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 > 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>
