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Comment by wereHamster | Hacker News Reader

It's not obvious. If it was then GCC would not do it. Please don't use the word 'obvious' when it is to you but not to the majority of population.

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It is obvious, though I meant "obvious and correct" not "obviously correct"

Who can clarify? Google says: Qbvious - easily perceived or understood; clear, self-evident, or apparent.

The confusion is between "the assembly clang generates is the obviously correct assembly for this operation" (true) and "the obviously correct behaviour for a compiler is to generate the assembly that clang does" (disputable, as while the assembly GCC generates is less clear, there may be advantages to doing it that way in e.g. performance). "Clang does the obvious, correct thing." is ambiguous between these two meanings.

I disagree with "ambiguous", you sneakily changed "obvious" to "obviously" in the second sentence.

  obvious, correct != obviously correct

Sounds like by "obvious" the parent means that Clang uses the most trivial approach.

Have you looked at the actual GCC codebase? It's very easy to say something is obvious when you're looking at a problem which someone else has nicely isolated; it's much harder to dive into a complex codebase which has a very wide support matrix and say it's worth the effort to change working code instead of so many other things.

More bluntly, before now wouldn't most people have said it was “obvious” that Intel would support their own documented features?

The code produced by Clang is a direct translation of the C code. That's the obvious part of it

Most people here are not familiar with x86 assembly and its caveats it seems. Reading the Intel and AMD optimization manuals might be a good start

(and yes, the bug is not in GCC it's on the Intel processor)

> Most people here are not familiar with x86 assembly and its caveats it seems. Reading the Intel and AMD optimization manuals might be a good start.

Please don't make unsupported assertions that everyone but you is speaking out ignorance. It doesn't add anything to the conversation, especially when dealing with older codebases unless you can prove that this is and never has been the correct way to write that code. Otherwise it's just another way to say “CPU optimizations change over time and an open-source project doesn't have a team of experts tracking microbenchmarks to decide when to switch”.

Should be easy to prove me wrong if it's as easy as you say, I don't see why you're so annoyed by it

> “CPU optimizations change over time and an open-source project doesn't have a team of experts tracking microbenchmarks to decide when to switch”

Yes, that's what's happening, but the problem comes from the P6 architecture (though Netburst doesn't have those problems), this problem has been known for around 20 years

I suppose the choice of word was to describe what code a human would intuitively/idomatically write for this situation. Whereas compiler passes are often wont to sum up to unintuitive/non-obvious code that is "correct" in that it's a valid lowering of the higher level code but not likely what a human would've written. This can sometimes be a pro or a con in terms of the resulting code's performance.

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