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 planede on Tuesday January 16, 2024 @10:24AM (#39014261)

You can't fully disable fast-math per-library, moreover a library compiled with fast-math might also introduce inaccuracies in a seemingly unrelated library or application code in the same executable. The reason is that fast-math enables some dynamic initialization of the library that changes the floating point environment in some ways.

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Re:Std: Clamp generates less efficient assembly than std:min... by dahart on Tuesday January 16, 2024 @10:46AM (#39014620)

> You can’t fully disable fast-math per library

Can you elaborate? What fast-math can sneak into a library that disabled fast-math at compile time?

> fast-math enables some dynamic initialization of the library that changes the floating point environment in some ways.

I wasn’t aware of this, I would love to see some documentation discussing exactly what happens, can you send a link?

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Re:Std: Clamp generates less efficient assembly than std:min... by jcranmer on Tuesday January 16, 2024 @11:12AM (#39014997)

https://github.com/llvm/llvm-project/issues/57589

Turn on fast-math, it flips the FTZ/DAZ bit for the entire application. Even if you turned it on for just a shared library!

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Re:Std: Clamp generates less efficient assembly than std:min... by account42 on Wednesday January 17, 2024 @05:02AM (#39025719)

That's only one small part of -ffast-math/-Ofast though and not a very scary one at that.

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Re:Std: Clamp generates less efficient assembly than std:min... by mort96 on Wednesday January 17, 2024 @06:39PM (#39035225)

But it's an example of -ffast-math affecting separately compiled libraries.

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Re:Std: Clamp generates less efficient assembly than std:min... by mort96 on Tuesday January 16, 2024 @10:53AM (#39014711)

> Can you elaborate? What fast-math can sneak into a library that disabled fast-math at compile time?

A lot of library code is in headers (especially in C++!). The code in headers is compiled by your compiler using your compile options.

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Re:Std: Clamp generates less efficient assembly than std:min... by dahart on Tuesday January 16, 2024 @10:55AM (#39014749)

Ah, of course, very good point. A header-only library doesn’t have separate compile options. This is a great reason for a float-sensitive library to not be header-only, right?

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Re:Std: Clamp generates less efficient assembly than std:min... by mort96 on Tuesday January 16, 2024 @11:02AM (#39014852)

It's not just about being header-only, lots of libraries which aren't header-only still have code in headers. The library may choose to put certain functions in headers for performance reasons (to let compiler inline them), or, in C++, function templates and class templates generally have to be in headers.

But yeah, it's probably a good idea to not put code which breaks under -ffast-math in headers if possible.

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Re:Std: Clamp generates less efficient assembly than std:min... by londons_explore on Tuesday January 16, 2024 @10:37AM (#39014459)

you're gonna hive to give us a concrete real world example to convince most of us...

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