1 files changed, 361 insertions, 0 deletions
diff --git a/lib/CodeGen/GlobalMerge.cpp b/lib/CodeGen/GlobalMerge.cpp
new file mode 100644
index 0000000..027ee38
--- /dev/null
+++ b/lib/CodeGen/GlobalMerge.cpp
@@ -0,0 +1,361 @@
+//===-- GlobalMerge.cpp - Internal globals merging  -----------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// This pass merges globals with internal linkage into one. This way all the
+// globals which were merged into a biggest one can be addressed using offsets
+// from the same base pointer (no need for separate base pointer for each of the
+// global). Such a transformation can significantly reduce the register pressure
+// when many globals are involved.
+//
+// For example, consider the code which touches several global variables at
+// once:
+//
+// static int foo[N], bar[N], baz[N];
+//
+// for (i = 0; i < N; ++i) {
+//    foo[i] = bar[i] * baz[i];
+// }
+//
+//  On ARM the addresses of 3 arrays should be kept in the registers, thus
+//  this code has quite large register pressure (loop body):
+//
+//  ldr     r1, [r5], #4
+//  ldr     r2, [r6], #4
+//  mul     r1, r2, r1
+//  str     r1, [r0], #4
+//
+//  Pass converts the code to something like:
+//
+//  static struct {
+//    int foo[N];
+//    int bar[N];
+//    int baz[N];
+//  } merged;
+//
+//  for (i = 0; i < N; ++i) {
+//    merged.foo[i] = merged.bar[i] * merged.baz[i];
+//  }
+//
+//  and in ARM code this becomes:
+//
+//  ldr     r0, [r5, #40]
+//  ldr     r1, [r5, #80]
+//  mul     r0, r1, r0
+//  str     r0, [r5], #4
+//
+//  note that we saved 2 registers here almostly "for free".
+// ===---------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/IR/Attributes.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Target/TargetLowering.h"
+#include "llvm/Target/TargetLoweringObjectFile.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "global-merge"
+
+cl::opt<bool>
+EnableGlobalMerge("enable-global-merge", cl::Hidden,
+                  cl::desc("Enable global merge pass"),
+                  cl::init(true));
+
+static cl::opt<bool>
+EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
+                         cl::desc("Enable global merge pass on constants"),
+                         cl::init(false));
+
+// FIXME: this could be a transitional option, and we probably need to remove
+// it if only we are sure this optimization could always benefit all targets.
+static cl::opt<bool>
+EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
+     cl::desc("Enable global merge pass on external linkage"),
+     cl::init(false));
+
+STATISTIC(NumMerged      , "Number of globals merged");
+namespace {
+  class GlobalMerge : public FunctionPass {
+    const TargetMachine *TM;
+
+    bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
+                 Module &M, bool isConst, unsigned AddrSpace) const;
+
+    /// \brief Check if the given variable has been identified as must keep
+    /// \pre setMustKeepGlobalVariables must have been called on the Module that
+    ///      contains GV
+    bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
+      return MustKeepGlobalVariables.count(GV);
+    }
+
+    /// Collect every variables marked as "used" or used in a landing pad
+    /// instruction for this Module.
+    void setMustKeepGlobalVariables(Module &M);
+
+    /// Collect every variables marked as "used"
+    void collectUsedGlobalVariables(Module &M);
+
+    /// Keep track of the GlobalVariable that must not be merged away
+    SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
+
+  public:
+    static char ID;             // Pass identification, replacement for typeid.
