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0 pointsplanede2y ago0 comments

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.

0 comments

dahart2y 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?

mort962y 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.

dahart2y 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?

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jcranmer2y ago

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!

account422y ago

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

1 more reply

londons_explore2y ago

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

j / k navigate · click thread line to collapse

0 comments

dahart2y 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?

mort962y 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.

dahart2y 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?

1 more reply

jcranmer2y ago

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!

account422y ago

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

1 more reply

londons_explore2y ago

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

j / k navigate · click thread line to collapse