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Comment by joatmon-snoo | Hacker News Reader

My first experience with UB:

    foo(bar(), bar());

(where bar() was stateful)

Discovering that g++ and clang++ compiled to one that did what I wanted and one that didn't was an interesting experience.

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That's not UB. Argument evaluation order is unspecified, not undefined.

He meant unspecified behavior. ;)

Starting with C++17 evaluation order in this case (and most other cases) is finally specified and standardized!

This is the accepted proposal:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p014...

The SGI compiler (now Open64) evaluates function arguments from right to left, unlike most other compilers. As a result we (PathScale) had to educate numerous users that the order is not guaranteed.

It gets even funnier in OCaml where they nominally defined it UB in order to get the freedom to make the only existing implementation always 100% reliably evaluate right-to-left, which reportedly was better for performance or something :)

And before somebody wonders about currying and eager evaluation, the problem is not functions but type constructors, which aren't curried.

The functions arguments are pushed onto the stack. From within the function you can think of the stack as an array.

    void test(a, b, c)
        stack[0] == a
        stack[1] == b
        stack[2] == c

But to get the items in the stack in that order you have to...

    puch c
    push b
    push a

If you wanted to do that, and still allow for left to right evaluation you could...

    temp_a = a
    temp_b = b
    temp_c = c

    push c
    puch b
    push a

That would be 2x instructions for every (already expensive at that time) function call. Defining unordered evaluation as UB was to avoid having to do temp_a, temp_b, temp_c. Now compiles can do that and trivially optimize that sort of thing out.

Except that in most cases that's not how it is done.

Instead, the stackframe is created in one go with a single change to the stack pointer and then the values are filled in in whatever order the compiler / language desires.

Here is a typical example using GCC:

        movq    %rsp, %rbp
        subq    $16, %rsp
        movl    $12, -4(%rbp)
        addl    $1, -4(%rbp)
        movl    -4(%rbp), %eax
        movl    $14, %edx
        movl    %eax, %esi
        movl    $12, %edi
        call    f

Which moves the parameters using registers (subject to availability) as a further optimization. The 'subq' reserves space for the stackframe with addresses relative to rbp being the parameters.

The ocaml one is very annoying because the bytecode interpreter reliably evaluates arguments left-to-right but when you compile to native code it reliably evaluates right-to-left (or the other way around, I don't remember).

I don't think so. Both bytecode and native evaluate right-to-left.

I swear that I remember having programs that could behave differently in the bytecode and native compiler. Perhaps I'm remembering an older version of the compiler or another order-of-evaluation issue other than function parameters?

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