[PATCH] D129561: [RS4GC] Fix over-restrictive assert in atomic memcpy/move handling

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev added inline comments.


================
Comment at: llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp:1705
       auto GetBaseAndOffset = [&](Value *Derived) {
-        assert(PointerToBase.count(Derived));
         unsigned AddressSpace = Derived->getType()->getPointerAddressSpace();
----------------
apilipenko wrote:
> mkazantsev wrote:
> > mkazantsev wrote:
> > > mkazantsev wrote:
> > > > apilipenko wrote:
> > > > > I suggest handling the undef case explicitly. Just return undef if the input is undef. 
> > > > Same goes for poison I guess.
> > > It's a no-go. Aside from undef and poison, here can be things like
> > > ```
> > > i32 addrspace(1)* bitcast (i8 addrspace(1)* getelementptr inbounds (i8, i8 addrspace(1)* null, i64 16) to i32 addrspace(1)*)
> > > ```
> > > etc. So the expression can be arbitrarily complex.
> > General idea is that undef may not be the immediate value, but a part of any other expression. Therefore, there is no way to guarantee it's not involved somewhere. The assert is just over-conservative w.r.t. this situation.
> I see your point.
> 
> Looks like RS4GC doesn't track liveness for constants. Instead, it assumes null base for them. See https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp#L537
> I think we should do the same here to be consistent. 
No, we can't do it. Imagine you had smth like
```
i32 addrspace(1)* bitcast (i8 addrspace(1)* getelementptr inbounds (i8, i8 addrspace(1)* %base, i64 16) to i32 addrspace(1)*)

```
and then unreached code opt made it into
```
i32 addrspace(1)* bitcast (i8 addrspace(1)* getelementptr inbounds (i8, i8 addrspace(1)* %base, i64 undef) to i32 addrspace(1)*)

```

It's unreached code opts. We can't make any assumptions here.


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https://reviews.llvm.org/D129561