llvm/lib/Transforms/Utils/CtorUtils.cpp

   1 //===- CtorUtils.cpp - Helpers for working with global_ctors ----*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file defines functions that are used to process llvm.global_ctors.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/Transforms/Utils/CtorUtils.h"
  14 #include "llvm/ADT/BitVector.h"
  15 #include "llvm/IR/Constants.h"
  16 #include "llvm/IR/Function.h"
  17 #include "llvm/IR/GlobalVariable.h"
  18 #include "llvm/IR/Module.h"
  19 #include "llvm/Support/Debug.h"
  20 #include "llvm/Support/raw_ostream.h"
  21 #include <numeric>
  22
  23 #define DEBUG_TYPE "ctor_utils"
  24
  25 using namespace llvm;
  26
  27 /// Given a specified llvm.global_ctors list, remove the listed elements.
  28 static void removeGlobalCtors(GlobalVariable *GCL, const BitVector &CtorsToRemove) {
  29   // Filter out the initializer elements to remove.
  30   ConstantArray *OldCA = cast<ConstantArray>(GCL->getInitializer());
  31   SmallVector<Constant *, 10> CAList;
  32   for (unsigned I = 0, E = OldCA->getNumOperands(); I < E; ++I)
  33     if (!CtorsToRemove.test(I))
  34       CAList.push_back(OldCA->getOperand(I));
  35
  36   // Create the new array initializer.
  37   ArrayType *ATy =
  38       ArrayType::get(OldCA->getType()->getElementType(), CAList.size());
  39   Constant *CA = ConstantArray::get(ATy, CAList);
  40
  41   // If we didn't change the number of elements, don't create a new GV.
  42   if (CA->getType() == OldCA->getType()) {
  43     GCL->setInitializer(CA);
  44     return;
  45   }
  46
  47   // Create the new global and insert it next to the existing list.
  48   GlobalVariable *NGV =
  49       new GlobalVariable(CA->getType(), GCL->isConstant(), GCL->getLinkage(),
  50                          CA, "", GCL->getThreadLocalMode());
  51   GCL->getParent()->insertGlobalVariable(GCL->getIterator(), NGV);
  52   NGV->takeName(GCL);
  53
  54   // Nuke the old list, replacing any uses with the new one.
  55   if (!GCL->use_empty())
  56     GCL->replaceAllUsesWith(NGV);
  57
  58   GCL->eraseFromParent();
  59 }
  60
  61 /// Given a llvm.global_ctors list that we can understand,
  62 /// return a list of the functions and null terminator as a vector.
  63 static std::vector<std::pair<uint32_t, Function *>>
  64 parseGlobalCtors(GlobalVariable *GV) {
  65   ConstantArray *CA = cast<ConstantArray>(GV->getInitializer());
  66   std::vector<std::pair<uint32_t, Function *>> Result;
  67   Result.reserve(CA->getNumOperands());
  68   for (auto &V : CA->operands()) {
  69     ConstantStruct *CS = cast<ConstantStruct>(V);
  70     Result.emplace_back(cast<ConstantInt>(CS->getOperand(0))->getZExtValue(),
  71                         dyn_cast<Function>(CS->getOperand(1)));
  72   }
  73   return Result;
  74 }
  75
  76 /// Find the llvm.global_ctors list.
  77 static GlobalVariable *findGlobalCtors(Module &M) {
  78   GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
  79   if (!GV)
  80     return nullptr;
  81
  82   // Verify that the initializer is simple enough for us to handle. We are
  83   // only allowed to optimize the initializer if it is unique.
  84   if (!GV->hasUniqueInitializer())
  85     return nullptr;
  86
  87   // If there are no ctors, then the initializer might be null/undef/poison.
  88   // Ignore anything but an array.
  89   ConstantArray *CA = dyn_cast<ConstantArray>(GV->getInitializer());
  90   if (!CA)
  91     return nullptr;
  92
  93   for (auto &V : CA->operands()) {
  94     if (isa<ConstantAggregateZero>(V))
  95       continue;
  96     ConstantStruct *CS = cast<ConstantStruct>(V);
  97     if (isa<ConstantPointerNull>(CS->getOperand(1)))
  98       continue;
  99
 100     // Can only handle global constructors with no arguments.
 101     Function *F = dyn_cast<Function>(CS->getOperand(1));
 102     if (!F || F->arg_size() != 0)
 103       return nullptr;
 104   }
 105   return GV;
 106 }
 107
 108 /// Call "ShouldRemove" for every entry in M's global_ctor list and remove the
 109 /// entries for which it returns true.  Return true if anything changed.
 110 bool llvm::optimizeGlobalCtorsList(
 111     Module &M, function_ref<bool(uint32_t, Function *)> ShouldRemove) {
 112   GlobalVariable *GlobalCtors = findGlobalCtors(M);
 113   if (!GlobalCtors)
 114     return false;
 115
 116   std::vector<std::pair<uint32_t, Function *>> Ctors =
 117       parseGlobalCtors(GlobalCtors);
 118   if (Ctors.empty())
 119     return false;
 120
 121   bool MadeChange = false;
 122   // Loop over global ctors, optimizing them when we can.
 123   BitVector CtorsToRemove(Ctors.size());
 124   std::vector<size_t> CtorsByPriority(Ctors.size());
 125   std::iota(CtorsByPriority.begin(), CtorsByPriority.end(), 0);
 126   stable_sort(CtorsByPriority, [&](size_t LHS, size_t RHS) {
 127     return Ctors[LHS].first < Ctors[RHS].first;
 128   });
 129   for (unsigned CtorIndex : CtorsByPriority) {
 130     const uint32_t Priority = Ctors[CtorIndex].first;
 131     Function *F = Ctors[CtorIndex].second;
 132     if (!F)
 133       continue;
 134
 135     LLVM_DEBUG(dbgs() << "Optimizing Global Constructor: " << *F << "\n");
 136
 137     // If we can evaluate the ctor at compile time, do.
 138     if (ShouldRemove(Priority, F)) {
 139       Ctors[CtorIndex].second = nullptr;
 140       CtorsToRemove.set(CtorIndex);
 141       MadeChange = true;
 142       continue;
 143     }
 144   }
 145
 146   if (!MadeChange)
 147     return false;
 148
 149   removeGlobalCtors(GlobalCtors, CtorsToRemove);
 150   return true;
 151 }