llvm/lib/Transforms/IPO/GlobalSplit.cpp

   1 //===- GlobalSplit.cpp - global variable splitter -------------------------===//
   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 pass uses inrange annotations on GEP indices to split globals where
  10 // beneficial. Clang currently attaches these annotations to references to
  11 // virtual table globals under the Itanium ABI for the benefit of the
  12 // whole-program virtual call optimization and control flow integrity passes.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #include "llvm/Transforms/IPO/GlobalSplit.h"
  17 #include "llvm/ADT/SmallVector.h"
  18 #include "llvm/ADT/StringExtras.h"
  19 #include "llvm/IR/Constant.h"
  20 #include "llvm/IR/Constants.h"
  21 #include "llvm/IR/DataLayout.h"
  22 #include "llvm/IR/Function.h"
  23 #include "llvm/IR/GlobalValue.h"
  24 #include "llvm/IR/GlobalVariable.h"
  25 #include "llvm/IR/Intrinsics.h"
  26 #include "llvm/IR/LLVMContext.h"
  27 #include "llvm/IR/Metadata.h"
  28 #include "llvm/IR/Module.h"
  29 #include "llvm/IR/Operator.h"
  30 #include "llvm/IR/Type.h"
  31 #include "llvm/IR/User.h"
  32 #include "llvm/InitializePasses.h"
  33 #include "llvm/Pass.h"
  34 #include "llvm/Support/Casting.h"
  35 #include "llvm/Transforms/IPO.h"
  36 #include <cstdint>
  37 #include <vector>
  38
  39 using namespace llvm;
  40
  41 static bool splitGlobal(GlobalVariable &GV) {
  42   // If the address of the global is taken outside of the module, we cannot
  43   // apply this transformation.
  44   if (!GV.hasLocalLinkage())
  45     return false;
  46
  47   // We currently only know how to split ConstantStructs.
  48   auto *Init = dyn_cast_or_null<ConstantStruct>(GV.getInitializer());
  49   if (!Init)
  50     return false;
  51
  52   // Verify that each user of the global is an inrange getelementptr constant.
  53   // From this it follows that any loads from or stores to that global must use
  54   // a pointer derived from an inrange getelementptr constant, which is
  55   // sufficient to allow us to apply the splitting transform.
  56   for (User *U : GV.users()) {
  57     if (!isa<Constant>(U))
  58       return false;
  59
  60     auto *GEP = dyn_cast<GEPOperator>(U);
  61     if (!GEP || !GEP->getInRangeIndex() || *GEP->getInRangeIndex() != 1 ||
  62         !isa<ConstantInt>(GEP->getOperand(1)) ||
  63         !cast<ConstantInt>(GEP->getOperand(1))->isZero() ||
  64         !isa<ConstantInt>(GEP->getOperand(2)))
  65       return false;
  66   }
  67
  68   SmallVector<MDNode *, 2> Types;
  69   GV.getMetadata(LLVMContext::MD_type, Types);
  70
  71   const DataLayout &DL = GV.getParent()->getDataLayout();
  72   const StructLayout *SL = DL.getStructLayout(Init->getType());
  73
  74   IntegerType *Int32Ty = Type::getInt32Ty(GV.getContext());
  75
  76   std::vector<GlobalVariable *> SplitGlobals(Init->getNumOperands());
  77   for (unsigned I = 0; I != Init->getNumOperands(); ++I) {
  78     // Build a global representing this split piece.
  79     auto *SplitGV =
  80         new GlobalVariable(*GV.getParent(), Init->getOperand(I)->getType(),
  81                            GV.isConstant(), GlobalValue::PrivateLinkage,
  82                            Init->getOperand(I), GV.getName() + "." + utostr(I));
  83     SplitGlobals[I] = SplitGV;
  84
  85     unsigned SplitBegin = SL->getElementOffset(I);
  86     unsigned SplitEnd = (I == Init->getNumOperands() - 1)
  87                             ? SL->getSizeInBytes()
  88                             : SL->getElementOffset(I + 1);
  89
  90     // Rebuild type metadata, adjusting by the split offset.
  91     // FIXME: See if we can use DW_OP_piece to preserve debug metadata here.
