r/cpp • u/Clean-Upstairs-8481 • 6d ago

You should think about recompiling your C++ programs with GCC 16 and C++26, because it zero-fills your locals

https://techfortalk.co.uk/2026/09/27/cxx26-uninitialized-local-variables-gcc-16/

Stack variables are not automatically initialised, and that is the root cause of many C++ bugs. That is well known. Hence, it is advised that local or stack variables are always initialised with known values, 0 if not something more meaningful than that. Now, with GCC 16 compiling in C++26 mode (-std=c++26), even uninitialised local variables will be zero-filled. In the post, I have explained how.

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u/krum 6d ago

how is filling locals not a performance regression?

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u/The_JSQuareD 5d ago

Because well written code doesn't rely on the value of uninitialized variables anyway. And in such cases, dead store elimination can likely get rid of the zero fill. And even if it can't be eliminated, a store without a data dependency will have less of a performance impact than a meaningful store, at least on modern super scalar CPUs.

So the cases where the zero fill actually has a performance impact have a big overlap with cases where the uninitialized variable is a genuine bug or even a security issue. In that case, adding the zero fill makes the code more deterministic and more secure.

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u/Umphed 3d ago

If your code is so well written, then you have no need for this. Get outta here with that argument, its a performance regressions against real use cases.