Introducing MIR

Re:Introducing MIR by munificent on Wednesday April 27, 2016 @04:28PM (#11584241)

I actually use negative indices in real code[1].

If you implement a stack using an array and you track the top of stack by a pointer just past the last element, then you can index negatively into that pointer to access items on the stack:

    // Create an empty stack.
    int stack[100];
    int* top = &stack;

    // Push a couple of values.
    *top++ = 123;
    *top++ = 456;

    // Peek the values.
    printf("%d\n", top[-1]); // 456.
    printf("%d\n", top[-2]); // 123.

You could make the pointer arithmetic more explicit like this:

    printf("%d\n", *(top - 1)); // 456.

But I think that's needlessly verbose.

[1]: https://github.com/munificent/wren/blob/master/src/vm/wren_v...

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Re:Introducing MIR by jeffdavis on Thursday April 28, 2016 @03:55AM (#11587136)

For something like that, I would use the explicit pointer arithmetic. The array syntax implies (to me) "I am doing the normal thing with this normal array".

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