4 ms·
Another PSA is that dynamic libraries compiled with fast-math will also introduce inaccuracies in unrelated libraries in the same executable, as they introduce
by planede 3y ago
Another PSA is that dynamic libraries compiled with fast-math will also introduce inaccuracies in unrelated libraries in the same executable, as they introduce dynamic initialization that globally changes the floating point environment.
- pavlov 3y agoThis would only affect code that uses the old-school x87 floating point instructions, though? The x87 FPU unit indeed has scary global state that can make your doubles behave like floats in secret and silence. I would think practically all modern FPU code on x86-64 would be using the SIMD registers which have explicit widths.
- borodi 3y agoSo 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 https://github.com/llvm/llvm-project/issues/57589
- jcranmer 3y ago> 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.
- mhh__ 3y agoYou're mistaking something else for the rounding mode and subnormal handling flags.
- deleted 3y ago[deleted]