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Even in the middle of that turmoil, we managed to compile some code with Intel's ICC and make it go faster on AMD Opterons, breaking Intel's own numbers.

When my colleague said that they managed to go faster than intel with icc with some hand tuned parameters, I remember answering "youdidwat?".

Good times.

Also MKL:

https://danieldk.eu/Intel-MKL-on-AMD-Zen

Is this for the runtime of the compiled code or for the compiling machine? Do they generate slow code if the compiler is running on non-intel?

the runtime. patching cpuid makes the code go faster

For the compiled code. Its output deliberately runs slower on non-Intel CPUs.

Runtime of the compiled code. The ostensible intent is so that new processors can use new features like SIMD, while offering a fallback for older ones. In practice, they’re detecting an Intel processor, not just the specific feature.