[Bugpoint] Erase comdat annotations after removing a global's initializer.

[jlebar]

Summary

This is necessary to keep the verifier happy after bugpoint removes an
initializer from a global variable with a comdat annotation, because
globals without initializers may not have comdats.

[majnemer]

Do we need the same code in DeleteFunctionBody?

[jlebar]

In D21274#455768, @majnemer wrote:

Do we need the same code in DeleteFunctionBody?

Hm, I am not sure; I guess you're saying it's illegal to have a comdat annotation on a function declaration? It may only be present on a definition? But then, if we delete it on the declaration, do we also need to delete it on the corresponding global variable?

[majnemer]

In D21274#455769, @jlebar wrote:

In D21274#455768, @majnemer wrote:

Do we need the same code in DeleteFunctionBody?

Hm, I am not sure; I guess you're saying it's illegal to have a comdat annotation on a function declaration? It may only be present on a definition? But then, if we delete it on the declaration, do we also need to delete it on the corresponding global variable?

COMDATs are only valid on definitions.

[jlebar]

Also delete comdats when turning function defs into decls.

[majnemer]

LGTM

tools/bugpoint/CrashDebugger.cpp

ReduceCrashingGlobalVariables::TestGlobalVariables(
   outs() << ": ";
 
   // Loop over and delete any global variables which we aren't supposed to be
   // playing with...
   for (GlobalVariable &I : M->globals())
     if (I.hasInitializer() && !GVSet.count(&I)) {
       DeleteGlobalInitializer(&I);
       I.setLinkage(GlobalValue::ExternalLinkage);
+      I.setComdat(nullptr);
     }
 
   // Try running the hacked up program...
   if (TestFn(BD, M)) {
     BD.setNewProgram(M);        // It crashed, keep the trimmed version...
 
     // Make sure to use global variable pointers that point into the now-current
     // module.
if (!NoGlobalRM &&
     Module *M = CloneModule(BD.getProgram()).release();
     bool DeletedInit = false;
 
     for (Module::global_iterator I = M->global_begin(), E = M->global_end();
          I != E; ++I)
       if (I->hasInitializer()) {
         DeleteGlobalInitializer(&*I);
         I->setLinkage(GlobalValue::ExternalLinkage);
+        I->setComdat(nullptr);
         DeletedInit = true;
       }
 
     if (!DeletedInit) {
       delete M;  // No change made...
     } else {
       // See if the program still causes a crash...
       outs() << "\nChecking to see if we can delete global inits: ";

tools/bugpoint/ExtractFunction.cpp

void llvm::DeleteGlobalInitializer(GlobalVariable *GV) {
   GV->setInitializer(nullptr);
 }
 
 // DeleteFunctionBody - "Remove" the function by deleting all of its basic
 // blocks, making it external.
 //
 void llvm::DeleteFunctionBody(Function *F) {
   eliminateAliases(F);
+  // Function declarations can't have comdats.
+  F->setComdat(nullptr);
 
   // delete the body of the function...
   F->deleteBody();
   assert(F->isDeclaration() && "This didn't make the function external!");
 }
 
 /// GetTorInit - Given a list of entries for static ctors/dtors, return them
 /// as a constant array.