It uses the CPUID instruction to get the CPU brand, family, model number and stepping. That's the easy part. Knowing what to do with the model number is the hard part. I was not able to find any table of family, model and stepping numbers for CPUs. I had to go by various random reports I saw.
Here's what I ended up with:
// Nehalem = i3/i5/i7 and some Xeon
// K10 = Opterons with 4 or more cores, Phenom, Phenom II, Athlon II
// Intel Core i5 family 6, model 26 or 30
// Intel Core i7 family 6, model 26 or 30
// Intel Core i3 family 6, model 37
// AMD Phenom family 16, model 10
bool fUseSSE2 = ((fIntel && nFamily * 10000 + nModel >= 60026) ||
(fAMD && nFamily * 10000 + nModel >= 160010));
I saw some sporadic inconsistent model numbers for AMD CPUs, so I'm not sure if this will catch all capable AMDs.
If it's wrong, you can still override it with -4way or -4way=0.
It prints what it finds in debug.log. Search on CPUID.
This is only enabled if built with GCC.