[llvm-dev] [RFC] A new intrinsic, `llvm.blackbox`, to explicitly prevent constprop, die, etc optimizations

[Richard Diamond via llvm-dev]

On Tue, Nov 3, 2015 at 3:15 PM, Daniel Berlin <dberlin at dberlin.org> wrote:

>
>
> On Tue, Nov 3, 2015 at 12:29 PM, Richard Diamond <
> wichard at vitalitystudios.com> wrote:
>
>>
>>
>> On Mon, Nov 2, 2015 at 9:16 PM, Daniel Berlin <dberlin at dberlin.org>
>> wrote:
>>
>>> I'm very unclear and why you think a generic black box intrinsic will
>>> have any different performance impact ;-)
>>>
>>>
>>> I'm also unclear on what the goal with this intrinsic is.
>>> I understand the symptoms you are trying to solve - what exactly is the
>>> disease.
>>>
>>> IE you say "
>>>
>>> I'd like to propose a new intrinsic for use in preventing optimizations
>>> from deleting IR due to constant propagation, dead code elimination, etc."
>>>
>>> But why are you trying to achieve this goal?
>>>
>>
>> It's a cleaner design than current solutions (as far as I'm aware).
>>
>
> For what, exact, well defined goal?
>

> Trying to make certain specific optimizations not work does not seem like
> a goal unto itself.
> It's a thing you are doing to achieve something else, right?
> (Because if not, it has a very well defined and well supported solutions -
> set up a pass manager that runs the passes you want)
>
> What is the something else?
>
> IE what is the problem that led you to consider this solution.
>

I apologize if I'm not being clear enough. This contrived example
```rust
#[bench]
fn bench_xor_1000_ints(b: &mut Bencher) {
    b.iter(|| {
        (0..1000).fold(0, |old, new| old ^ new);
    });
}
```
is completely optimized away. Granted, IRL production (ignoring the
question of why this code was ever used in production in the first place)
this optimization is desired, but here it leads to bogus measurements (ie
0ns per iteration). By using `test::black_box`, one would have

```rust
#[bench]
fn bench_xor_1000_ints(b: &mut Bencher) {
    b.iter(|| {
        let n = test::black_box(1000);  // optional
        test::black_box((0..n).fold(0, |old, new| old ^ new));
    });
}
```
and the microbenchmark wouldn't have bogos 0ns measurements anymore.

Now, as I stated in the proposal, `test::black_box` currently uses no-op
inline asm to "read" from its argument in a way the optimizations can't
see. Conceptually, this seems like something that should be modelled in
LLVM's IR rather than by hacks higher up the IR food chain because the root
problem is caused by LLVM's optimization passes (most of the time this code
optimization is desired, just not here). Plus, it seems others have used
other tricks to achieve similar effects (ie volatile), so why shouldn't
there be something to model this behaviour?


> Benchmarks that can be const prop'd/etc away are often meaningless.
>>>
>>
>> A benchmark that's completely removed is even more meaningless, and the
>> developer may not even know it's happening.
>>
>
> Write good benchmarks?
>
> No, seriously, i mean, you want benchmarks that tests what users will see
> when the compiler works, not benchmarks that test what users see if the
> were to suddenly turn off parts of the optimizers ;)
>

But users are also not testing how fast deterministic code which LLVM is
completely removing can go. This intrinsic prevents LLVM from correctly
thinking the code is deterministic (or that a value isn't used) so that
measurements are (at the very least, the tiniest bit) meaningful.

I'm not saying this intrinsic will make all benchmarks meaningful (and I
>> can't), I'm saying that it would be useful in Rust in ensuring that
>> tests/benches aren't invalidated simply because a computation wasn't
>> performed.
>>
>> Past that, if you want to ensure a particular optimization does a
>>> particular thing on a benchmark, ISTM it would be better to generate the
>>> IR, run opt (or build your own pass-by-pass harness), and then run "the
>>> passes you want on it" instead of "trying to stop certain passes from doing
>>> things to it".
>>>
>>
>> True, but why would you want to force that speed bump onto other
>> developers? I'd argue that's more hacky than the inline asm.
>>
>> Speed bump? Hacky?
> It's a completely normal test harness?
>
> That's in fact, why llvm uses it as a test harness?
>

I mean I wouldn't write a harness or some other type of workaround for
something like this: Rust doesn't seem to be the first to have encountered
this issue, thus it is nonsensical to require every project using LLVM to
have a separate harness or other workaround so they don't run into this
issue. LLVM's own documentation suggests that adding an intrinsic is the
best choice moving forward anyway: "Adding an intrinsic function is far
easier than adding an instruction, and is transparent to optimization
passes. If your added functionality can be expressed as a function call, an
intrinsic function is the method of choice for LLVM extension." (from
http://llvm.org/docs/ExtendingLLVM.html). That sounds perfect to me.

At anyrate, I apologize for my original hand-wavy-ness; I am young and
inexperienced.
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