[planede]

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.

[dahart]

> 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?

[mort96]

> 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.

[dahart]

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?

[mort96]

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.

[jcranmer]

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!

[account42]

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

[mort96]

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

[londons_explore]

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