On Mon, 25 May 2015, Marek Polacek wrote: > +/* Warn if signed left shift overflows. Note that we don't warn > + about left-shifting 1 into the sign bit; cf. > + <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3367.html#1457> > + for C++ and <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1817.htm> > + for C. LOC is a location of the shift; OP0 and OP1 are the operands. > + Return true if an overflow is detected, false otherwise. */
But for C that was declared not a defect. See <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1931.htm#dr_463>. So for C99 and later we *should* consider this an overflow (for the purposes of pedwarns-if-pedantic in contexts where an integer constant expression is required; maybe -Wshift-overflow=2 for other warnings?). If then a future C standard changes things here (in the list of issues to be considered for a future C standard in Standing Document 3, <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1917.htm>), then, as a non-defect change, it should be considered a non-overflow in GCC only for future C standard versions as well as C90. (Although treating something as not an integer constant expression does have consequences beyond pedwarns-if-pedantic - a zero derived from that expression is not a null pointer constant and that can affect the types of conditional expressions - I don't expect any significant breakage of real code from that.) -- Joseph S. Myers jos...@codesourcery.com