+    explicit GlobalMerge(const TargetMachine *TM = nullptr)
+      : FunctionPass(ID), TM(TM) {
+      initializeGlobalMergePass(*PassRegistry::getPassRegistry());
+    }
+
+    bool doInitialization(Module &M) override;
+    bool runOnFunction(Function &F) override;
+    bool doFinalization(Module &M) override;
+
+    const char *getPassName() const override {
+      return "Merge internal globals";
+    }
+
+    void getAnalysisUsage(AnalysisUsage &AU) const override {
+      AU.setPreservesCFG();
+      FunctionPass::getAnalysisUsage(AU);
+    }
+  };
+} // end anonymous namespace
+
+char GlobalMerge::ID = 0;
+INITIALIZE_TM_PASS(GlobalMerge, "global-merge", "Merge global variables",
+                   false, false)
+
+bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
+                          Module &M, bool isConst, unsigned AddrSpace) const {
+  const TargetLowering *TLI = TM->getTargetLowering();
+  const DataLayout *DL = TLI->getDataLayout();
+
+  // FIXME: Infer the maximum possible offset depending on the actual users
+  // (these max offsets are different for the users inside Thumb or ARM
+  // functions)
+  unsigned MaxOffset = TLI->getMaximalGlobalOffset();
+
+  // FIXME: Find better heuristics
+  std::stable_sort(Globals.begin(), Globals.end(),
+                   [DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
+    Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType();
+    Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType();
+
+    return (DL->getTypeAllocSize(Ty1) < DL->getTypeAllocSize(Ty2));
+  });
+
+  Type *Int32Ty = Type::getInt32Ty(M.getContext());
+
+  assert(Globals.size() > 1);
+
+  // FIXME: This simple solution merges globals all together as maximum as
+  // possible. However, with this solution it would be hard to remove dead
+  // global symbols at link-time. An alternative solution could be checking
+  // global symbols references function by function, and make the symbols
+  // being referred in the same function merged and we would probably need
+  // to introduce heuristic algorithm to solve the merge conflict from
+  // different functions.
+  for (size_t i = 0, e = Globals.size(); i != e; ) {
+    size_t j = 0;
+    uint64_t MergedSize = 0;
+    std::vector<Type*> Tys;
+    std::vector<Constant*> Inits;
+
+    bool HasExternal = false;
+    GlobalVariable *TheFirstExternal = 0;
+    for (j = i; j != e; ++j) {
+      Type *Ty = Globals[j]->getType()->getElementType();
+      MergedSize += DL->getTypeAllocSize(Ty);
+      if (MergedSize > MaxOffset) {
+        break;
+      }
+      Tys.push_back(Ty);
+      Inits.push_back(Globals[j]->getInitializer());
+
+      if (Globals[j]->hasExternalLinkage() && !HasExternal) {
+        HasExternal = true;
+        TheFirstExternal = Globals[j];
+      }
+    }
+
+    // If merged variables doesn't have external linkage, we needn't to expose
+    // the symbol after merging.
+    GlobalValue::LinkageTypes Linkage = HasExternal
+                                            ? GlobalValue::ExternalLinkage
+                                            : GlobalValue::InternalLinkage;
+
+    StructType *MergedTy = StructType::get(M.getContext(), Tys);
+    Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
+
+    // If merged variables have external linkage, we use symbol name of the
+    // first variable merged as the suffix of global symbol name. This would
+    // be able to avoid the link-time naming conflict for globalm symbols.
+    GlobalVariable *MergedGV = new GlobalVariable(
+        M, MergedTy, isConst, Linkage, MergedInit,
+        HasExternal ? "_MergedGlobals_" + TheFirstExternal->getName()
+                    : "_MergedGlobals",
+        nullptr, GlobalVariable::NotThreadLocal, AddrSpace);
+
+    for (size_t k = i; k < j; ++k) {
+      GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
+      std::string Name = Globals[k]->getName();
+
+      Constant *Idx[2] = {
+        ConstantInt::get(Int32Ty, 0),
+        ConstantInt::get(Int32Ty, k-i)
+      };
+      Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx);
+      Globals[k]->replaceAllUsesWith(GEP);
+      Globals[k]->eraseFromParent();
+
+      if (Linkage != GlobalValue::InternalLinkage) {
+        // Generate a new alias...
+        auto *PTy = cast<PointerType>(GEP->getType());
+        GlobalAlias::create(PTy->getElementType(), PTy->getAddressSpace(),
+                            Linkage, Name, GEP, &M);
+      }
+
+      NumMerged++;
+    }
+    i = j;
+  }
+
+  return true;
+}
+
+void GlobalMerge::collectUsedGlobalVariables(Module &M) {
+  // Extract global variables from llvm.used array
+  const GlobalVariable *GV = M.getGlobalVariable("llvm.used");
+  if (!GV || !GV->hasInitializer()) return;
+
+  // Should be an array of 'i8*'.
