stor-layout: Create DECL_BIT_FIELD_REPRESENTATIVE even for bitfields in unions [PR101062]

The following testcase is miscompiled on x86_64-linux, the bitfield store
is implemented as a RMW 64-bit operation at d+24 when the d variable has
size of only 28 bytes and scheduling moves in between the R and W part
a store to a different variable that happens to be right after the d
variable.

The reason for this is that we weren't creating
DECL_BIT_FIELD_REPRESENTATIVEs for bitfields in unions.

The following patch does create them, but treats all such bitfields as if
they were in a structure where the particular bitfield is the only field.

2021-06-16  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/101062
	* stor-layout.c (finish_bitfield_representative): For fields in unions
	assume nextf is always NULL.
	(finish_bitfield_layout): Compute bit field representatives also in
	unions, but handle it as if each bitfield was the only field in the
	aggregate.

	* gcc.dg/pr101062.c: New test.

2 files changed, 44 insertions, 11 deletions

diff --git a/gcc/stor-layout.c b/gcc/stor-layout.c
index 94b8b21..4e3a41b 100644
--- a/gcc/stor-layout.c
+++ b/gcc/stor-layout.c
@@ -2072,9 +2072,14 @@ finish_bitfield_representative (tree repr, tree field)
   bitsize = (bitsize + BITS_PER_UNIT - 1) & ~(BITS_PER_UNIT - 1);
 
   /* Now nothing tells us how to pad out bitsize ...  */
-  nextf = DECL_CHAIN (field);
-  while (nextf && TREE_CODE (nextf) != FIELD_DECL)
-    nextf = DECL_CHAIN (nextf);
+  if (TREE_CODE (DECL_CONTEXT (field)) == RECORD_TYPE)
+    {
+      nextf = DECL_CHAIN (field);
+      while (nextf && TREE_CODE (nextf) != FIELD_DECL)
+	nextf = DECL_CHAIN (nextf);
+    }
+  else
+    nextf = NULL_TREE;
   if (nextf)
     {
       tree maxsize;
@@ -2167,13 +2172,6 @@ finish_bitfield_layout (tree t)
   tree field, prev;
   tree repr = NULL_TREE;
 
-  /* Unions would be special, for the ease of type-punning optimizations
-     we could use the underlying type as hint for the representative
-     if the bitfield would fit and the representative would not exceed
-     the union in size.  */
-  if (TREE_CODE (t) != RECORD_TYPE)
-    return;
-
   for (prev = NULL_TREE, field = TYPE_FIELDS (t);
        field; field = DECL_CHAIN (field))
     {
@@ -2233,7 +2231,13 @@ finish_bitfield_layout (tree t)
       if (repr)
 	DECL_BIT_FIELD_REPRESENTATIVE (field) = repr;
 
-      prev = field;
+      if (TREE_CODE (t) == RECORD_TYPE)
+	prev = field;
+      else if (repr)
+	{
+	  finish_bitfield_representative (repr, field);
+	  repr = NULL_TREE;
+	}
     }
 
   if (repr)
diff --git a/gcc/testsuite/gcc.dg/pr101062.c b/gcc/testsuite/gcc.dg/pr101062.c
new file mode 100644
index 0000000..6c37ed8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr101062.c
@@ -0,0 +1,29 @@
+/* PR middle-end/101062 */
+/* { dg-do run } */
+/* { dg-options "-O2 -fno-toplevel-reorder -frename-registers" } */
+
+union U { signed b : 5; };
+int c;
+volatile union U d[7] = { { 8 } };
+short e = 1;
+
+__attribute__((noipa)) void
+foo ()
+{
+  d[6].b = 0;
+  d[6].b = 0;
+  d[6].b = 0;
+  d[6].b = 0;
+  d[6].b = 0;
+  e = 0;
+  c = 0;
+}
+
+int
+main ()
+{
+  foo ();
+  if (e != 0)
+    __builtin_abort ();
+  return 0;
+}