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Discussion on: What was the weirdest bug you ever encountered?

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Dave Cridland

Personally, the weirdest and hardest to solve bugs for me are when the C++ optimizer takes a shortcut that ends up ignoring my code. These are usually cases where, for example, a pointer cannot be NULL according to the standard and therefore my NULL check is ignored.

I know of two other bugs which are gloriously Byzantine. I don't know Trey Harris, but his bug is legendary: The 500 Mile Email

I do know (and used to work for) Dave Baggett, and have heard this story first-hand. My Hardest Bug Ever

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Michel Renaud

Those optimizers can be sneaky. I don't know how long you've been doing this, but I remember when the first C and C++ optimizing compilers for PC came out (late '80s, roughly) that they did a lot of nasty things and you had to be very careful with what optimizations you picked. Let's just say it was a rocky start.

I was on a project with some C++ just three years ago and it's a good thing my supervisor was borderline genius because he caught VC++ doing some really strange things. The ways he figured it out was beyond the skills of pretty much everyone else I know.

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Dave Cridland

I've been using C/C++ since around 1990 or so. The old problems were usually weird compiler bugs when optimizations went wrong dramatically, from what I can recall. But I was thinking more of cases like this, which tripped me up recently - I'll simplify, of course:

int * p = nullptr; // Start with a NULL pointer.
int & r = *p; // Later on, convert to a reference, probably in a function call.
int * i = &r; // Convert the reference back to a pointer.
if (i != nullptr) { // If it's not NULL
  std::cout << "i is now " << *i << std::endl; // Print it.
}

That code crashes with a segfault trying to deference a NULL pointer. This is because the standard says that if the address of a reference is the NULL pointer, it's Undefined Behaviour. So most modern compilers now choose to optimize away the test, even without optimization turned on...

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Michel Renaud

"Undefined behaviour" is hell.

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Andreas Author

Thank you so much for sharing! The 500 mile email is indeed legendary! 😂