[Bug middle-end/101062] [10 Regression] wrong code with "-O2 -fno-toplevel-reorder -frename-registers"

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101062

--- Comment #21 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:3df2f5828752a91343d01defb215de326b5ddb4c

commit r10-10623-g3df2f5828752a91343d01defb215de326b5ddb4c
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.

    (cherry picked from commit 3587c2c241eda0f3ab54ea60d46e9caf12d69b5a)