https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95141
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #2)
> So there's already (OVF) at
>
> ((long unsigned int) IA1 & 158(OVF)) & 1
>
> but we only check
>
> 375 if (TREE_OVERFLOW_P (ret)
> 376 && !TREE_OVERFLOW_P (op0)
> 377 && !TREE_OVERFLOW_P (op1))
> 378 overflow_warning (EXPR_LOC_OR_LOC (expr, input_location),
> ret, expr);
>
> which doesn't catch the pre-existing overflow on op0 which then propagates.
>
> The original overflow is introduced folding short 158 to signed char -98(OVF)
> via convert.c:do_narrow:
>
> 437 /* We should do away with all this once we have a proper
> 438 type promotion/demotion pass, see PR45397. */
> 439 expr = maybe_fold_build2_loc (dofold, loc, ex_form, typex,
> 440 convert (typex, arg0),
> 441 convert (typex, arg1));
>
> specifically the convert (typex, arg1).
>
> Now TREE_OVERFLOW in general is quite a fragile thing, but it's tempting to
> adjust the overflow_warning guard for this case ...
>
> The do_narrow code also specifically looks for overflow cases that matter
> and does not perform narrowing then so clearing TREE_OVERFLOW there would
> also look reasonable.
>
> Thus like the following? Should be cheaper than adding walk_tree to the
> diagnostic guard.
>
> diff --git a/gcc/convert.c b/gcc/convert.c
> index 42509c518a9..ed00ded1a89 100644
> --- a/gcc/convert.c
> +++ b/gcc/convert.c
> @@ -436,9 +436,16 @@ do_narrow (location_t loc,
> }
> /* We should do away with all this once we have a proper
> type promotion/demotion pass, see PR45397. */
> + /* Above we checked for all cases where overflow matters, avoid
> + geneating overflowed constants here which otherwise propagate
> + and cause spurious warnings, see PR95141. */
> + tree converted_arg1 = convert (typex, arg1);
> + if (TREE_OVERFLOW_P (converted_arg1)
> + && !TREE_OVERFLOW_P (arg1))
> + converted_arg1 = drop_tree_overflow (converted_arg1);
> expr = maybe_fold_build2_loc (dofold, loc, ex_form, typex,
> convert (typex, arg0),
> - convert (typex, arg1));
> + converted_arg1);
> return convert (type, expr);
> }
Regresses
FAIL: gcc.dg/overflow-warn-5.c (test for warnings, line 6)
which looks like a useful warning to preserve. Lame "walk-tree" variant
catching this case:
diff --git a/gcc/c/c-fold.c b/gcc/c/c-fold.c
index 63becfeaf2c..bd21d247051 100644
--- a/gcc/c/c-fold.c
+++ b/gcc/c/c-fold.c
@@ -374,6 +374,7 @@ c_fully_fold_internal (tree expr, bool in_init, bool
*maybe_const_operands,
ret = fold (expr);
if (TREE_OVERFLOW_P (ret)
&& !TREE_OVERFLOW_P (op0)
+ && !(BINARY_CLASS_P (op0) && TREE_OVERFLOW_P (TREE_OPERAND (op0, 1)))
&& !TREE_OVERFLOW_P (op1))
overflow_warning (EXPR_LOC_OR_LOC (expr, input_location), ret, expr);
if (code == LSHIFT_EXPR
