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minetest / irrlicht

Minetest's fork of Irrlicht
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Keep RTTI for Minetest IPO/LTO #268

Closed okias closed 10 months ago

okias commented 11 months ago

Minetest using RTTI, so we cannot apply the flag here if we want to start using IPO/LTO.

If we keep -fno-rtti here, compiler loses part of informations and cannot do efficient optimizations.

The benefit of not using RTTI on Irrlicht disappears with when Minetest and Irrlicht gets IPO/LTO linked together.

sfan5 commented 11 months ago

Not that it ultimately matters but this is my first time hearing of RTTI being relevant for optimization/LTO and I couldn't find anything on it. Do you have some reference material for me?

okias commented 11 months ago

Do you have some reference material for me?

The best I see is https://stackoverflow.com/questions/15101859/subclassing-class-from-shared-library-compiled-with-fno-rtti/15213718#15213718

Without LTO: We compile Irrlicht, we compile minetest, and link it together. That's all. Linker doesn't care (two types of separate objects in one file)

With LTO: Both code gets compiled and at linking time another set of optimizations is being done. The linker sees one set of definitions from minetest (RTTI) objects and a second from Irrlicht (no-RTTI), which makes it impossible to do right optimizations + triggers ODR warnings because it sees two a bit different definitions.

sfan5 commented 10 months ago

Hm, that's a bit weird because I'm pretty sure we don't subclass most types.

To be clear, did you check that the warnings disappear when enabling RTTI in Irrlicht?

okias commented 10 months ago

To be clear, did you check that the warnings disappear when enabling RTTI in Irrlicht?

Yup.