Using picolibc with libstdc++_nano.a

[Damian-Nordic]

Hi,

I wanted to switch a bunch of C++ applications from newlibc nano to picolibc but I noticed it resulted in ~4 kB flash usage growth. I realized it was because nano.specs replaces -lstdc++ flag with -lstdc++_nano, while picolibc.specs does not. The full libstdc++ contains a lot of code for handling exceptions and stack unwinding, which is often not needed in embedded applications.

I forced the build system to use -lstdc++_nano by adding set_linker_property(PROPERTY c++_library "-lstdc++_nano") in CMakeLists, and the flash usage dropped by ~10kB.

Would it make sense to default to lstdc++_nano when linking with picolibc or at least support the nano variant to be used by applications that disable C++ exceptions?

[keith-packard]

Good idea. We'll have to re-build libstdc++ to use the picolibc ABI though; libstdc++_nano is built against newlib-nano, which has a different ABI.

[LuDuda]

This looks like a pretty nice optimization!

@keith-packard would you be able to help with re-building process? Is this something we can plan for next releases of the Zephyr toolchain?

[keith-packard]

This looks like a pretty nice optimization!

@keith-packard would you be able to help with re-building process? Is this something we can plan for next releases of the Zephyr toolchain?

It's pretty easy to adapt crosstool-ng to build another version of any library; it would be great if you'd dig in and take a look; I'd be happy to provide some guidance.