+  const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());
+
+  for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
+    if (const GlobalVariable *G =
+        dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
+      MustKeepGlobalVariables.insert(G);
+}
+
+void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
+  collectUsedGlobalVariables(M);
+
+  for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn;
+       ++IFn) {
+    for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
+         IBB != IEndBB; ++IBB) {
+      // Follow the invoke link to find the landing pad instruction
+      const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator());
+      if (!II) continue;
+
+      const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst();
+      // Look for globals in the clauses of the landing pad instruction
+      for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses();
+           Idx != NumClauses; ++Idx)
+        if (const GlobalVariable *GV =
+            dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
+                                     ->stripPointerCasts()))
+          MustKeepGlobalVariables.insert(GV);
+    }
+  }
+}
+
+bool GlobalMerge::doInitialization(Module &M) {
+  if (!EnableGlobalMerge)
+    return false;
+
+  DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
+                                                        BSSGlobals;
+  const TargetLowering *TLI = TM->getTargetLowering();
+  const DataLayout *DL = TLI->getDataLayout();
+  unsigned MaxOffset = TLI->getMaximalGlobalOffset();
+  bool Changed = false;
+  setMustKeepGlobalVariables(M);
+
+  // Grab all non-const globals.
+  for (Module::global_iterator I = M.global_begin(),
+         E = M.global_end(); I != E; ++I) {
+    // Merge is safe for "normal" internal or external globals only
+    if (I->isDeclaration() || I->isThreadLocal() || I->hasSection())
+      continue;
+
+    if (!(EnableGlobalMergeOnExternal && I->hasExternalLinkage()) &&
+        !I->hasInternalLinkage())
+      continue;
+
+    PointerType *PT = dyn_cast<PointerType>(I->getType());
+    assert(PT && "Global variable is not a pointer!");
+
+    unsigned AddressSpace = PT->getAddressSpace();
+
+    // Ignore fancy-aligned globals for now.
+    unsigned Alignment = DL->getPreferredAlignment(I);
+    Type *Ty = I->getType()->getElementType();
+    if (Alignment > DL->getABITypeAlignment(Ty))
+      continue;
+
+    // Ignore all 'special' globals.
+    if (I->getName().startswith("llvm.") ||
+        I->getName().startswith(".llvm."))
+      continue;
+
+    // Ignore all "required" globals:
+    if (isMustKeepGlobalVariable(I))
+      continue;
+
+    if (DL->getTypeAllocSize(Ty) < MaxOffset) {
+      if (TargetLoweringObjectFile::getKindForGlobal(I, *TM).isBSSLocal())
+        BSSGlobals[AddressSpace].push_back(I);
+      else if (I->isConstant())
+        ConstGlobals[AddressSpace].push_back(I);
+      else
+        Globals[AddressSpace].push_back(I);
+    }
+  }
+
+  for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
+       I = Globals.begin(), E = Globals.end(); I != E; ++I)
+    if (I->second.size() > 1)
+      Changed |= doMerge(I->second, M, false, I->first);
+
+  for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
+       I = BSSGlobals.begin(), E = BSSGlobals.end(); I != E; ++I)
+    if (I->second.size() > 1)
+      Changed |= doMerge(I->second, M, false, I->first);
+
+  if (EnableGlobalMergeOnConst)
+    for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
+         I = ConstGlobals.begin(), E = ConstGlobals.end(); I != E; ++I)
+      if (I->second.size() > 1)
+        Changed |= doMerge(I->second, M, true, I->first);
+
+  return Changed;
+}
+
+bool GlobalMerge::runOnFunction(Function &F) {
+  return false;
+}
+
+bool GlobalMerge::doFinalization(Module &M) {
+  MustKeepGlobalVariables.clear();
+  return false;
+}
+
+Pass *llvm::createGlobalMergePass(const TargetMachine *TM) {
+  return new GlobalMerge(TM);
+}