https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102291
Jakub Jelinek <jakub at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |jason at gcc dot gnu.org, | |jsm28 at gcc dot gnu.org, | |mpolacek at gcc dot gnu.org --- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> --- I'm afraid I'm getting lost in what convert does though with the TREE_OVERFLOW flag. In the testcase, we have a cast of BIT_IOR_EXPR with (whatever, 0x80) and 0 int arguments to unsigned char. do_narrow is called and does: /* Don't do unsigned arithmetic where signed was wanted, or vice versa. Exception: if both of the original operands were unsigned then we can safely do the work as unsigned. Exception: shift operations take their type solely from the first argument. Exception: the LSHIFT_EXPR case above requires that we perform this operation unsigned lest we produce signed-overflow undefinedness. And we may need to do it as unsigned if we truncate to the original size. */ and as BIT_IOR_EXPR isn't listed, type == typex is unsigned and TREE_TYPE (arg0) and TREE_TYPE (arg1) are signed goes the: if (TYPE_UNSIGNED (typex)) typex = signed_type_for (typex); way. Those conversions create TYPE_OVERFLOW constant, even when there was nothing wrong in the source and because of the COMPOUND_EXPRs in there we don't manage to optimize those TYPE_OVERFLOW constants away and warn on it. For BIT_AND_EXPR/BIT_IOR_EXPR/BIT_XOR_EXPR, I must say I miss the reason why it would be ever a good idea to use typex = signed_type_for (typex), when we want unsigned result, just converting to that right away doesn't have any downsides I can come up with. But I wonder even about the other operations, do we really want to introduce TYPE_OVERFLOWs in such cases where the operations are originally on wider signed operands and are cast to some narrower unsigned type?