On Mon, 21 Apr 2025, Jason Merrill wrote:
> Tested x86_64-pc-linux-gnu, OK for trunk?
>
> -- 8< --
>
> While working on PR119162 it occurred to me that it would be simpler to
> detect the problem of a value referring to a heap allocation if we stopped
> setting TREE_STATIC on them so they naturally are not considered to have a
> constant address. With that change we no longer need to specifically avoid
> caching a value that refers to a deleted pointer.
>
> But with this change maybe_nonzero_address is not sure whether the variable
> could have address zero. I don't understand why it returns 1 only for
> variables in the current function, rather than all non-symtab decls; an auto
> variable from some other function also won't have address zero. Maybe this
> made more sense when it was in tree_single_nonzero_warnv_p before r7-5868?
>
> But assuming there is some reason for the current behavior, this patch only
> changes the handling of non-symtab decls when folding_cxx_constexpr.
FWIW the maybe_nonzero_address change seems to fix PR85944, yay! Except for
the PR115207 dup, which we still reject with
115207.C:11:18: error: ‘(((const value*)(&<anonymous>)) != ((const value*)(&
test.array<4>::_items)))’ is not a constant expression
11 | if (this != &other) {
| ~~~~~^~~~~~~~~
Could this mean another spot in the middle-end needs relaxing in order to
handle address comparisons of context-less local vars?
>
> PR c++/119162
>
> gcc/cp/ChangeLog:
>
> * constexpr.cc (find_deleted_heap_var): Remove.
> (cxx_eval_call_expression): Don't call it. Don't set TREE_STATIC on
> heap vars.
> (cxx_eval_outermost_constant_expr): Don't mess with varpool.
>
> gcc/ChangeLog:
>
> * fold-const.cc (maybe_nonzero_address): Return 1 for non-symtab
> vars if folding_cxx_constexpr.
> ---
> gcc/cp/constexpr.cc | 29 -----------------------------
> gcc/fold-const.cc | 25 ++++++++++++++++---------
> 2 files changed, 16 insertions(+), 38 deletions(-)
>
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 8a11e6265f2..5b7b70f7e65 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -1550,7 +1550,6 @@ static tree cxx_eval_bare_aggregate (const
> constexpr_ctx *, tree,
> static tree cxx_fold_indirect_ref (const constexpr_ctx *, location_t, tree,
> tree,
> bool * = NULL);
> static tree find_heap_var_refs (tree *, int *, void *);
> -static tree find_deleted_heap_var (tree *, int *, void *);
>
> /* Attempt to evaluate T which represents a call to a builtin function.
> We assume here that all builtin functions evaluate to scalar types
> @@ -2975,14 +2974,6 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
> tree t,
> : heap_uninit_identifier,
> type);
> DECL_ARTIFICIAL (var) = 1;
> - TREE_STATIC (var) = 1;
> - // Temporarily register the artificial var in varpool,
> - // so that comparisons of its address against NULL are folded
> - // through nonzero_address even with
> - // -fno-delete-null-pointer-checks or that comparison of
> - // addresses of different heap artificial vars is folded too.
> - // See PR98988 and PR99031.
> - varpool_node::finalize_decl (var);
> ctx->global->heap_vars.safe_push (var);
> ctx->global->put_value (var, NULL_TREE);
> return fold_convert (ptr_type_node, build_address (var));
> @@ -3454,11 +3445,6 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
> tree t,
> cacheable = false;
> break;
> }
> - /* And don't cache a ref to a deleted heap variable (119162). */
> - if (cacheable
> - && (cp_walk_tree_without_duplicates
> - (&result, find_deleted_heap_var, NULL)))
> - cacheable = false;
> }
>
> /* Rewrite all occurrences of the function's RESULT_DECL with the
> @@ -9025,20 +9011,6 @@ find_heap_var_refs (tree *tp, int *walk_subtrees, void
> */*data*/)
> return NULL_TREE;
> }
>
> -/* Look for deleted heap variables in the expression *TP. */
> -
> -static tree
> -find_deleted_heap_var (tree *tp, int *walk_subtrees, void */*data*/)
> -{
> - if (VAR_P (*tp)
> - && DECL_NAME (*tp) == heap_deleted_identifier)
> - return *tp;
> -
> - if (TYPE_P (*tp))
> - *walk_subtrees = 0;
> - return NULL_TREE;
> -}
> -
> /* Find immediate function decls in *TP if any. */
>
> static tree
> @@ -9275,7 +9247,6 @@ cxx_eval_outermost_constant_expr (tree t, bool
> allow_non_constant,
> r = t;
> non_constant_p = true;
> }
> - varpool_node::get (heap_var)->remove ();
> }
> }
>
> diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
> index c9471ea44b0..35fcf5087fb 100644
> --- a/gcc/fold-const.cc
> +++ b/gcc/fold-const.cc
> @@ -9917,22 +9917,29 @@ pointer_may_wrap_p (tree base, tree offset,
> poly_int64 bitpos)
> static int
> maybe_nonzero_address (tree decl)
> {
> + if (!DECL_P (decl))
> + return -1;
> +
> /* Normally, don't do anything for variables and functions before symtab is
> built; it is quite possible that DECL will be declared weak later.
> But if folding_initializer, we need a constant answer now, so create
> the symtab entry and prevent later weak declaration. */
> - if (DECL_P (decl) && decl_in_symtab_p (decl))
> - if (struct symtab_node *symbol
> - = (folding_initializer
> - ? symtab_node::get_create (decl)
> - : symtab_node::get (decl)))
> - return symbol->nonzero_address ();
> + if (decl_in_symtab_p (decl))
> + {
> + if (struct symtab_node *symbol
> + = (folding_initializer
> + ? symtab_node::get_create (decl)
> + : symtab_node::get (decl)))
> + return symbol->nonzero_address ();
> + }
> + else if (folding_cxx_constexpr)
> + /* Anything that doesn't go in the symtab has non-zero address. */
> + return 1;
>
> /* Function local objects are never NULL. */
> - if (DECL_P (decl)
> - && (DECL_CONTEXT (decl)
> + if (DECL_CONTEXT (decl)
> && TREE_CODE (DECL_CONTEXT (decl)) == FUNCTION_DECL
> - && auto_var_in_fn_p (decl, DECL_CONTEXT (decl))))
> + && auto_var_in_fn_p (decl, DECL_CONTEXT (decl)))
> return 1;
>
> return -1;
>
> base-commit: 9ac98b5742ebce41d7da9fda10852a0bc958f1cb
> --
> 2.49.0
>
>
