[PATCH] D101264: [IR] Optimize mayBeDerefined for known interposable linkages. NFC

[Fangrui Song via Phabricator via llvm-commits]

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For weak/linkonce/common/extern_weak (they are also interposable by default), just return true.
For internal/private/appending, there cannot be different copies, just return false.
Only for external, we need to call `isInterposable`: if it is dso_local, the
definition resolves to a symbol within the same linkage, and we can consider its
definition as exact; otherwise the definition is not exact.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D101264

Files:
  llvm/include/llvm/IR/GlobalValue.h


Index: llvm/include/llvm/IR/GlobalValue.h
===================================================================
--- llvm/include/llvm/IR/GlobalValue.h
+++ llvm/include/llvm/IR/GlobalValue.h
@@ -127,19 +127,23 @@
   /// time.
   bool mayBeDerefined() const {
     switch (getLinkage()) {
-    case WeakODRLinkage:
-    case LinkOnceODRLinkage:
-    case AvailableExternallyLinkage:
-      return true;
-
     case WeakAnyLinkage:
     case LinkOnceAnyLinkage:
     case CommonLinkage:
     case ExternalWeakLinkage:
-    case ExternalLinkage:
-    case AppendingLinkage:
+
+    // The three cannot be overridden but can be de-refined.
+    case WeakODRLinkage:
+    case LinkOnceODRLinkage:
+    case AvailableExternallyLinkage:
+      return true;
+
     case InternalLinkage:
     case PrivateLinkage:
+    case AppendingLinkage:
+      return false;
+
+    case ExternalLinkage:
       return isInterposable();
     }
 


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