[Bug c/114930] [14/15 regression] ICE in fld_incomplete_type_of when building libwebp with -std=c23 -flto

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114930

--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:777cc6a01d1cf783a36d0fa67ab20f0312f35d7a

commit r15-1597-g777cc6a01d1cf783a36d0fa67ab20f0312f35d7a
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Jun 25 08:35:56 2024 +0200

    c: Fix ICE related to incomplete structures in C23 [PR114930]

    Here is a version of the c_update_type_canonical fixes which passed
    bootstrap/regtest.
    The non-trivial part is the handling of the case when
    build_qualified_type (TYPE_CANONICAL (t), TYPE_QUALS (x))
    returns a type with NULL TYPE_CANONICAL.  That should happen only
    if TYPE_CANONICAL (t) == t, because otherwise c_update_type_canonical
should
    have been already called on the other type.  c, the returned type, is
usually x
    and in that case it should have TYPE_CANONICAL set to itself, or worst
    for whatever reason x is not the right canonical type (say it has
attributes
    or whatever disqualifies it from check_qualified_type).  In that case
    either it finds some pre-existing type from the variant chain of t which
    is later in the chain and we haven't processed it yet (but then
    get_qualified_type moves it right after t in:
            /* Put the found variant at the head of the variant list so
               frequently searched variants get found faster.  The C++ FE
               benefits greatly from this.  */
            tree t = *tp;
            *tp = TYPE_NEXT_VARIANT (t);
            TYPE_NEXT_VARIANT (t) = TYPE_NEXT_VARIANT (mv);
            TYPE_NEXT_VARIANT (mv) = t;
            return t;
    optimization), or creates a fresh new type using build_variant_type_copy,
    which again places the new type right after t:
      /* Add the new type to the chain of variants of TYPE.  */
      TYPE_NEXT_VARIANT (t) = TYPE_NEXT_VARIANT (m);
      TYPE_NEXT_VARIANT (m) = t;
      TYPE_MAIN_VARIANT (t) = m;
    At this point we want to make c its own canonical type (i.e. TYPE_CANONICAL
    (c) = c;), but also need to process pointers to it and only then return
back
    to processing x.  Processing the whole chain from c again could be costly,
    we could have hundreds of types in the chain already processed, and while
    the loop would just quickly skip them
      for (tree x = t, l = NULL_TREE; x; l = x, x = TYPE_NEXT_VARIANT (x))
        {
          if (x != t && TYPE_STRUCTURAL_EQUALITY_P (x))
    ...
          else if (x != t)
            continue;
    it feels costly.  So, this patch instead moves c from right after t
    to right before x in the chain (that shouldn't change anything, because
    clearly build_qualified_type didn't find any matches in the chain before
    x) and continues processing the c at that position, so should handle the
    x that encountered this in the next iteration.

    We could avoid some of the moving in the chain if we processed the chain
    twice, once deal only with x != t && TYPE_STRUCTURAL_EQUALITY_P (x)
    && TYPE_CANONICAL (t) == t && check_qualified_type (t, x, TYPE_QUALS (x))
    types (in that case set TYPE_CANONICAL (x) = x) and once the rest.  There
    is still the theoretical case where build_qualified_type would return
    a new type and in that case we are back to the moving the type around and
    needing to handle it though.

    2024-06-25  Jakub Jelinek  <jakub@redhat.com>
                Martin Uecker  <uecker@tugraz.at>

            PR c/114930
            PR c/115502
    gcc/c/
            * c-decl.cc (c_update_type_canonical): Assert t is main variant
            with 0 TYPE_QUALS.  Simplify and don't use check_qualified_type.
            Deal with the case where build_qualified_type returns
            TYPE_STRUCTURAL_EQUALITY_P type.
    gcc/testsuite/
            * gcc.dg/pr114574-1.c: Require lto effective target.
            * gcc.dg/pr114574-2.c: Likewise.
            * gcc.dg/pr114930.c: New test.
            * gcc.dg/pr115502.c: New test.