GlobalDCE: Delete available_externally initializers if it allows removing the value the initializer is referring to.

This is useful for functions that are not actually available externally but
referenced by a vtable of some kind. Clang emits functions like this for the MS
ABI.

PR20182.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212337 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/IPO/GlobalDCE.cpp

@@ -44,14 +44,19 @@ namespace {
     bool runOnModule(Module &M) override;
 
   private:
-    SmallPtrSet<GlobalValue*, 32> AliveGlobals;
+    SmallPtrSet<Constant *, 32> AliveGlobals;
     SmallPtrSet<Constant *, 8> SeenConstants;
+    SmallPtrSet<GlobalVariable *, 8> DiscardableGlobalInitializers;
 
     /// GlobalIsNeeded - mark the specific global value as needed, and
     /// recursively mark anything that it uses as also needed.
     void GlobalIsNeeded(GlobalValue *GV);
     void MarkUsedGlobalsAsNeeded(Constant *C);
 
+    /// \brief Checks if C is alive or is a ConstantExpr that refers to an alive
+    /// value.
+    bool ContainsUsedGlobal(Constant *C);
+
     bool RemoveUnusedGlobalValue(GlobalValue &GV);
   };
 }
@@ -162,6 +167,19 @@ bool GlobalDCE::runOnModule(Module &M) {
       I->setAliasee(nullptr);
     }
 
+  // Look for available externally constants that we can turn into normal
+  // externals by deleting their initalizers. This allows us to remove other
+  // globals that are referenced by the initializer.
+  if (!DiscardableGlobalInitializers.empty()) {
+    for (GlobalVariable *GV : DiscardableGlobalInitializers) {
+      if (!ContainsUsedGlobal(GV->getInitializer())) {
+        GV->setInitializer(nullptr);
+        GV->setLinkage(GlobalValue::ExternalLinkage);
+        Changed = true;
+      }
+    }
+  }
+
   if (!DeadFunctions.empty()) {
     // Now that all interferences have been dropped, delete the actual objects
     // themselves.
@@ -209,8 +227,12 @@ void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
     // If this is a global variable, we must make sure to add any global values
     // referenced by the initializer to the alive set.
-    if (GV->hasInitializer())
+    if (GV->hasInitializer()) {
-      MarkUsedGlobalsAsNeeded(GV->getInitializer());
+      if (GV->hasAvailableExternallyLinkage())
+        DiscardableGlobalInitializers.insert(GV);
+      else
+        MarkUsedGlobalsAsNeeded(GV->getInitializer());
+    }
   } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
     // The target of a global alias is needed.
     MarkUsedGlobalsAsNeeded(GA->getAliasee());
@@ -248,6 +270,21 @@ void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
   }
 }
 
+bool GlobalDCE::ContainsUsedGlobal(Constant *C) {
+  // C contains a used global If C is alive or we visited it while marking
+  // values alive.
+  if (AliveGlobals.count(C) || SeenConstants.count(C))
+    return true;
+
+  // Now check all operands of a ConstantExpr.
+  for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
+    Constant *Op = dyn_cast<Constant>(*I);
+    if (Op && ContainsUsedGlobal(Op))
+      return true;
+  }
+  return false;
+}
+
 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
 // If found, check to see if the constant pointer ref is safe to destroy, and if

test/Transforms/GlobalDCE/externally_available.ll

@@ -1,10 +1,43 @@
-; RUN: opt < %s -globaldce -S | not grep test_
+; RUN: opt < %s -globaldce -S | FileCheck %s
 
 ; test_function should not be emitted to the .s file.
+; CHECK-NOT: @test_function
 define available_externally i32 @test_function() {
   ret i32 4
 }
 
 ; test_global should not be emitted to the .s file.
+; CHECK-NOT: @test_global
 @test_global = available_externally global i32 4
 
+; CHECK: @x = external constant void ()*
+@x = available_externally constant void()* @f
+; CHECK: @y = external constant i32
+@y = available_externally constant i32 ptrtoint (void()* @g to i32)
+; @h is still alive, so don't remove the initializer too eagerly.
+; CHECK: @z = available_externally constant i8 ptrtoint (void (i8)* @h to i8)
+@z = available_externally constant i8 ptrtoint (void(i8)* @h to i8)
+
+; CHECK-NOT: @f
+define linkonce_odr void @f() {
+  ret void
+}
+
+; CHECK-NOT: @g
+define linkonce_odr void @g() {
+  ret void
+}
+
+; CHECK: define linkonce_odr void @h
+define linkonce_odr void @h(i8) {
+  ret void
+}
+
+define i32 @main() {
+  %f = load void()** @x
+  call void %f()
+  %g = load i32* @y
+  %h = load i8* @z
+  call void @h(i8 %h)
+  ret i32 %g
+}