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

[Reid Kleckner via llvm-dev]

One thing that volatile doesn't do is escape results that have been written
to memory.

The proposed blackbox intrinsic is modeled as reading and writing any
pointed to memory, which is useful.

I also think blackbox will be a lot easier for people to use than empty
volatile inline asm and volatile loads and stores. That alone seems worth
something. :)

On Mon, Nov 9, 2015 at 6:04 PM, Daniel Berlin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>> I think the fundamental thing you're missing is that benchmarks are an
>> exercise in if/then:
>>
>
> I don't believe i'm actually missing anything.
>
>
>>
>> *If* a user exercises this API, *then* how well would it perform?
>
>
> In this case, he exercised the API, and the compiler optimized it away.
> If he wants to test whether the API, exercised in some other way, will be
> optimized, he should test that.
>
> Your argument that this can't be tested is *almost always* false.  It's
> true sometimes, but that's *actually pretty rare*.
>
>
>> Of course, in the case of a user, the data could come from anywhere, and
>> go anywhere - the terminal, a network socket, whatever.
>>
>> However, in a benchmark, all the data comes from (and goes) to places the
>> compiler and see.
>>
>
> This is not necessarily true, but as I said, the way around this is to use
> volatile.
>
>
>>
>> Thus, it's necessary to make the compiler _pretend_ the data came from
>> and goes to a "black box", in order for the benchmarks to even *remotely*
>> resemble what they're meant to test.
>>
>
>
>> This is actually distinct from #1, #2, _and_ #3 above - quite simply,
>> what is needed is a way to simulate a "real usage" scenario without
>> actually contacting the external world.
>>
>>
> No. It is not distinct in any way, and you haven't shown why it is.
> It is exactly  "i want this operation to happen exactly X times despite
> whether the compiler thinks it is necessary or can be removed"
>
> That is volatile.
>
>
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