fhahn added inline comments.
================
Comment at: llvm/lib/IR/Instruction.cpp:580
+ if (auto *CB = dyn_cast<CallBase>(this))
+ return objcarc::hasRetainRVOrClaimRVAttr(CB);
+ return false;
----------------
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?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D92808/new/
https://reviews.llvm.org/D92808
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