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Use the 64b inner:monotonize() implementation not the 128b one for aarch64 aarch64 prior to v8.4 (FEAT_LSE2) doesn't have an instruction that guarantees untorn 128b reads except for completing a 128b load/store exclusive pair (ldxp/stxp) or compare-and-swap (casp) successfully. The requirement to complete a 128b read+write atomic is actually more expensive and more unfair than the previous implementation of monotonize() which used a Mutex on aarch64, especially at large core counts. For aarch64 switch to the 64b atomic implementation which is about 13x faster for a benchmark that involves many calls to Instant::now().