Go's Escape Analysis and Why My Function Return Worked

Assuming to everything allocates on the heap, will solve this specific confusion.

My understanding is that C will let you crash quite fast if the stack becomes too large, go will dynamically grow the stack as needed. So it's possible to think you're working on the heap, but you are actually threshing the runtime with expensive stack grow calls. Go certainly tries to be smart about it with various strategies, but a rapid stack grow rate will have it's cost.

Why? It is the same as in C.

    #include <stdio.h>
    #include <stdlib.h>

    struct slice {
        int *data;
        size_t len;
        size_t cap;
    };

    struct slice read() {
        int *data = malloc(2);
        data[0] = 1;
        data[1] = 2;
        return (struct slice){ data, 2, 2 };
    }

    int main() {
        struct slice s = read();
        for (int i = 0; i < s.len; i++) {
            printf("%d\n", s.data[i]);
        }
        free(s.data);
    }

But yeah, to your point, returning a slice in a GC language is not some exotic thing.

Since 1.17 it’s not impossible for escape analysis to come into play for slices but afaik that is only a consideration for slices with a statically known size under 64KiB.

That said, when it matters it matters a lot. In those times I wish it was more visible in Go code, but I would want it to not get in the way the rest of the time. But I’m ok with the status quo of hunting down my notes on escape analysis every few months and taking a few minutes to get reacquainted.

Side note: I love how you used “from above” and “from below”. It makes me feel angelic as somebody who came from above; even if Java and Ruby hardly seemed like heaven.

even in C, the concept of returning a pointer to a stack allocated variable is explicitly considered undefined behavior (not illegal, explicitly undefined by the standard, and yes that means unsafe to use). It be one thing if the the standard disallowed it.

but that's only because the memory location pointed to by the pointer will be unknown (even perhaps immediately). the returning of the variable's value itself worked fine. In fact, one can return a stack allocated struct just fine.

TLDR: I don't see what the difference between returning a stack allocated struct in C and a stack allocated slice in Go is to a C programmer. (my guess is that the C programmer thinks that a stack allocated slice in Go is a pointer to a slice, when it isn't, it's a "struct" that wraps a pointer)

Back in Python 2.1 days, there was no guarantee that a locally scoped variable would continue to exist past the end of the method. It was not guaranteed to vanish or go fully out of scope, but you could not rely on it being available afterwards. I remember this changing from 2.3 onwards (because we relied on the behaviour at work) - from that point onwards you could reliably "catch" and reuse a variable after the scope it was declared in had ended, and the runtime would ensure that the "second use" maintained the reference count correctly. GC did not get in the way or concurrently disappear the variable from underneath you anymore.

Then from 2008 onwards the same stability was extended to more complex data types. Again, I remember this from having work code give me headaches for yanking supposedly out-of-scope variable into thin air, and the only difference being a .1 version difference between the work laptop (where things worked as you'd expect) and the target SoC device (where they didn't).

And these days, if you're bothering with C you probably care about these things. Accidentally promoting from the stack to the heap would be annoying.