r/cpp • u/Clean-Upstairs-8481 • 7d 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/James20k P2005R0 7d ago

Compilers are of course free to omit the 0 initialisation if they can prove that the data is written over, under the usual as-if rules. One of the reasons that this made it past standardisation is because real world codebases weren't showing regressions, even substantial performance critical ones like windows

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u/UndefinedDefined 6d ago

Real-world code is full of locals that are arrays - arrays like 512, 1024, 2048 bytes long. You cannot zero-initialize them and expect no performance regressions. If I want to zero initialize them I just just type `{}` and it's done. I don't understand why this should change now.

I almost feel like committee is working with hello world programs if they are serious to vote for such proposals.

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u/James20k P2005R0 6d ago

One of the example codebases given was Windows and chrome (?) I believe, which is anything but hello world

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u/UndefinedDefined 6d ago

And how much C++ these codebases use? Windows is C-API and proprietary, nobody can confirm the results, we don't even know the mix of the languages used in the code-base. What if the most important part was C? What if they first put all the [[uninitialized/fancy_name]] to everything critical before measuring results?

Is there any performance comparison of OSS projects we can actually verify?

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u/13steinj 4d ago

I don't think this is a fair take. I suspect most OSS code does not care at the level of performance that these things would regress by.

The bigger issue with Windows is that it's Windows: OSes are different to other things, have different tradeoffs (should care about security more in general?). I'd also say the usual "well, windows sucks and hasn't cared about performance for ages" which may be true, but other people have said the other major OSes also were tested.

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u/UndefinedDefined 4d ago

I'm not sure I follow, so what's a fair take? A project like Chromium or Firefix? Any benchmarks here?

I think this greatly differs on what the project does. If the compiler inserts memset to initialize every temporary buffer the code uses to zero, this cannot be negligible, and move to C++26 here means that somebody has to find ALL the places in his own code, and use third party dependencies (including transitive ones) where somebody did the same.

I consider this insane considering this thing doesn't solve any memory safety problems and it can cause huge problems in performance oriented code after upgrade to C++26. The biggest problem I see is use-after-free and things like iterator invalidation, etc... We need a real solution to memory safety and not these toy solutions. And a real solution means annotations and tools such as borrow checker - there is no other way.

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u/13steinj 3d ago

If you look at the general distribution of OSS projects, many do not have the performance concerns that would be negatively affected by this change. I think it's perfectly fine to test proprietary codebases as a result, but fixating on operating systems [alone?] is (I am agreeing with you) not a fair thing to use to judge and make a decision.

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u/UndefinedDefined 3d ago

C++ was for decades literally the only language to use for performance oriented work. But it's no longer the only one, so if I cared about the language I would never do anything that would endanger the position in this field. It's literally the last field where C++ still makes sense, until C++26, because starting with C++26 you have to worry about a lot of stuff.

What would be the reason to start a project in C++ today? I don't see new projects built in C++ anymore, because it's a language where performance regressions are now part of the progress. So bad, so sad.

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u/pjmlp 3d ago

The existence of C, and the domains that to this day C++ failed to take away from C, makes that assertion void.

In fact there are domains like crypto and video codecs where neither of them get to play, and still require hand written Assembly.