this post was submitted on 17 Aug 2026
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C/C++ compilers are non deterministic due to support of super macros that change run to run, non-deterministic optimisation strategies or ordering due to parallelism, and linkers often produce different outputs every time they are run where subtle bugs can cause crashes when addresses don't line up how you expect. And that's without mentioning projects that use a configuration step.
It's actually a big problem in producing reproducible builds for security.
Can't most serious compilers produce reproducible builds these days given the same build environment. I know there has been a drive towards reproducible builds in general for security verification purposes.
I’m not very good with C/C++ so please correct me, isn’t that what’s called a “race condition”? Parallelism can cause non-determinism but not in the same sense LLMs generate non-deterministic output. Compilers are not statistical machines.
You don't need parallelism to have a race condition, just not handling an event with expected timing can cause one - like when two keys are pressed within one polling cycle and you depend on one being pressed before the other for some logic like up and right arrow for a diagonal but they register as right and up so the diagonal movement doesn't trigger
Compiler optimisation strategies sometimes use statistical machines and link time optimisation does use random number generators for producing output