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Significant compression speed diffs between Clang and GCC in long mode

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optimization
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Description

As noted in https://www.phoronix.com/scan.php?page=article&item=clang12-gcc11-icelake&num=2, there is a huge difference in compression speed between clang and gcc in long mode.

![image](https://user-images.githubusercontent.com/22410393/120888705-1af9b380-c5fa-11eb-9264-eaac5359957d.png)

Standard mode looks kinda fine.

![image](https://user-images.githubusercontent.com/22410393/120888763-71ff8880-c5fa-11eb-918e-be532647b9ed.png)

Missed vectorization? Any chance to improve it?

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