[PATCH] Bug 20788 - bugpoint does not respect the "Alias must point to a definition"

[Nick Johnson]

Fix bug 20788.

GlobalAliases may reference function definitions, but not function declarations.

bugpoint would sometimes create invalid IR by deleting a function's body (thus mutating a function definition into a declaration) without first 'fixing' any GlobalAliases that reference that function definition.

This patch iteratively prevents that issue.  Before deleting a function's body, it scans the module for GlobalAliases which reference that function.  When found, it eliminates them using replaceAllUsesWith.

http://reviews.llvm.org/D9529

Files:
  tools/bugpoint/ExtractFunction.cpp

Index: tools/bugpoint/ExtractFunction.cpp
===================================================================
--- tools/bugpoint/ExtractFunction.cpp
+++ tools/bugpoint/ExtractFunction.cpp
@@ -184,6 +184,26 @@
 // blocks, making it external.
 //
 void llvm::DeleteFunctionBody(Function *F) {
+  // First, check whether a GlobalAlias references this declaration.
+  // GlobalAlias MAY NOT reference declarations.
+  for(;;) {
+    // 1. Find aliases
+    SmallVector<GlobalAlias*,1> aliases;
+    Module *M = F->getParent();
+    for(Module::alias_iterator i=M->alias_begin(), E=M->alias_end(); i!=E; ++i)
+      if( i->getAliasee() == F )
+        aliases.push_back(&*i);
+    if( aliases.empty() )
+      break;
+    // 2. Resolve aliases
+    for(unsigned i=0, N=aliases.size(); i<N; ++i) {
+      aliases[i]->replaceAllUsesWith( aliases[i]->getAliasee() );
+      aliases[i]->eraseFromParent();
+    }
+    // 3. Repeat until no more aliases found; there might
+    // be an alias to an alias...
+  }
+
   // delete the body of the function...
   F->deleteBody();
   assert(F->isDeclaration() && "This didn't make the function external!");

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