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

lib/Transforms/Utils/CtorUtils.cpp

@@ -0,0 +1,181 @@
+//===- CtorUtils.cpp - 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.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/CtorUtils.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/Debug.h"
+
+#define DEBUG_TYPE "ctor_utils"
+
+namespace llvm {
+
+namespace {
+/// Given a specified llvm.global_ctors list, install the
+/// specified array.
+void installGlobalCtors(GlobalVariable *GCL,
+                        const std::vector<Function *> &Ctors) {
+  // If we made a change, reassemble the initializer list.
+  Constant *CSVals[2];
+  CSVals[0] = ConstantInt::get(Type::getInt32Ty(GCL->getContext()), 65535);
+  CSVals[1] = nullptr;
+
+  StructType *StructTy =
+      cast<StructType>(GCL->getType()->getElementType()->getArrayElementType());
+
+  // Create the new init list.
+  std::vector<Constant *> CAList;
+  for (unsigned i = 0, e = Ctors.size(); i != e; ++i) {
+    if (Ctors[i]) {
+      CSVals[1] = Ctors[i];
+    } else {
+      Type *FTy = FunctionType::get(Type::getVoidTy(GCL->getContext()), false);
+      PointerType *PFTy = PointerType::getUnqual(FTy);
+      CSVals[1] = Constant::getNullValue(PFTy);
+      CSVals[0] =
+          ConstantInt::get(Type::getInt32Ty(GCL->getContext()), 0x7fffffff);
+    }
+    CAList.push_back(ConstantStruct::get(StructTy, CSVals));
+  }
+
+  // Create the array initializer.
+  Constant *CA =
+      ConstantArray::get(ArrayType::get(StructTy, CAList.size()), CAList);
+
+  // If we didn't change the number of elements, don't create a new GV.
+  if (CA->getType() == GCL->getInitializer()->getType()) {
+    GCL->setInitializer(CA);
+    return;
+  }
+
+  // Create the new global and insert it next to the existing list.
+  GlobalVariable *NGV =
+      new GlobalVariable(CA->getType(), GCL->isConstant(), GCL->getLinkage(),
+                         CA, "", GCL->getThreadLocalMode());
+  GCL->getParent()->getGlobalList().insert(GCL, NGV);
+  NGV->takeName(GCL);
+
+  // Nuke the old list, replacing any uses with the new one.
+  if (!GCL->use_empty()) {
+    Constant *V = NGV;
+    if (V->getType() != GCL->getType())
+      V = ConstantExpr::getBitCast(V, GCL->getType());
+    GCL->replaceAllUsesWith(V);
+  }
+  GCL->eraseFromParent();
+}
+
+/// Given a llvm.global_ctors list that we can understand,
+/// return a list of the functions and null terminator as a vector.
+std::vector<Function*> parseGlobalCtors(GlobalVariable *GV) {
+  if (GV->getInitializer()->isNullValue())
+    return std::vector<Function *>();
+  ConstantArray *CA = cast<ConstantArray>(GV->getInitializer());
+  std::vector<Function *> Result;
+  Result.reserve(CA->getNumOperands());
+  for (User::op_iterator i = CA->op_begin(), e = CA->op_end(); i != e; ++i) {
+    ConstantStruct *CS = cast<ConstantStruct>(*i);
+    Result.push_back(dyn_cast<Function>(CS->getOperand(1)));
+  }
+  return Result;
+}
+
+/// Find the llvm.global_ctors list, verifying that all initializers have an
+/// init priority of 65535.
+GlobalVariable *findGlobalCtors(Module &M) {
+  GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
+  if (!GV)
+    return nullptr;
+
+  // Verify that the initializer is simple enough for us to handle. We are
+  // only allowed to optimize the initializer if it is unique.
+  if (!GV->hasUniqueInitializer())
+    return nullptr;
+
+  if (isa<ConstantAggregateZero>(GV->getInitializer()))
+    return GV;
+  ConstantArray *CA = cast<ConstantArray>(GV->getInitializer());
+
+  for (User::op_iterator i = CA->op_begin(), e = CA->op_end(); i != e; ++i) {
+    if (isa<ConstantAggregateZero>(*i))
+      continue;
+    ConstantStruct *CS = cast<ConstantStruct>(*i);
+    if (isa<ConstantPointerNull>(CS->getOperand(1)))
+      continue;
+
+    // Must have a function or null ptr.
+    if (!isa<Function>(CS->getOperand(1)))
+      return nullptr;
+
+    // Init priority must be standard.
+    ConstantInt *CI = cast<ConstantInt>(CS->getOperand(0));
+    if (CI->getZExtValue() != 65535)
+      return nullptr;
+  }
+
+  return GV;
+}
+} // namespace
+
+/// 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) {
+  GlobalVariable *GlobalCtors = findGlobalCtors(M);
+  if (!GlobalCtors)
+    return false;
+
+  std::vector<Function *> Ctors = parseGlobalCtors(GlobalCtors);
+  if (Ctors.empty())
+    return false;
+
+  bool MadeChange = false;
+
+  // Loop over global ctors, optimizing them when we can.
+  for (unsigned i = 0; i != Ctors.size(); ++i) {
+    Function *F = Ctors[i];
+    // Found a null terminator in the middle of the list, prune off the rest of
+    // the list.
+    if (!F) {
+      if (i != Ctors.size() - 1) {
+        Ctors.resize(i + 1);
+        MadeChange = true;
+      }
+      break;
+    }
+    DEBUG(dbgs() << "Optimizing Global Constructor: " << *F << "\n");
+
+    // We cannot simplify external ctor functions.
+    if (F->empty())
+      continue;
+
+    // If we can evaluate the ctor at compile time, do.
+    if (ShouldRemove(Context, F)) {
+      Ctors.erase(Ctors.begin() + i);
+      MadeChange = true;
+      --i;
+      continue;
+    }
+  }
+
+  if (!MadeChange)
+    return false;
+
+  installGlobalCtors(GlobalCtors, Ctors);
+  return true;
+}
+
+} // End llvm namespace