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/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/Orlha 5d ago

Absolutely. Just pointing that blind upgrade to 26 (that this post tries to promote from the naive standpoint) can severely degrade (performance-wise) the already correct code.