[llvm] [WholeProgramDevirt] Add check for AvailableExternal and give up devirt (PR #143468)

[Tianle Liu via llvm-commits]

tianleliu wrote:

Hi @teresajohnson Thanks for your review.
> I don't see them checking isDeclarationForLinker or the linkage type directly.
I mean vtable's all function's body would be checked. If any of the functions is a declaration, devirtualization of tryVirtualConstProp would drop.
For example of "@_ZTVSt9exception = available_externally constant { [2 x ptr] } { [2 x ptr] [ptr foo1, ptr @foo2] }". If foo1 or foo2 is a declaration, devirtualization would be drop.
Code in tryVirtualConstProp, https://github.com/llvm/llvm-project/blob/968d8eaa44c500259fe8d56ad77ec1c71cad35e2/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp#L1868
In code, checking function by isDeclaration() but not isDeclarationForLinker(). So I guess if function is available_externally, the devirt would not drop. But I am curious what a real case that a external function is regarded as available_externally?

**You are right** for trySingleImplDevirt that it does not check function's body. No matter if vtable is available_externally or not, it would do devirtualization if vtable's function are the same. The example is as below:
`@_ZTVSt9exception = **available_externally** constant { [2 x ptr] } { [2 x ptr] [ptr null, ptr @foo] }
@_ZTV1A.0 = constant { [2 x ptr] } { [2 x ptr] [ptr null, ptr @foo] }`
trySingleImplDevirt() would work and translate to `tail call i16 @foo(ptr null)`
So for trySingleImplDevirt, it does not check available_externally. 
**If no check of available_externally is reasonable for trySingleImplDevirt, I think I should move the checking in DevirtModule::tryICallBranchFunnel**.

> Do you have a test case where all vtables are available externally, and how does that work?
I write a all vtables are available externally and it crashes in LowerTypeTests. The crash is because it skips dealing with available externally. And code is at https://github.com/llvm/llvm-project/blob/9d491bc602c2d9730cb42fe25f0753471a3af389/llvm/lib/Transforms/IPO/LowerTypeTests.cpp#L2227
Even I walk around it, there is still linkage type issue that not pass VerifierPass because all available externally would combined to privateLinkage GV at https://github.com/llvm/llvm-project/blob/9d491bc602c2d9730cb42fe25f0753471a3af389/llvm/lib/Transforms/IPO/LowerTypeTests.cpp#L887
I am not sure if they are bugs for LowerTypeTests?
**If assuming LowerTypeTests is right, I guess any available externally for icall.branch.funnel should be stopped**.

So do you agree that I postpone available externally in tryICallBranchFunnel, and any one of available externally vtable would stop icall.branch.funnel?

https://github.com/llvm/llvm-project/pull/143468