erff fails with clang and -O0
$ CC=clang CFLAGS=-O0 ./check.sh --exhaustive -rndn erff
FAIL x=-0x1.c40052p-68 ref=-0x1.fe076p-68 y=-0x1.e2b8dp-5
FAIL x=-0x1.c80062p-72 ref=-0x1.014574p-71 y=-0x1.e2b8dp-5
FAIL x=-0x1.e40006p-100 ref=-0x1.11115cp-99 y=-0x1.e2b8dp-5
It seems to work at level -O1 and higher.