1 files changed, 159 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Utils/CtorUtils.cpp b/llvm/lib/Transforms/Utils/CtorUtils.cpp
new file mode 100644
index 000000000000..069a86f6ab33
--- /dev/null
+++ b/llvm/lib/Transforms/Utils/CtorUtils.cpp
@@ -0,0 +1,159 @@
+//===- CtorUtils.cpp - Helpers for working with global_ctors ----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines functions that are used to process llvm.global_ctors.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/CtorUtils.h"
+#include "llvm/ADT/BitVector.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+
+#define DEBUG_TYPE "ctor_utils"
+
+using namespace llvm;
+
+/// Given a specified llvm.global_ctors list, remove the listed elements.
+static void removeGlobalCtors(GlobalVariable *GCL, const BitVector &CtorsToRemove) {
+  // Filter out the initializer elements to remove.
+  ConstantArray *OldCA = cast<ConstantArray>(GCL->getInitializer());
+  SmallVector<Constant *, 10> CAList;
+  for (unsigned I = 0, E = OldCA->getNumOperands(); I < E; ++I)
+    if (!CtorsToRemove.test(I))
+      CAList.push_back(OldCA->getOperand(I));
+
+  // Create the new array initializer.
+  ArrayType *ATy =
+      ArrayType::get(OldCA->getType()->getElementType(), CAList.size());
+  Constant *CA = ConstantArray::get(ATy, CAList);
+
+  // If we didn't change the number of elements, don't create a new GV.
+  if (CA->getType() == OldCA->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->getIterator(), 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.
+static 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 (auto &V : CA->operands()) {
+    ConstantStruct *CS = cast<ConstantStruct>(V);
+    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.
+static 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 (auto &V : CA->operands()) {
+    if (isa<ConstantAggregateZero>(V))
+      continue;
+    ConstantStruct *CS = cast<ConstantStruct>(V);
+    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;
+}
+
+/// 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 llvm::optimizeGlobalCtorsList(
+    Module &M, function_ref<bool(Function *)> ShouldRemove) {
+  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.
+  unsigned NumCtors = Ctors.size();
+  BitVector CtorsToRemove(NumCtors);
+  for (unsigned i = 0; i != Ctors.size() && NumCtors > 0; ++i) {
+    Function *F = Ctors[i];
+    // Found a null terminator in the middle of the list, prune off the rest of
+    // the list.
+    if (!F)
+      continue;
+
+    LLVM_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(F)) {
+      Ctors[i] = nullptr;
+      CtorsToRemove.set(i);
+      NumCtors--;
+      MadeChange = true;
+      continue;
+    }
+  }
+
+  if (!MadeChange)
+    return false;
+
+  removeGlobalCtors(GlobalCtors, CtorsToRemove);
+  return true;
+}