On 2015-08-21 13:27:22 -0400, Tom Lane wrote: > Andres Freund <and...@anarazel.de> writes: > > On 2015-08-21 13:03:42 -0400, Tom Lane wrote: > >> The behavior is well-defined, at least as long as we don't shift far > >> enough to have integer overflow > > > Unfortunately not: > > 5.8.2: The value of E1 << E2 is E1 left-shifted E2 bit positions; > > vacated bits are zero-filled. If E1 has an unsigned type, the value of > > the result is E1 � 2 E2 , reduced modulo one more than the maximum value > > representable in the result type. Otherwise, if E1 has a signed type and > > non-negative value, and E1 � 2 E2 is representable in the result type, > > then that is the resulting value; >>otherwise, the behavior is undefined<<. > > [ rolls eyes... ] There isn't any machine in the world where the behavior > isn't well-defined short of overflow.
> Why do the C spec authors persist in pretending otherwise? Yea, it's way past time that C is redefined being based on 2-s completement. And why this is declared undefined rather than implementation defined is completely beyond me. FWIW, icc apparently has been observed to recognize that a negative value cannot be shifted and thus optimized based on the assumption that the number is positive... -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
