https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69984
--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> --- (In reply to Edmar Wienskoski from comment #6) > Hummm, You are almost convincing me, one last question, > be patient with me. > > As Andrew posted: > C = A * B > should be equivalent to: > C = (unsigned long)( ((int)A) * ((int)B) ) > > The variables are promoted *before* the multiplication. > How come (int)65535 multiplied by itself is undefined behavior ? Because signed integer overflow is undefined. That is if there is an overflow for int*int, the outcome is undefined. Since GCC Knows both sides of the multiplier is positive, GCC treats the outcome of the multiply to be also positive (due to overflow being undefined).
