Std: Clamp generates less efficient assembly than std:min(max,std:max(min,v))

Re:Std: Clamp generates less efficient assembly than std:min... by jeffbee on Tuesday January 16, 2024 @08:32AM (#39013066)

Clang generates the shortest of these if you target sandybridge, or x86-64-v3, or later. The real article that's buried in this article is that compilers target k8-generic unless you tell them otherwise, and the features and cost model of opteron are obsolete.

Always specify your target.

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Re:Std: Clamp generates less efficient assembly than std:min... by josephg on Tuesday January 16, 2024 @09:20AM (#39013490)

Yep. Adding "-C target-cpu=native" to rustc on my desktop computer consistently gets a ~10-15% performance boost compared to the default target. The default target is extremely conservative. As far as I can tell, it doesn't take advantage of any CPU features added in the last 20 years. (The k8 came out in 2003.)

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Re:Std: Clamp generates less efficient assembly than std:min... by wongarsu on Tuesday January 16, 2024 @09:54AM (#39013885)

Red Hat Enterprise Linux has upgraded their default target to x86-64-v2 and is considering switching to x86-64-v3 for RHEL 10 (which should release around 2026?). I'd take that as a sign that those might be reasonable choices for newly released software.

Some linux distros also give you the option to either get a version compatible with ancient hardware or the optimized x86-64-v3 version, which seems like a good compromise.

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Re:Std: Clamp generates less efficient assembly than std:min... by jeffbee on Tuesday January 16, 2024 @09:28AM (#39013577)

Those Gentoo people were onto something.

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Re:Std: Clamp generates less efficient assembly than std:min... by alexey-salmin on Tuesday January 16, 2024 @10:01AM (#39013971)

Funny that it stopped being the case for a while around 2006. AMD64 became widespread while also being very new, closing the gap between "default" and "native".

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Re:Std: Clamp generates less efficient assembly than std:min... by skykooler on Tuesday January 16, 2024 @02:01PM (#39017351)

Of course, gentoo just started using prebuilt packages a few months ago…

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Re:Std: Clamp generates less efficient assembly than std:min... by x1f604 on Thursday January 18, 2024 @04:07AM (#39039616)

Even with -march=x86-64-v4 at -O3 the compiler still generates fewer lines of assembly for the incorrect clamp compared to the correct clamp for this "realistic" code:

https://godbolt.org/z/hd44KjMMn

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