[aw1621107]

I had always thought expression templates at the very least needed the optimizer to inline/flatten the tree of function calls that are built up. For instance, for something like x + y * z I'd expect an expression template type like sum<vector, product<vector, vector>> where sum would effectively have:

    vector l;
    product& r;
    auto operator[](size_t i) {
        return l[i] + r[i];
    }

And then product<vector, vector> would effectively have:

    vector l;
    vector r;
    auto operator[](size_t i) {
        return l[i] * r[i];
    }

That would require the optimizer to inline the latter into the former to end up with a single expression, though. Is there a different way to express this that doesn't rely on the optimizer for inlining?

[menaerus]

Expression templates do not rely on optimizer since you're not dealing with the computations directly but rather expressions (nodes) through which you are deferring the computation part until the very last moment (when you have a fully built an expression of expressions, basically almost an AST). This guarantees that you get zero cost when you really need it. What you're describing is something keen of copy elision and function folding though inlining which is pretty much basics in any c++ compiler and happens automatically without special care.

[aw1621107]

> since you're not dealing with the computations directly but rather expressions (nodes) through which you are deferring the computation part until the very last moment (when you have a fully built an expression of expressions, basically almost an AST).

Right, I understand that. What is not exactly clear to me is how you get from the tree of deferred expressions to the "flat" optimized expression without involving the optimizer.

Take something like the above example for instance - w = x + y * z for vectors w/x/y/z. How do you get from that to effectively

    for (size_t i = 0; i < w.size(); ++i) {
        w[i] = x[i] + y[i] * z[i];
    }

without involving the optimizer at all?

[menaerus]

The example is false because that's not how you would write an expression template for given computation so the question being how is it that the optimizer is not involved is also not quite set in the correct context so I can't give you an answer for that. Of course that the optimizer is generally going to be involved, as it is for all the code and not the expression templates, but expression templates do not require the optimizer in the way you're trying to suggest. Expression templates do not rely on O1, O2 or O3 levels being set - they work the same way in O0 too and that may be the hint you were looking for.

[aw1621107]

> The example is false because that's not how you would write an expression template for given computation

OK, so how would you write an expression template for the given computation, then?

> Expression templates do not rely on O1, O2 or O3 levels being set - they work the same way in O0 too and that may be the hint you were looking for.

This claim confuses me given how expression templates seem to work in practice?

For example, consider Todd Veldhuizen's 1994 paper introducing expression templates [0]. If you take the examples linked at the top of the page and plug them into Godbolt (with slight modifications to isolate the actual work of interest) you can see that with -O0 you get calls to overloaded operators instead of the nice flattened/unrolled/optimized operations you get with -O1.

You see something similar with Eigen [2] - you get function calls to "raw" expression template internals with -O0, and you need to enable the optimizer to get unrolled/flattened/etc. operations.

Similar thing yet again with Blaze [3].

At least to me, it looks like expression templates produce quite different outputs when the optimizer is enabled vs. disabled, and the -O0 outputs very much don't resemble the manually-unrolled/flattened-like output one might expect (and arguably gets with optimizations enabled). Did all of these get expression templates wrong as well?

[0]: https://web.archive.org/web/20050210090012/http://osl.iu.edu...

[1]: https://cpp.godbolt.org/z/Pdcqdrobo

[2]: https://cpp.godbolt.org/z/3x69scorG

[3]: https://cpp.godbolt.org/z/7vh7KMsnv

[menaerus]

Look, I have just completed work on some high performance serialization library which avoids computing heavy expressions and temporary allocations all by using expression templates and no, optimization levels are not needed. The code works as advertised at O0 - that's the whole deal around it. If you have a genuine question you should ask one but please do not disguise so that it only goes to prove your point. I am not that naive. All I can say is that your understanding of expression templates is not complete and therefore you draw incorrect conclusions. Silly example you provided shows that you don't understand how expression template code looks like and yet you're trying to prove your point all over and over again. Also, most of the time I am writing my comments on my mobile so I understand that my responses sometime appear too blunt but in any case I will obviously not going to write, run or check the code as if I had been on my work. My comments here is not work, and I am not here to win arguments, but most of the time learn from other people's experiences, and sometimes dispute conclusions based on those experiences too. If you don't believe me, or you believe expression templates work differently, then so be it.

[aw1621107]

> If you have a genuine question you should ask one but please do not disguise so that it only goes to prove your point.

I think my question is pretty simple: "How does an optimizer-independent expression template implementation work?" Evidently the resources I've found so far describe "optimizer-dependent expression templates", and apparently none of the "expression template" implementations I've had reason to look at disabused me of that notion.

> My comments here is not work, and I am not here to win arguments, but most of the time learn from other people's experiences, and sometimes dispute conclusions based on those experiences too.

Sure, and I like to learn as well from the more knowledgeable/experienced folk here, but as much as I want to do so here I'm finding it difficult since there's precious little for me to go off of beyond basically just being told I'm wrong.

> If you don't believe me, or you believe expression templates work differently, then so be it.

I want to understand how you understand expression templates, but between the above and not being able to find useful examples of your description of expression templates I'm at a bit of a loss.