4 ms·

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

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.

londons_explore 3y ago
you're gonna hive to give us a concrete real world example to convince most of us...
dahart 3y ago
> 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 3y ago
  > 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 3y ago
    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 3y ago
      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 3y ago
  https://github.com/llvm/llvm-project/issues/57589 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 3y ago
    That's only one small part of -ffast-math/-Ofast though and not a very scary one at that.
    - mort96 3y ago
      But it's an example of -ffast-math affecting separately compiled libraries.
deleted 3y ago
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londons_explore 3y ago

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

dahart 3y ago

> 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 3y ago
> 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 3y ago
  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 3y ago
    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 3y ago
https://github.com/llvm/llvm-project/issues/57589 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 3y ago
  That's only one small part of -ffast-math/-Ofast though and not a very scary one at that.
  - mort96 3y ago
    But it's an example of -ffast-math affecting separately compiled libraries.

deleted 3y ago

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