  92     for (MDNode *Type : Types) {
  93       uint64_t ByteOffset = cast<ConstantInt>(
  94               cast<ConstantAsMetadata>(Type->getOperand(0))->getValue())
  95               ->getZExtValue();
  96       // Type metadata may be attached one byte after the end of the vtable, for
  97       // classes without virtual methods in Itanium ABI. AFAIK, it is never
  98       // attached to the first byte of a vtable. Subtract one to get the right
  99       // slice.
 100       // This is making an assumption that vtable groups are the only kinds of
 101       // global variables that !type metadata can be attached to, and that they
 102       // are either Itanium ABI vtable groups or contain a single vtable (i.e.
 103       // Microsoft ABI vtables).
 104       uint64_t AttachedTo = (ByteOffset == 0) ? ByteOffset : ByteOffset - 1;
 105       if (AttachedTo < SplitBegin || AttachedTo >= SplitEnd)
 106         continue;
 107       SplitGV->addMetadata(
 108           LLVMContext::MD_type,
 109           *MDNode::get(GV.getContext(),
 110                        {ConstantAsMetadata::get(
 111                             ConstantInt::get(Int32Ty, ByteOffset - SplitBegin)),
 112                         Type->getOperand(1)}));
 113     }
 114
 115     if (GV.hasMetadata(LLVMContext::MD_vcall_visibility))
 116       SplitGV->setVCallVisibilityMetadata(GV.getVCallVisibility());
 117   }
 118
 119   for (User *U : GV.users()) {
 120     auto *GEP = cast<GEPOperator>(U);
 121     unsigned I = cast<ConstantInt>(GEP->getOperand(2))->getZExtValue();
 122     if (I >= SplitGlobals.size())
 123       continue;
 124
 125     SmallVector<Value *, 4> Ops;
 126     Ops.push_back(ConstantInt::get(Int32Ty, 0));
 127     for (unsigned I = 3; I != GEP->getNumOperands(); ++I)
 128       Ops.push_back(GEP->getOperand(I));
 129
 130     auto *NewGEP = ConstantExpr::getGetElementPtr(
 131         SplitGlobals[I]->getInitializer()->getType(), SplitGlobals[I], Ops,
 132         GEP->isInBounds());
 133     GEP->replaceAllUsesWith(NewGEP);
 134   }
 135
 136   // Finally, remove the original global. Any remaining uses refer to invalid
 137   // elements of the global, so replace with undef.
 138   if (!GV.use_empty())
 139     GV.replaceAllUsesWith(UndefValue::get(GV.getType()));
 140   GV.eraseFromParent();
 141   return true;
 142 }
 143
 144 static bool splitGlobals(Module &M) {
 145   // First, see if the module uses either of the llvm.type.test or
 146   // llvm.type.checked.load intrinsics, which indicates that splitting globals
 147   // may be beneficial.
 148   Function *TypeTestFunc =
 149       M.getFunction(Intrinsic::getName(Intrinsic::type_test));
 150   Function *TypeCheckedLoadFunc =
 151       M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
 152   if ((!TypeTestFunc || TypeTestFunc->use_empty()) &&
 153       (!TypeCheckedLoadFunc || TypeCheckedLoadFunc->use_empty()))
 154     return false;
 155
 156   bool Changed = false;
 157   for (auto I = M.global_begin(); I != M.global_end();) {
 158     GlobalVariable &GV = *I;
 159     ++I;
 160     Changed |= splitGlobal(GV);
 161   }
 162   return Changed;
 163 }
 164
 165 namespace {
 166
 167 struct GlobalSplit : public ModulePass {
 168   static char ID;
 169
 170   GlobalSplit() : ModulePass(ID) {
 171     initializeGlobalSplitPass(*PassRegistry::getPassRegistry());
 172   }
 173
 174   bool runOnModule(Module &M) override {
 175     if (skipModule(M))
 176       return false;
 177
 178     return splitGlobals(M);
 179   }
 180 };
 181
 182 } // end anonymous namespace
 183
 184 char GlobalSplit::ID = 0;
 185
 186 INITIALIZE_PASS(GlobalSplit, "globalsplit", "Global splitter", false, false)
 187
 188 ModulePass *llvm::createGlobalSplitPass() {
 189   return new GlobalSplit;
 190 }
 191
 192 PreservedAnalyses GlobalSplitPass::run(Module &M, ModuleAnalysisManager &AM) {
 193   if (!splitGlobals(M))
 194     return PreservedAnalyses::all();
 195   return PreservedAnalyses::none();
 196 }