Teach GlobalDCE how to remove empty global_ctor entries.

This moves most of GlobalOpt's constructor optimization
code out of GlobalOpt into Transforms/Utils/CDtorUtils.{h,cpp}. The
public interface is a single function OptimizeGlobalCtorsList() that
takes a predicate returning which constructors to remove.

GlobalOpt calls this with a function that statically evaluates all
constructors, just like it did before. This part of the change is
behavior-preserving.

Also add a call to this from GlobalDCE with a filter that removes global
constructors that contain a "ret" instruction and nothing else – this
fixes PR19590.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207856 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Transforms/Utils/CtorUtils.h

@@ -0,0 +1,35 @@
+//===- CtorUtils.h - Helpers for working with global_ctors ------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines functions that are used to process llvm.global_ctors.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_UTILS_CTOR_UTILS_H
+#define LLVM_TRANSFORMS_UTILS_CTOR_UTILS_H
+
+#include <functional>
+#include <vector>
+
+namespace llvm {
+
+class GlobalVariable;
+class Function;
+class Module;
+
+typedef bool (*ShouldRemoveCtor)(void *, Function *);
+
+/// Call "ShouldRemove" for every entry in M's global_ctor list and remove the
+/// entries for which it returns true.  Return true if anything changed.
+bool optimizeGlobalCtorsList(Module &M, ShouldRemoveCtor ShouldRemove,
+                             void *Context);
+
+} // End llvm namespace
+
+#endif