Apple Radar ticket 9908625 has been filed. The clang 2.1 optimizer causes overflows in Object/intobject.c:int_pow() to be missed, so 2**63 turns into a negative integer. The attached test program narrows it down. llvm-gcc is fine, so this ticket is mostly an FYI.
idiotbox:Python-2.7.2 cloomis$ ./seqpoint-OK 4000000000 4000000000
a=4000000000, b=4000000000, c(a*b)=-2446744073709551616, c/b=-611686018
overflow detected: 1
to turn into:
idiotbox:Python-2.7.2 cloomis$ ./seqpoint 4294967296 4200000000
a=4294967296, b=4200000000, c(a*b)=-407881430509551616, c/b=4294967296
overflow detected: 0