[PATCH] stor-layout: Don't create DECL_BIT_FIELD_REPRESENTATIVE for QUAL_UNION_TYPE [PR101062]

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Jun 16, 2021 at 09:45:17AM +0200, Jakub Jelinek via Gcc-patches wrote:
> 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.

While the patch passed bootstrap/regtest on the trunk, when trying to
backport it to 11 branch the bootstrap failed with
atree.ads:3844:34: size for "Node_Record" too small
errors.  Turns out the error is not about size being too small, but actually
about size being non-constant, and comes from:
/* In a FIELD_DECL of a RECORD_TYPE, this is a pointer to the storage
   representative FIELD_DECL.  */
#define DECL_BIT_FIELD_REPRESENTATIVE(NODE) \
  (FIELD_DECL_CHECK (NODE)->field_decl.qualifier)

/* For a FIELD_DECL in a QUAL_UNION_TYPE, records the expression, which
   if nonzero, indicates that the field occupies the type.  */
#define DECL_QUALIFIER(NODE) (FIELD_DECL_CHECK (NODE)->field_decl.qualifier)
so by setting up DECL_BIT_FIELD_REPRESENTATIVE in QUAL_UNION_TYPE we
actually set or modify DECL_QUALIFIER and then construct size as COND_EXPRs
with those bit field representatives (e.g. with array type) as conditions
which doesn't fold into constant.

The following patch fixes it by not creating DECL_BIT_FIELD_REPRESENTATIVEs
for QUAL_UNION_TYPE as there is nowhere to store them,

Bootstrapped/regtested on x86_64-linux and i686-linux (both trunk and
11 branch - there on top of the earlier patch backport).  Ok for trunk and
the backport?

Shall we change tree.h to document that DECL_BIT_FIELD_REPRESENTATIVE
is valid also on UNION_TYPE?
I see:
tree-ssa-alias.c-  if (TREE_CODE (type1) == RECORD_TYPE
tree-ssa-alias.c:      && DECL_BIT_FIELD_REPRESENTATIVE (field1))
tree-ssa-alias.c:    field1 = DECL_BIT_FIELD_REPRESENTATIVE (field1);
tree-ssa-alias.c-  if (TREE_CODE (type2) == RECORD_TYPE
tree-ssa-alias.c:      && DECL_BIT_FIELD_REPRESENTATIVE (field2))
tree-ssa-alias.c:    field2 = DECL_BIT_FIELD_REPRESENTATIVE (field2);
Shall we change that to || == UNION_TYPE or do we assume all fields
are overlapping in a UNION_TYPE already?
At other spots (asan, ubsan, expr.c) it is unclear what will happen
if they see a QUAL_UNION_TYPE with a DECL_QUALIFIER (or does the Ada FE
lower that somehow)?

2021-06-18  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/101062
	* stor-layout.c (finish_bitfield_layout): Don't add bitfield
	representatives in QUAL_UNION_TYPE.

--- gcc/stor-layout.c.jj	2021-06-16 12:17:49.254221954 +0200
+++ gcc/stor-layout.c	2021-06-17 11:36:13.011822974 +0200
@@ -2172,6 +2172,9 @@ finish_bitfield_layout (tree t)
   tree field, prev;
   tree repr = NULL_TREE;
 
+  if (TREE_CODE (t) == QUAL_UNION_TYPE)
+    return;
+
   for (prev = NULL_TREE, field = TYPE_FIELDS (t);
        field; field = DECL_CHAIN (field))
     {


	Jakub