Re: [PATCH] x86: add more x86-64 micro-architecture levels

[Hanabishi]

On 9/18/24 21:14, John wrote:
> I am not sure.  Are some of the other things -march=-x86-64-v3 driving them?

Looking up a full table, v3 adds more than just AVX.

x86-64-v2:
CMPXCHG16B
LAHF-SAHF
POPCNT
SSE3
SSE4_1
SSE4_2
SSSE3

x86-64-v3:
AVX
AVX2
BMI1
BMI2
F16C
FMA
LZCNT
MOVBE
OSXSAVE

x86-64-v4:
AVX512F
AVX512BW
AVX512CD
AVX512DQ
AVX512VL

Maybe some other enabled instructions could issue some benefit.

v4 seems to be useless for us though.

> As to the code you referenced re: disabling the SIMD extensions.  Do you know why that is in place?

Not really. There is a link above pointing to a bug report discussing GCC quirks. I am not an expert in that.

One day, out of curiosity, I tried to override it and build the kernel with '-mavx' (free performance, yay!).
Well, it didn't even start and crashed immediately.

I don't know if something has changed since then, but I guess there are reasons.