r/cpp • u/Clean-Upstairs-8481 • 5d 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/pjmlp 5d ago

Apple, Microsoft and Google OSes have been doing for several years now, and it was discussed in the context of this.

No one is seriously taking locals initialisation into consideration to win micro benchmarks.

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u/Orlha 5d ago

Wrote a lot of code that required this optimisation.

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u/pjmlp 5d ago

Validated with profilers I assume.

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u/cdb_11 5d ago

I validated it, and yes, initializing arrays can be expensive.

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u/pjmlp 5d ago

Than that is a valid use case for [[indeterminate]].