What GCC version was used to build stage4 packages?

GCC 6.4.0

What CFLAGS value was used?

"-march=haswell -O3 -pipe"

As per the Gentoo documentation: https://wiki.gentoo.org/wiki/GCC_optimization#What_are_CFLAGS_and_CXXFLAGS.3F, we used 'O3' as the highest level of optimization possible. All packages for stage4 were built using 'O3' without any compilation failures. Additionally, per the following documentation: https://wiki.gentoo.org/wiki/Safe_CFLAGS#Haswell we used 'haswell' to specify CPU architecture type because we want stage4 to work best with a specific CPU architecture, in this case, Haswell.

What (if any) CPU_FLAGS_X86 were used?

We used the following instruction sets for CPU flags: "3dnow aes avx avx2 bmi bmi2 f16c fma3 fsgbase mmx mmxext movbe pclmul popcnt rdrnd sse sse2 sse3 sse4_1 sse4_2 ssse3" We've aggregated this information from running 'cpuinfo2cpuflags-x86' command and referring to the 'haswell' section in the following documentation: https://gcc.gnu.org/onlinedocs/gcc-6.4.0/gcc/x86-Options.html