It will be some time before we can drop support for all pre-10.12 OS X versions, so I'm afraid we can't rely on 10.12-only syscalls now.

But mach_absolute_time is available everywhere. The only problem is that I don't know if its Mach absolute time unit changes dynamically (in the lifetime of a process) or not. note, AbsoluteToNanosecond is deprecated, but we can use absolutetime_to_nanosecond instead. However, we still need to figure out how to reimplement its functionality in Go. I'm hoping it doesn't require kernel specific memory accesses.

mach_absolute_time is using rdtsc with some kernel provided magic numbers from comm page. // from xnu-3248.60.11.1.2~2 _mach_absolute_time: 00007fff5fc25726 pushq %rbp 00007fff5fc25727 movq %rsp, %rbp 00007fff5fc2572a movabsq $0x7fffffe00050, %rsi ## imm = 0x7FFFFFE00050 00007fff5fc25734 movl 0x18(%rsi), %r8d 00007fff5fc25738 testl %r8d, %r8d 00007fff5fc2573b je 0x7fff5fc25734 00007fff5fc2573d lfence 00007fff5fc25740 rdtsc 00007fff5fc25742 lfence 00007fff5fc25745 shlq $0x20, %rdx 00007fff5fc25749 orq %rdx, %rax 00007fff5fc2574c movl 0xc(%rsi), %ecx 00007fff5fc2574f andl $0x1f, %ecx 00007fff5fc25752 subq (%rsi), %rax 00007fff5fc25755 shlq %cl, %rax 00007fff5fc25758 movl 0x8(%rsi), %ecx 00007fff5fc2575b mulq %rcx 00007fff5fc2575e shrdq $0x20, %rdx, %rax 00007fff5fc25763 addq 0x10(%rsi), %rax 00007fff5fc25767 cmpl 0x18(%rsi), %r8d 00007fff5fc2576b jne 0x7fff5fc25734 00007fff5fc2576d popq %rbp 00007fff5fc2576e retq 00007fff5fc2576f nop I think it's the same as our current runtime.nanotime implementation.