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Comment by _wmd | Hacker News Reader

Linux hit a related situation: a harmless null pointer dereference was treated by GCC as a signal that a subsequent isnull test could not be true, causing the test to be optimized away. https://lwn.net/Articles/575563/

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My opinion on that, is that such code MUST NOT be optimized away. Instead it should be a compile error.

You might wish for that, but the ship has sailed. Undefined behavior means that the implementation can do whatever it can. That said, I do expect tools, both sanitizers and static analyzers to improve to detect more of these kinds of cases.

Well, not exactly. There are things that are UB according to the standard but that particular compilers give an option to make defined: see `-fwrapv`, for example.

There have been static analyzers that will detect this for years. They report "check for null after use" or some such.

The original intention of standardization was that compilers would gradually reach consensus on what the behaviour in certain cases should be, and once that happened the standard would be updated to standardize that behaivour. Compilers are allowed - indeed encouraged - to provide additional guarantees beyond the minimum defined in the standard (indeed part of the point of UB is that a compiler is allowed to specify what the behaviour is in that case).

The problem, as far as I understand it (though I’m a layman), is that by the time the dead code optimization pass runs, the code has been transformed so much that there’s no obvious way for the compiler to tell the difference between “obvious programmer-intended null check that we shouldn’t optimize out” and “spurious dead code introduced by macro expansion” or (in C++) “by template instantiation”.

Couldn't user defined branches be tagged by such a compiler and if a tagged branch is eliminated the error generated with a reference to the tagged line in question?

That is a good idea and I’ll admit that I’m not sure why it isn’t implemented.

Why should it be a compile error? The pointer may be null, but is not guaranteed to be.

If you mean that C++ should require a null check before dereferencing any pointer that is not guaranteed to be non-null, then that would break most existing C++ code out there, so it's a non-starter.

in the particular situation they're talking about, you have a pointer to a struct, which you dereference by accessing one of its fields. the null check happens after the dereference, almost certainly a mistake.

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