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

[Artur Pilipenko via Phabricator via llvm-commits]

apilipenko 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();
----------------
mkazantsev wrote:
> 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.
In this case, the pointer should be in the live set.


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https://reviews.llvm.org/D129561