Fix the IR attributes of swift_getObjectType.

[eeckstein]

It's not readnone, because it reads the metatype from an object.
Readnone would let the llvm ARC optimizer reschedule the call with a release-call for the object.

fixes SR-6560.
rdar://problem/35998785

[eeckstein]

@swift-ci test

[rjmccall]

Your reasoning seems sound. Essentially, anything that depends on object liveness can be thought of as reading the reference count of those objects.

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 70aeb71

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 70aeb71

[eeckstein]

@swift-ci test

[jrose-apple]

If only we had a notion of "this can be done earlier, but not later".

[eeckstein]

@swift-ci nominate

• Explanation: Fixes a use-after-free issue related to class types
• Scope of Issue: may show up if dealing with class types
• Origination: it's an old bug
• Risk: Low
• Testing: CI bots

[atrick]

I'm not opposed to the conservative fix, but it does indicate a problem or deficiency in the LLVM ARC pass. Reference counts are part of the "runtime memory", so LLVM doesn't see them. Any pass that reorders runtime calls needs a different interpretation of the memory effects.

[eeckstein]

Any LLVM pass could reorder this operations if the function is marked as readnone.
Memory effects of ref count operations are visible in LLVM.