[gcc r14-2622] c++: constexpr bit_cast with empty field

[gcc r14-2622] c++: constexpr bit_cast with empty field

[Jason Merrill]

https://gcc.gnu.org/g:b41a927bcbdf27723b9d420f0b403f2af12129f1

commit r14-2622-gb41a927bcbdf27723b9d420f0b403f2af12129f1
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Jul 14 12:25:51 2023 -0400

    c++: constexpr bit_cast with empty field
    
    The change to only cache constexpr calls that are
    reduced_constant_expression_p tripped on bit-cast3.C, which failed that
    predicate due to the presence of an empty field in the result of
    native_interpret_aggregate, which reduced_constant_expression_p rejects to
    avoid confusing output_constructor.
    
    This patch proposes to skip such fields in native_interpret_aggregate, since
    they aren't actually involved in the value representation.
    
    gcc/ChangeLog:
    
            * fold-const.cc (native_interpret_aggregate): Skip empty fields.
    
    gcc/cp/ChangeLog:
    
            * constexpr.cc (cxx_eval_bit_cast): Check that the result of
            native_interpret_aggregate doesn't need more evaluation.

Diff:
---
 gcc/cp/constexpr.cc | 9 +++++++++
 gcc/fold-const.cc   | 3 ++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 9d85c3be5cc..6e8f1c2b61e 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -1440,6 +1440,8 @@ enum value_cat {
 
 static tree cxx_eval_constant_expression (const constexpr_ctx *, tree,
 					  value_cat, bool *, bool *, tree * = NULL);
+static tree cxx_eval_bare_aggregate (const constexpr_ctx *, tree,
+				     value_cat, bool *, bool *);
 static tree cxx_fold_indirect_ref (const constexpr_ctx *, location_t, tree, tree,
 				   bool * = NULL);
 static tree find_heap_var_refs (tree *, int *, void *);
@@ -4803,6 +4805,13 @@ cxx_eval_bit_cast (const constexpr_ctx *ctx, tree t, bool *non_constant_p,
 	{
 	  clear_type_padding_in_mask (TREE_TYPE (t), mask);
 	  clear_uchar_or_std_byte_in_mask (loc, r, mask);
+	  if (CHECKING_P)
+	    {
+	      tree e = cxx_eval_bare_aggregate (ctx, r, vc_prvalue,
+						non_constant_p, overflow_p);
+	      gcc_checking_assert (e == r);
+	      r = e;
+	    }
 	}
     }
 
diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
index a02ede79fed..7e5494dfd39 100644
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -8935,7 +8935,8 @@ native_interpret_aggregate (tree type, const unsigned char *ptr, int off,
     return NULL_TREE;
   for (tree field = TYPE_FIELDS (type); field; field = DECL_CHAIN (field))
     {
-      if (TREE_CODE (field) != FIELD_DECL || DECL_PADDING_P (field))
+      if (TREE_CODE (field) != FIELD_DECL || DECL_PADDING_P (field)
+	  || is_empty_type (TREE_TYPE (field)))
 	continue;
       tree fld = field;
       HOST_WIDE_INT bitoff = 0, pos = 0, sz = 0;