Build CPython (master in this case - though I originally noticed the problem when building a 3.6 tree) as follows with clang installed:

build$ LD=clang-5.0 LDFLAGS=-fsanitize=undefined CC=clang-5.0 CXX=clang-5.0 CFLAGS=-fsanitize=undefined CXXFLAGS=-fsanitize=undefined ../gpshead/configure
build$ make -j12

...

notice many of the warnings scroll by during the build itself as it executes the interpreter

then execute it yourself at the end and you'll get a bunch of these:

../gpshead/Objects/dictobject.c:547:12: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:1145:18: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:2817:15: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:831:27: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:1144:18: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:1034:15: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:728:11: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:1064:9: runtime error: index 64 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:2960:31: runtime error: index 64 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:1489:11: runtime error: index 32 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:637:27: runtime error: index 128 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:788:27: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:1671:22: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:554:31: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:1223:15: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:876:27: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:2396:15: runtime error: index 32 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:2078:10: runtime error: index 128 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:3584:38: runtime error: index 16 out of bounds for type 'int8_t [8]'
../gpshead/Objects/dictobject.c:3502:38: runtime error: index 64 out of bounds for type 'int8_t [8]'

At issue is the hash table here: https://github.com/python/cpython/blob/3.7/Objects/dict-common.h

which is intentionally meant to be indexed "out of bounds" off the end of the struct.

I'm not a strict C language definition so I don't know if that is _supposed_ to be defined behavior as we all tend to assume it is in C or not.  If it is supposed to be okay, we should be able to annotate it as such to avoid the warning under ubsan builds.

If it is not, we need to change the way this is written.