[1/*] I wanted to post a brief illustration of how hard it is to use CLANG when you're trying to write anything that needs to be carefully optimized. I am not cherry-picking this - this happens to me literally all the time with CLANG.
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[12/*] I am not an optimization person. I maybe spend a week every three months looking at ASM. This has happened to me _three times in the last year_. It's almost 100% of the time I've looked at CLANG ASM, I have hit something like this.
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[13/*] I'm not sure what the solution to this problem is. But I do think it would be a nice start for people to recognize that it _is_ a problem, because I think it really is.
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[14/*] As an example of a possible direction to go, perhaps we could have a new directive to the compiler that you could bracket pieces of code with that you have crafted carefully and analyzed.
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[15/*] This directive could do some things, like limit reordering, try to translate intrinsics as directly as possible, etc., with an emphasis on _predictability_ rather than speed.
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[16/*] This would allow programmers to have some confidence that when they've worked out the best way to do something, they can lock that in and know it won't get undone by crazy optimizer steps.
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[17/*] The only alternative to something like this would be to write these parts in inline assembler, which honestly, would be fine with me - except the syntax for inline assembler is so horrid that it makes it very unpleasant to use.
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[18/*] So another option would be to get serious about inline assembler, and make a pleasant-to-use, well specific ASM syntax that can be placed in C. I'd be fine with either.
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[19/*] What I'm increasingly less fine with is constantly having to struggle with CLANG to get it to _stop_ doing crazy things to routines that were already basically optimal if it just translated them directly. It's very frustrating, and sometimes literally can't be fixed :(
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[20/*] That's all. I figured this has happened enough times now that I should post something about it.
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In Haskell, where optimisations can also vary widely between versions, we use a tool called "inspection-testing" where instead of testing that the asm is exactly as expected (which is fragile), we test that the high-level and manually-optimized code compile to the same thing.
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Not sure I completely understand... is there a writeup somewhere?
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