Linus Torvalds on aliasing

I never really understood why people complain about Linus personality. If you don't like it go somewhere else nobody is forcing you to code for Linux.

Fork it and make your own.

Theo de Raadt has this potential on the alternative timeline.

I'm in no way offended by Torvalds' typical expletive-laden rants. But they do strike me as a terrible way to communicate.

These outbursts seem to happen every few weeks, and have long seized to surprise. IF anything actually bad ever comes up, he has no further escalation available to raise awareness.

It also reminds me of these choleric bosses screaming at someone every day: it's almost always a sign that they're insecure, sociopaths, or can't handle their responsibilities.

Suppose you want to look at the bits of a floating point number, as though it were an integer. The type punning way to do that would be to access a double in memory through an integer pointer:

    double d = 3.14;
    int64_t *p = (int64_t *) &d; // !!!
    int64_t n = *p;

The union aliasing way would be to makeset the double field in a union and read the integer field back:

    typedef union {
        double d;
        int64_t n;
    } int_or_double;

    int_or_double u;
    u.d = 3.14
    int64_t n = u.n

According to the standard, the union aliasing approach is allowed, but the type punning one is undefined behavior.

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The standard team decided to forbid type punning to allow compilers to implement more optimizations. They still allow union aliasing because they felt there should still be a way to do low-level type casts if for those that absolutely want to.

Several compiler optimizations center around proving which variables don't change. Allowing type punning makes it harder for the compiler to do this.