Make GlobalOpt be conservative with TLS variables (PR14309)

For global variables that get the same value stored into them
everywhere, GlobalOpt will replace them with a constant. The problem is
that a thread-local GlobalVariable looks like one value (the address of
the TLS var), but is different between threads.

This patch introduces Constant::isThreadDependent() which returns true
for thread-local variables and constants which depend on them (e.g. a GEP
into a thread-local array), and teaches GlobalOpt not to track such
values.

llvm-svn: 168037

2 files changed, 34 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index 678189b3d6c..591278fa62c 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -225,6 +225,7 @@ static bool AnalyzeGlobal(const Value *V, GlobalStatus &GS,
 
         // Don't hack on volatile stores.
         if (SI->isVolatile()) return true;
+
         GS.Ordering = StrongerOrdering(GS.Ordering, SI->getOrdering());
 
         // If this is a direct store to the global (i.e., the global is a scalar
@@ -234,6 +235,14 @@ static bool AnalyzeGlobal(const Value *V, GlobalStatus &GS,
           if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(
                                                            SI->getOperand(1))) {
             Value *StoredVal = SI->getOperand(0);
+
+            if (Constant *C = dyn_cast<Constant>(StoredVal)) {
+              if (C->isThreadDependent()) {
+                // The stored value changes between threads; don't track it.
+                return true;
+              }
+            }
+
             if (StoredVal == GV->getInitializer()) {
               if (GS.StoredType < GlobalStatus::isInitializerStored)
                 GS.StoredType = GlobalStatus::isInitializerStored;
diff --git a/llvm/lib/VMCore/Constants.cpp b/llvm/lib/VMCore/Constants.cpp
index f96fb1d4019..eae96ef86c2 100644
--- a/llvm/lib/VMCore/Constants.cpp
+++ b/llvm/lib/VMCore/Constants.cpp
@@ -245,6 +245,31 @@ bool Constant::canTrap() const {
   }
 }
 
+/// isThreadDependent - Return true if the value can vary between threads.
+bool Constant::isThreadDependent() const {
+  SmallPtrSet<const Constant*, 64> Visited;
+  SmallVector<const Constant*, 64> WorkList;
+  WorkList.push_back(this);
+  Visited.insert(this);
+
+  while (!WorkList.empty()) {
+    const Constant *C = WorkList.pop_back_val();
+
+    if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(C)) {
+      if (GV->isThreadLocal())
+        return true;
+    }
+
+    for (unsigned I = 0, E = C->getNumOperands(); I != E; ++I) {
+      const Constant *D = cast<Constant>(C->getOperand(I));
+      if (Visited.insert(D))
+        WorkList.push_back(D);
+    }
+  }
+
+  return false;
+}
+
 /// isConstantUsed - Return true if the constant has users other than constant
 /// exprs and other dangling things.
 bool Constant::isConstantUsed() const {