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

[Reid Kleckner via llvm-dev]

On Wed, Nov 11, 2015 at 10:41 AM, Daniel Berlin <dberlin at dberlin.org> wrote:

> On Wed, Nov 11, 2015 at 10:32 AM, Reid Kleckner <rnk at google.com> wrote:
>
>> I think the idea is to model the intrinsic as a normal external function
>> call:
>>
> - Can read/write escaped memory
>>
> - Escapes pointer args
>> - Functionattrs cannot infer anything about it
>> - Returns a pointer which may alias any escaped data
>>
>> As you point out so nicely, there is already a list of stuff that
> external function calls may do, but we may be able to prove things about
> them anyway due to attributes, etc.
>
> So it's not just an external function call, it's a super-magic one.
>

Right, an external function, with a definition that the compiler will never
find.


> Now, can we handle that?
> Sure.
>
> For example,  i can move external function calls if i can prove things
> about their dependencies, and the above list is not sufficient to prevent
> me from moving (or PRE'ing) most of the blackbox calls that just take
> normal non-pointer args.
> Is that going to be okay?
>
> (Imagine, for example, LTO modes where i can guarantee i have the entire
> program, etc.
> You still want blackbox to be magically special in these modes, even
> though nothing else is).
>

Sure, the compiler can reorder all the memory accesses to non-escaped
memory as it sees fit across the barrier. That's part of the normal
modelling of external calls.

I don't know how you could CSE it, though. Any call you can't reason about
can always use inline asm to talk to external devices or issue a write
syscall. I don't know how you could practically deploy a super-duper LTO
mode that doesn't allow that as part of its model.

The following CFG simplification would be legal, as it also fits the normal
model of an external call:
if (cond) y =llvm.blackbox(x)
else y = llvm.blackbox(x)
-->
y = llvm.blackbox(x)

I don't see how this is special. It just provides an overloaded intrinsic
whose definition we promise to never reason about. Other than that it
follows the same familiar rules that function calls do.
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