GlobalOpt does not treat externally_initialized globals correctly

[olista01]

Summary

GlobalOpt currently merges stores into the initialisers of internal, externally_initialized globals, but should not do so as the value of the global may change between the initialiser and any code in the module being run.

[olista01]

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[olista01]

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[olista01]

Accepted by James Molloy on the mailing list.

[olista01]

Committed revision 250035.

lib/Transforms/IPO/GlobalOpt.cpp

if (!GS.IsLoaded) {
     ++NumMarked;
     return true;
   } else if (!GV->getInitializer()->getType()->isSingleValueType()) {
     const DataLayout &DL = GV->getParent()->getDataLayout();
     if (GlobalVariable *FirstNewGV = SRAGlobal(GV, DL)) {
       GVI = FirstNewGV; // Don't skip the newly produced globals!
       return true;
     }
-  } else if (GS.StoredType == GlobalStatus::StoredOnce) {
+  } else if (GS.StoredType == GlobalStatus::StoredOnce && GS.StoredOnceValue) {
     // If the initial value for the global was an undef value, and if only
     // one other value was stored into it, we can just change the
     // initializer to be the stored value, then delete all stores to the
     // global.  This allows us to mark it constant.
     if (Constant *SOVConstant = dyn_cast<Constant>(GS.StoredOnceValue))
       if (isa<UndefValue>(GV->getInitializer())) {
         // Change the initial value here.
         GV->setInitializer(SOVConstant);

lib/Transforms/Utils/GlobalStatus.cpp

if (const Constant *CU = dyn_cast<Constant>(U)) {
         return false;
     } else
       return false;
   return true;
 }
 
 static bool analyzeGlobalAux(const Value *V, GlobalStatus &GS,
                              SmallPtrSetImpl<const PHINode *> &PhiUsers) {
+  if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
+    if (GV->isExternallyInitialized())
+      GS.StoredType = GlobalStatus::StoredOnce;
+
   for (const Use &U : V->uses()) {
     const User *UR = U.getUser();
     if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(UR)) {
       GS.HasNonInstructionUser = true;
 
       // If the result of the constantexpr isn't pointer type, then we won't
       // know to expect it in various places.  Just reject early.
       if (!isa<PointerType>(CE->getType()))

test/Transforms/GlobalOpt/externally-initialized.ll

+; RUN: opt < %s -S -globalopt | FileCheck %s
+
+; This global is externally_initialized, which may modify the value between
+; it's static initializer and any code in this module being run, so the only
+; write to it cannot be merged into the static initialiser.
+; CHECK: @a = internal unnamed_addr externally_initialized global i32 undef
+@a = internal externally_initialized global i32 undef
+
+; This global is stored to by the external initialization, so cannot be
+; constant-propagated and removed, despite the fact that there are no writes
+; to it.
+; CHECK: @b = internal unnamed_addr externally_initialized global i32 undef
+@b = internal externally_initialized global i32 undef
+
+
+define void @foo() {
+; CHECK-LABEL: foo
+entry:
+; CHECK: store i32 42, i32* @a
+  store i32 42, i32* @a
+  ret void
+}
+define i32 @bar() {
+; CHECK-LABEL: bar
+entry:
+; CHECK: %val = load i32, i32* @a
+  %val = load i32, i32* @a
+  ret i32 %val
+}
+
+define i32 @baz() {
+; CHECK-LABEL: baz
+entry:
+; CHECK: %val = load i32, i32* @b
+  %val = load i32, i32* @b
+  ret i32 %val
+}