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Is that true? The compiler is being explicitly instructed to (try to) detect undefined behavior where it happens. This is the difference between tools like UBSan and tools like Valgrind. Valgrind is low-level, it’s a virtual x86, so the optimizer runs first. UBSan is high-level, it instruments the code using the compiler, and presumably the compiler can be directed to preserve the instrumentation.

UBsan is part of the compiler, so it inserts runtime checks before the optimization passes which rely on undefined behaviour run.

I see the same errors using Clang with -O1, -O2, and -O3.

At -O0, the compiler will generally not do any of that analysis.

GCC doesn’t enable -fdelete-null-pointer-checks until -O2, so even -O1 might work fairly well.