On 11/17/15 11:26, Markus Armbruster wrote: > Paolo Bonzini <pbonz...@redhat.com> writes: > >> There's no reason for the compiler to exploit the undefinedness of left >> shifts, In fact GCC explicitly documents that they do not use at all >> all this possibility. They also say this is subject to change, but > > Suggest to scratch one of two "all" :) > >> they have been saying this for 10 years (since the wording appeared in >> the GCC 4.0 manual). >> >> Any workaround for this particular case of undefined behavior uglifies >> the code: using unsigned is unsafe because the value becomes positive >> when extended; using -(a << b) does not express as well that the >> intention is to compute -a * 2^N. >> >> Clang has just added an obnoxious, pointless, *totally useless*, unsafe >> warning about this. It's obnoxious and pointless because the compiler >> is not using the latitude that the standard gives it, so it just adds >> noise. It is useless and unsafe because it does not catch the widely >> more common case where the LHS is a variable, and thus gives a false
"wildly more", I think. >> sense of security. >> >> The noisy nature of the warning means that it should have never been >> added to -Wall. The uselessness means that it probably should not >> have even been added to -Wextra. >> >> Document this explicitly, and shut up the stupid warning. >> </rant> >> >> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> > > Reviewed-by: Markus Armbruster <arm...@redhat.com> >
