[gcc r12-2320] gimplify: Fix endless recursion on volatile empty type reads/writes [PR101437]

[gcc r12-2320] gimplify: Fix endless recursion on volatile empty type reads/writes [PR101437]

[Jakub Jelinek]

https://gcc.gnu.org/g:f6dde32b9d487dd6e343d0a1e1d1f60783f5e735

commit r12-2320-gf6dde32b9d487dd6e343d0a1e1d1f60783f5e735
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Jul 15 10:17:06 2021 +0200

    gimplify: Fix endless recursion on volatile empty type reads/writes [PR101437]
    
    Andrew's recent change to optimize away during gimplification not just
    assignments of zero sized types, but also assignments of empty types,
    caused infinite recursion in the gimplifier.
    If such assignment is optimized away, we gimplify separately the to_p
    and from_p operands and throw away the result.  When gimplifying the
    operand that is volatile, we run into the gimplifier code below, which has
    different handling for types with non-BLKmode mode, tries to gimplify
    those as vol.N = expr, and for BLKmode just throws those away.
    Zero sized types will always have BLKmode and so are fine, but for the
    non-BLKmode ones like struct S in the testcase, the vol.N = expr
    gimplification will reach again the gimplify_modify_expr code, see it is
    assignment of empty type and will gimplify again vol.N separately
    (non-volatile, so ok) and expr, on which it will recurse again.
    
    The following patch breaks that infinite recursion by ignoring bare
    volatile loads from empty types.
    If a volatile load or store for aggregates are supposed to be member-wise
    loads or stores, then there are no non-padding members in the empty types that
    should be copied and so it is probably ok.
    
    2021-07-15  Jakub Jelinek  <jakub@redhat.com>
    
            PR middle-end/101437
            * gimplify.c (gimplify_expr): Throw away volatile reads from empty
            types even if they have non-BLKmode TYPE_MODE.
    
            * gcc.c-torture/compile/pr101437.c: New test.

Diff:
---
 gcc/gimplify.c                                 |  3 ++-
 gcc/testsuite/gcc.c-torture/compile/pr101437.c | 29 ++++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/gcc/gimplify.c b/gcc/gimplify.c
index 75a4a9d59fd..93a2121fea1 100644
--- a/gcc/gimplify.c
+++ b/gcc/gimplify.c
@@ -15060,7 +15060,8 @@ gimplify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
 	  *expr_p = NULL;
 	}
       else if (COMPLETE_TYPE_P (TREE_TYPE (*expr_p))
-	       && TYPE_MODE (TREE_TYPE (*expr_p)) != BLKmode)
+	       && TYPE_MODE (TREE_TYPE (*expr_p)) != BLKmode
+	       && !is_empty_type (TREE_TYPE (*expr_p)))
 	{
 	  /* Historically, the compiler has treated a bare reference
 	     to a non-BLKmode volatile lvalue as forcing a load.  */
diff --git a/gcc/testsuite/gcc.c-torture/compile/pr101437.c b/gcc/testsuite/gcc.c-torture/compile/pr101437.c
new file mode 100644
index 00000000000..96e7df89046
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/pr101437.c
@@ -0,0 +1,29 @@
+/* PR middle-end/101437 */
+
+struct S { int : 1; };
+
+void
+foo (volatile struct S *p)
+{
+  struct S s = {};
+  *p = s;
+}
+
+void
+bar (volatile struct S *p)
+{
+  *p;
+}
+
+void
+baz (volatile struct S *p)
+{
+  struct S s;
+  s = *p;
+}
+
+void
+qux (volatile struct S *p, volatile struct S *q)
+{
+  *p = *q;
+}