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

[Daniel Berlin via llvm-dev]

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.


>
>> 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 ;)


> 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 guess i don't see why an intrinsic with not well defined semantics, used
in weird ways to try to outsmart some but not all optimizations, is "less
hacky" than a harness that says "hey, i want to see the effects of running
mem2reg and code gen on this, without running constprop. So i'm just going
to run mem2reg and codegen on this, and see the results!".
Because the former is just a way to try to magic the compiler, and the
second expresses exactly what you want.
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