The Dangers of Super Smart Compilers

Re:The Dangers of Super Smart Compilers by mikeash on Sunday December 20, 2015 @12:54PM (#10767684)

This function can invoke undefined behavior:

    int f(int x) { return x + 1; }

So can this one:

    int get(struct Something *s) { return s->field; }

Figuring out when undefined behavior needs a warning and when it's unnecessary because it's known by the programmer to be impossible is a Hard Problem. Especially since the compiler has no idea whether the programmer wrote f() knowing that they'll never pass in INT_MAX, or whether the programmer actually does expect to pass in INT_MAX and expects to receive INT_MIN as the result.

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Re:The Dangers of Super Smart Compilers by Spivak on Sunday December 20, 2015 @06:19PM (#10768732)

In practice, does anyone really assume that their system doesn't have two's compliment overflow behavior?

Hell during my intro to C course I took at university they told me I should actually detect overflow by seeing if the signs change.

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Re:The Dangers of Super Smart Compilers by mikeash on Sunday December 20, 2015 @07:30PM (#10768929)

I sure hope a lot of people assume their system doesn't have two's compliment overflow behavior, because that is absolutely not guaranteed to be the case, and there are real-world C compilers which will not exhibit that behavior in all circumstances.

For example, since overflow is undefined, the compiler can assume that an expression like x > x + 1 is always true. Testing with the version of clang that comes with Xcode 7.2, it does this when compiling with -O3.

(Note that this is only true of signed overflow. Unsigned overflow is defined behavior, and all unsigned arithmetic is done modulo 2^n where n is the number of bits in the type.)

The underlying machine is, of course, two's complement. But nothing says the compiler must compile code to match how the underlying machine behaves.

Unfortunately, whoever taught your intro to C course apparently didn't understand C. This is a frighteningly common occurrence, of course. Most people seem to learn C is a very inconsistent way, and imagine a lot of things about it rather than learning how the language actually works.

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