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.

0 files changed, 0 insertions, 0 deletions