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Peter Anvin" References: <20240915124944.GAZubX6LAcjQjN-yEb@fat_crate.local> <90d5a756-e534-490b-b451-7c855183ebc3@gmail.com> <96f609c7-f185-49c8-a9d5-a26bfd093b09@gmail.com> From: Hanabishi In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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.