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

[Tianle Liu via llvm-commits]

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@@ -1103,6 +1104,16 @@ bool DevirtModule::tryFindVirtualCallTargets(
         GlobalObject::VCallVisibilityPublic)
       return false;
 
+    // Record if the first GV is AvailableExternally
+    if (TargetsForSlot.empty())
+      hasAvailableExternally = TM.Bits->GV->hasAvailableExternallyLinkage();
+
+    // When the first GV is AvailableExternally, check if all other GVs are
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tianleliu wrote:

This is a conservative checking that does not stop devirtualization for all available externally.
Because when doing the check of available externally, checking of VCallVisibility != VCallVisibilityPublic is pass, which means all virtual function calls from this available externally vtable are in the current LTO unit. So I guess a icall.branch.funnel whose parameters all relate to available externally vtable is allowed to generated? 

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