Infinity and NaN can also be a performance problem but it depends on your processor. Though of course if you get those during matrix multiplication something has probably gone wrong.

The point of NaNs and infinities is that sometimes it is easier/faster/cleaner to just let those things propagate through the calculation than to do conditional logic that in the end just tends to just simulate the same effect.

True, that consideration is more valid for element-by-element operations rather than things like matrix multiplications. But depending on the operation it might make sense to rely on NaN propagation rather than trying to prescan you matrix for NaNs.

The only instructions you can’t set to flush are the legacy x87 opcodes, which you shouldn’t be using in a performance-sensitive context anyway.

    #define CSR_FLUSH_TO_ZERO         (1 << 15)
    unsigned csr = __builtin_ia32_stmxcsr();
    csr |= CSR_FLUSH_TO_ZERO;
    __builtin_ia32_ldmxcsr(csr);