lib/Target/ARM/ARMGlobalMerge.cpp

//===-- ARMGlobalMerge.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".
// ===----------------------------------------------------------------------===//

#define DEBUG_TYPE "arm-global-merge"
#include "ARM.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Attributes.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/Intrinsics.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetLowering.h"
using namespace llvm;

namespace {
  class LLVM_LIBRARY_VISIBILITY ARMGlobalMerge : public FunctionPass {
    /// TLI - Keep a pointer of a TargetLowering to consult for determining
    /// target type sizes.
    const TargetLowering *TLI;

    bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
                 Module &M, bool) const;

  public:
    static char ID;             // Pass identification, replacement for typeid.
    explicit ARMGlobalMerge(const TargetLowering *tli)
      : FunctionPass(&ID), TLI(tli) {}

    virtual bool doInitialization(Module &M);
    virtual bool runOnFunction(Function& F);

    const char *getPassName() const {
      return "Merge internal globals";
    }

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
      FunctionPass::getAnalysisUsage(AU);
    }

    struct GlobalCmp {
      const TargetData *TD;

      GlobalCmp(const TargetData *td):
        TD(td) { };

      bool operator() (const GlobalVariable* GV1,
                       const GlobalVariable* GV2) {
        const Type* Ty1 = cast<PointerType>(GV1->getType())->getElementType();
        const Type* Ty2 = cast<PointerType>(GV2->getType())->getElementType();

        return (TD->getTypeAllocSize(Ty1) < TD->getTypeAllocSize(Ty2));
      }
    };
  };
} // end anonymous namespace

char ARMGlobalMerge::ID = 0;

bool ARMGlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
                             Module &M, bool isConst) const {
  const TargetData *TD = TLI->getTargetData();

  // 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(), GlobalCmp(TD));

  const Type *Int32Ty = Type::getInt32Ty(M.getContext());

  for (size_t i = 0, e = Globals.size(); i != e; ) {
    size_t j = 0;
    uint64_t MergedSize = 0;
    std::vector<const Type*> Tys;
    std::vector<Constant*> Inits;
    for (j = i; MergedSize < MaxOffset && j != e; ++j) {
      const Type* Ty = Globals[j]->getType()->getElementType();
      Tys.push_back(Ty);
      Inits.push_back(Globals[j]->getInitializer());
      MergedSize += TD->getTypeAllocSize(Ty);
    }

    StructType* MergedTy = StructType::get(M.getContext(), Tys);
    Constant* MergedInit = ConstantStruct::get(MergedTy, Inits);
    GlobalVariable* MergedGV = new GlobalVariable(M, MergedTy, isConst,
                                                  GlobalValue::InternalLinkage,
                                                  MergedInit, "merged");
    for (size_t k = i; k < j; ++k) {
      SmallVector<Constant*, 2> Idx;
      Idx.push_back(ConstantInt::get(Int32Ty, 0));
      Idx.push_back(ConstantInt::get(Int32Ty, k-i));

      Constant* GEP =
        ConstantExpr::getInBoundsGetElementPtr(MergedGV,
                                               &Idx[0], Idx.size());

      Globals[k]->replaceAllUsesWith(GEP);
      Globals[k]->eraseFromParent();
    }
    i = j;
  }

  return true;
}


bool ARMGlobalMerge::doInitialization(Module& M) {
  SmallVector<GlobalVariable*, 16> Globals, ConstGlobals;
  const TargetData *TD = TLI->getTargetData();
  unsigned MaxOffset = TLI->getMaximalGlobalOffset();
  bool Changed = false;

  // 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 globals only
    if (!I->hasLocalLinkage() || I->isThreadLocal() || I->hasSection())
      continue;

    // Ignore fancy-aligned globals for now.
    if (I->getAlignment() != 0)
      continue;

    if (TD->getTypeAllocSize(I->getType()) < MaxOffset) {
      if (I->isConstant())
        ConstGlobals.push_back(I);
      else
        Globals.push_back(I);
    }
  }

  if (Globals.size() > 1)
    Changed |= doMerge(Globals, M, false);
  if (ConstGlobals.size() > 1)
    Changed |= doMerge(ConstGlobals, M, true);

  return Changed;
}

bool ARMGlobalMerge::runOnFunction(Function& F) {
  return false;
}

FunctionPass *llvm::createARMGlobalMergePass(const TargetLowering *tli) {
  return new ARMGlobalMerge(tli);
}