[gcc r11-8615] stor-layout: Don't create DECL_BIT_FIELD_REPRESENTATIVE for QUAL_UNION_TYPE [PR101062]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:3587c2c241eda0f3ab54ea60d46e9caf12d69b5a

commit r11-8615-g3587c2c241eda0f3ab54ea60d46e9caf12d69b5a
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jun 18 11:20:40 2021 +0200

    stor-layout: Don't create DECL_BIT_FIELD_REPRESENTATIVE for QUAL_UNION_TYPE [PR101062]
    
    > 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,
    
    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.

Diff:
---
 gcc/stor-layout.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/gcc/stor-layout.c b/gcc/stor-layout.c
index 4e3a41b34e9..9f850c3951e 100644
--- a/gcc/stor-layout.c
+++ b/gcc/stor-layout.c
@@ -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))
     {