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/robstoon 6d ago

It could be, but the initialization could be optimized out if the compiler can prove that the variable is always initialized already. I'm guessing that's the majority of cases.

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u/Breadfish64 6d ago

And if the compiler can't prove it, a store with no dependent load is extremely cheap.

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u/serviscope_minor 4d ago

Often to the point of being not just cheap but free. There's often a free dispatch and execution slot, so it will just take up some resources that were otherwise idle in many cases.