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

[Mehdi Amini via llvm-dev]

> On Nov 6, 2015, at 8:35 AM, Richard Diamond via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
> 
> On Tue, Nov 3, 2015 at 3:15 PM, Daniel Berlin <dberlin at dberlin.org <mailto:dberlin at dberlin.org>> wrote:
> 
> 
> On Tue, Nov 3, 2015 at 12:29 PM, Richard Diamond <wichard at vitalitystudios.com <mailto:wichard at vitalitystudios.com>> wrote:
> 
> 
> On Mon, Nov 2, 2015 at 9:16 PM, Daniel Berlin <dberlin at dberlin.org <mailto: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 behavior?

How would black_box be different from existing mechanism (inline asm, volatile, …)? 
If the effect on the optimizer is not different then there is no reason to introduce a new intrinsic just for the sake of it. It has some cost: any optimization has to take this into account.

On this topic, I think Chandler’s talk at CppCon seems relevant: https://www.youtube.com/watch?v=nXaxk27zwlk

>  
> 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 <http://llvm.org/docs/ExtendingLLVM.html>). That sounds perfect to me.

The doc is about if you *need* to extend LLVM, then you should try with intrinsic instead of adding an instruction, it is the “need” part that is not clear here. The doc also states that an intrinsic is transparent to optimization passes, but it is not the case here since you want to prevent optimizations from happening (and you haven’t really specified how to decide what can an optimization do around this intrinsic, because if you don’t teach the optimizer about it, it will treat it as an external function call).

— 
Mehdi


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