So it was a bit more pervasive than this, the issue was that flushing subnormals (values very close to 0) to 0 is a register that gets set, so if a library is built with the fastmath flags and it gets loaded, it sets the register, causing the whole process to flush it's subnormals. i.e https://github.com/llvm/llvm-project/issues/57589

> This would only affect code that uses the old-school x87 floating point instructions, though?

Actually, no, the x87 FPU instructions are the only ones that won't be affected.

It sets the FTZ/DAZ bits, which exist for SSE instructions but not x87 instructions.

You're mistaking something else for the rounding mode and subnormal handling flags.