ahatanak added inline comments.
================
Comment at: llvm/lib/IR/Instruction.cpp:580
+ if (auto *CB = dyn_cast<CallBase>(this))
+ return objcarc::hasRetainRVOrClaimRVAttr(CB);
+ return false;
----------------
fhahn wrote:
> ahatanak wrote:
> > fhahn wrote:
> > > rjmccall wrote:
> > > > nikic wrote:
> > > > > This change looks pretty fishy. Objective C shouldn't be hijacking
> > > > > LLVMs core instruction model in this way. If it writes to memory,
> > > > > this should either be reflected in the attributes, or modeled using
> > > > > operand bundles.
> > > > >
> > > > > @fhahn Did you review these changes? If not, I'd suggest to revert
> > > > > this patch and get a review on the LLVM changes.
> > > > This could definitely be an operand bundle, and I suppose the presence
> > > > of a bundle does force a conservative assumption on passes.
> > > > @fhahn Did you review these changes?
> > >
> > > Nope I didn't have time to look at this so far.
> > >
> > > <snip>
> > >
> > > Can functions marked as `readonly`/`readnone` be called with the objc
> > > attributes?
> > >
> > > I'm not very familiar with ObjC, but even for a function that just
> > > returns a passed in object id, don't we need to retain & release the
> > > object in the function? Which means the function cannot be `readonly`
> > > because we need to call `@llvm.objc*` functions? If that's the case,
> > > could we just check in the verifier that the attributes are never used
> > > with `readonly` functions?
> > >
> > > If there are indeed cases where we can call `readonly` functions, operand
> > > bundles would probably be safest. It would probably also good to have at
> > > least a few alias-analysis tests, to make sure things work as expected.
> > A function compiled using ARC can call a function compiled using MRR, which
> > can be readonly/readnone. Also, a function compiled using ARC can be marked
> > as readonly/readnone (if an optimization pass wants to do so) after ARC
> > optimizer removes the retainRV/autoreleaseRV pair.
> >
> > ```
> > define i8* @foo() {
> > %1 = tail call i8* @readonlyMRRFunc()
> > ; This function can be readonly if ARC optimizer removes the following
> > two instructions.
> > %2 = call i8* @llvm.objc.retainAutoreleasedReturnValue(i8* %1)
> > %3 = tail call i8* @llvm.objc.autoreleaseReturnValue(i8* %2)
> > ret i8*
> > }
> > ```
> > A function compiled using ARC can call a function compiled using MRR, which
> > can be readonly/readnone
>
> Ok, sounds like a bundle would be a good option then?
Yes. I think bundle should be used here.
I'm considering passing an integer flag, which distinguishes between retain and
claim (e.g., 0 for retain and 1 for claim), to the bundles:
```
call i8* @foo() [ "clang.arc.rv"(i64 0) ]
```
Do you see any problems with this approach? Alternatively, we could pass the
pointer to the runtime function (e.g., pointer to
`@llvm.objc.retainAutoreleasedReturnValue`).
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D92808/new/
https://reviews.llvm.org/D92808
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