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sparkie a month ago  [ - ]

Defer is a restricted form of COMEFROM with automatic labels. You COMEFROM the end of the next `defer` block in the same scope, or from the end of the function (before `return`) if there is no more `defer`. The order of execution of defer-blocks is backwards (bottom-to-top) rather than the typical top-to-bottom.

    puts("foo");
    defer { puts("bar"); }
    puts("baz");
    defer { puts("qux"); }
    puts("corge");
    return;
Will evaluate:

    puts("foo");
    puts("baz");
    puts("corge");
    puts("qux");
    puts("bar");
    return;
vlowther a month ago  [ - ]

That is the most cursed description I have seen on how defer works. Ever.

sparkie a month ago  [ - ]

This is how it would look with explicit labels and comefrom:

    puts("foo");
    before_defer0:
    comefrom after_defer1;
    puts("bar");
    after_defer0:
    comefrom before_defer0;
    puts("baz");
    before_defer1:
    comefrom before_ret;
    puts("qux");
    after_defer1:
    comefrom before_defer1;
    puts("corge");
    before_ret:
    comefrom after_defer0;
    return;
---

`defer` is obviously not implemented in this way, it will re-order the code to flow top-to-bottom and have fewer branches, but the control flow is effectively the same thing.

In theory a compiler could implement `comefrom` by re-ordering the basic blocks like `defer` does, so that the actual runtime evaluation of code is still top-to-bottom.