On Fri, Jul 11, 2025 at 02:34:24PM -0400, Jason Merrill wrote:
> But by the time we get to cp_fold, DECL_READ_P should have already been set
> appropriately when we built the thing we're now folding. And calling
Clearly it hasn't been, otherwise I'd need to patch different spots as well.
> mark_exp_read on foo(op0) won't mark op0 anyway; it doesn't recurse. I
> don't see any regressions on Wunused* after
mark_exp_read recurses a little bit, on some simple cases.
E.g. COND_EXPR, COMPOUND_EXPR, casts, INDIRECT_REF, ...
For more complex trees the expectation is that mark_exp_read will
be called when creating or folding those trees (like CALL_EXPR etc.).
Admittedly it isn't very clean but mostly happens to work fine for years.
> diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
> index 9a98628d9e8..addbc29d104 100644
> --- a/gcc/cp/cp-gimplify.cc
> +++ b/gcc/cp/cp-gimplify.cc
> @@ -3221,16 +3221,8 @@ cp_fold (tree x, fold_flags_t flags)
> clear_decl_read = false;
> if (code == MODIFY_EXPR
> && (VAR_P (op0) || TREE_CODE (op0) == PARM_DECL)
> - && !DECL_READ_P (op0)
> - && (VAR_P (op0) ? warn_unused_but_set_variable
> - : warn_unused_but_set_parameter) > 2
> - && BINARY_CLASS_P (TREE_OPERAND (x, 1))
> - && TREE_OPERAND (TREE_OPERAND (x, 1), 0) == op0)
> - {
> - mark_exp_read (TREE_OPERAND (TREE_OPERAND (x, 1), 1));
Guess another option instead of mark_exp_read would be some
cp_walk_tree_without_duplicates that would return if it finds
any references to op0 in that subexpression.
> > Well, I'm not sure it is actually an error. finish_unary_op_expr doesn't
> > use cp_fully_fold result as an operand of {PRE,POST}{INC,DEC}REMENT_EXPR
> > (that would be wrong, we don't want the operand to fold into non-lvalue), it
> > is called only to find out if overflow warning should be emitted.
>
> But it only warns if the whole expression folds to a constant, which can
> never happen for these tree codes. So folding the operand is useless.
Seems you're right, can tweak it to punt for those codes earlier instead.
In fact, I wonder if the finish_unary_op_expr function is ever called on
POST{IN,DE}CREMENT_EXPR.
Jakub
