git-authenticate.sh: New files.
* Makefile.am (MODULES): Add the *.scm files.
(SH_TESTS): Add 'tests/guix-git-authenticate.sh'.
* doc/guix.texi (Channels)[Specifying Channel Authorizations]: Mention
'guix git authenticate'.
(Invoking guix git authenticate): New node.
* po/guix/POTFILES.in: Add 'guix/scripts/git.scm' and
'guix/scripts/git/authenticate.scm'.
</span></span></td><td>Ludovic Courtès</td></tr>