lib/Transforms/Utils/CanonicalizeAliases.cpp

   1 //===- CanonicalizeAliases.cpp - ThinLTO Support: Canonicalize Aliases ----===//
   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 // Currently this file implements partial alias canonicalization, to
  10 // flatten chains of aliases (also done by GlobalOpt, but not on for
  11 // O0 compiles). E.g.
  12 //  @a = alias i8, i8 *@b
  13 //  @b = alias i8, i8 *@g
  14 //
  15 // will be converted to:
  16 //  @a = alias i8, i8 *@g  <-- @a is now an alias to base object @g
  17 //  @b = alias i8, i8 *@g
  18 //
  19 // Eventually this file will implement full alias canonicalation, so that
  20 // all aliasees are private anonymous values. E.g.
  21 //  @a = alias i8, i8 *@g
  22 //  @g = global i8 0
  23 //
  24 // will be converted to:
  25 //  @0 = private global
  26 //  @a = alias i8, i8* @0
  27 //  @g = alias i8, i8* @0
  28 //
  29 // This simplifies optimization and ThinLTO linking of the original symbols.
  30 //===----------------------------------------------------------------------===//
  31
  32 #include "llvm/Transforms/Utils/CanonicalizeAliases.h"
  33
  34 #include "llvm/IR/Operator.h"
  35 #include "llvm/IR/ValueHandle.h"
  36
  37 using namespace llvm;
  38
  39 namespace {
  40
  41 static Constant *canonicalizeAlias(Constant *C, bool &Changed) {
  42   if (auto *GA = dyn_cast<GlobalAlias>(C)) {
  43     auto *NewAliasee = canonicalizeAlias(GA->getAliasee(), Changed);
  44     if (NewAliasee != GA->getAliasee()) {
  45       GA->setAliasee(NewAliasee);
  46       Changed = true;
  47     }
  48     return NewAliasee;
  49   }
  50
  51   auto *CE = dyn_cast<ConstantExpr>(C);
  52   if (!CE)
  53     return C;
  54
  55   std::vector<Constant *> Ops;
  56   for (Use &U : CE->operands())
  57     Ops.push_back(canonicalizeAlias(cast<Constant>(U), Changed));
  58   return CE->getWithOperands(Ops);
  59 }
  60
  61 /// Convert aliases to canonical form.
  62 static bool canonicalizeAliases(Module &M) {
  63   bool Changed = false;
  64   for (auto &GA : M.aliases())
  65     canonicalizeAlias(&GA, Changed);
  66   return Changed;
  67 }
  68
  69 // Legacy pass that canonicalizes aliases.
  70 class CanonicalizeAliasesLegacyPass : public ModulePass {
  71
  72 public:
  73   /// Pass identification, replacement for typeid
  74   static char ID;
  75
  76   /// Specify pass name for debug output
  77   StringRef getPassName() const override { return "Canonicalize Aliases"; }
  78
  79   explicit CanonicalizeAliasesLegacyPass() : ModulePass(ID) {}
  80
  81   bool runOnModule(Module &M) override { return canonicalizeAliases(M); }
  82 };
  83 char CanonicalizeAliasesLegacyPass::ID = 0;
  84
  85 } // anonymous namespace
  86
  87 PreservedAnalyses CanonicalizeAliasesPass::run(Module &M,
  88                                                ModuleAnalysisManager &AM) {
  89   if (!canonicalizeAliases(M))
  90     return PreservedAnalyses::all();
  91
  92   return PreservedAnalyses::none();
  93 }
  94
  95 INITIALIZE_PASS_BEGIN(CanonicalizeAliasesLegacyPass, "canonicalize-aliases",
  96                       "Canonicalize aliases", false, false)
  97 INITIALIZE_PASS_END(CanonicalizeAliasesLegacyPass, "canonicalize-aliases",
  98                     "Canonicalize aliases", false, false)
  99
 100 namespace llvm {
 101 ModulePass *createCanonicalizeAliasesPass() {
 102   return new CanonicalizeAliasesLegacyPass();
 103 }
 104 } // namespace llvm