minor improvements#328
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When an enqueue fails due to racing with a dequeue, it needs to undo the insertion of a state into the upcoming queue tail. This CAS is guaranteed to succeed. So there is no need to take the overhead of a `CMPXCHG` in this path. We can instead rephrase this code to preserve the assertion but weaken the operations involved into non-atomic `MOV`s. Profiling with some medium sized models indicates this has no significant effect on performance. But it may help models that have a different pattern where threads often contend on the same queue.
The style this code is written in is intended to prioritise:
1. White space independence (e.g. no // comments). This code is
currently copied as-is into the output but in future we might do
some sort of white space coalescing.
2. Density/readability. Hence why this change removes bracing on
single-line ifs, gaining back a line of display.
I think eventually I will probably decide this does not matter and accept all clang-format suggestions. But I am still in some phase of cope.
On x86-64, users should be compiling with `-mcx16`. But if they do not, double-word CAS is implemented by a libatomic call. In this case it is unsafe to use AVX stores/loads for double-word write/read (respectively) because, although they are atomic, they do not synchronise with the (possibly lock acquiring) double-word CAS.
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