path: root/lib/Target/ARM/ARMConstantIslandPass.cpp

//===-- ARMConstantIslandPass.cpp - ARM constant islands --------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains a pass that splits the constant pool up into 'islands'
// which are scattered through-out the function.  This is required due to the
// limited pc-relative displacements that ARM has.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "arm-cp-islands"
#include "ARM.h"
#include "ARMMachineFunctionInfo.h"
#include "ARMInstrInfo.h"
#include "llvm/CodeGen/MachineConstantPool.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

STATISTIC(NumCPEs,     "Number of constpool entries");
STATISTIC(NumSplit,    "Number of uncond branches inserted");
STATISTIC(NumCBrFixed, "Number of cond branches fixed");
STATISTIC(NumUBrFixed, "Number of uncond branches fixed");

namespace {
  /// ARMConstantIslands - Due to limited PC-relative displacements, ARM
  /// requires constant pool entries to be scattered among the instructions
  /// inside a function.  To do this, it completely ignores the normal LLVM
  /// constant pool; instead, it places constants wherever it feels like with
  /// special instructions.
  ///
  /// The terminology used in this pass includes:
  ///   Islands - Clumps of constants placed in the function.
  ///   Water   - Potential places where an island could be formed.
  ///   CPE     - A constant pool entry that has been placed somewhere, which
  ///             tracks a list of users.
  class VISIBILITY_HIDDEN ARMConstantIslands : public MachineFunctionPass {
    /// NextUID - Assign unique ID's to CPE's.
    unsigned NextUID;

    /// BBSizes - The size of each MachineBasicBlock in bytes of code, indexed
    /// by MBB Number.  The two-byte pads required for Thumb alignment are
    /// counted as part of the following block (i.e., the offset and size for
    /// a padded block will both be ==2 mod 4).
    std::vector<unsigned> BBSizes;
    
    /// BBOffsets - the offset of each MBB in bytes, starting from 0.
    /// The two-byte pads required for Thumb alignment are counted as part of
    /// the following block.
    std::vector<unsigned> BBOffsets;

    /// WaterList - A sorted list of basic blocks where islands could be placed
    /// (i.e. blocks that don't fall through to the following block, due
    /// to a return, unreachable, or unconditional branch).
    std::vector<MachineBasicBlock*> WaterList;

    /// CPUser - One user of a constant pool, keeping the machine instruction
    /// pointer, the constant pool being referenced, and the max displacement
    /// allowed from the instruction to the CP.
    struct CPUser {
      MachineInstr *MI;
      MachineInstr *CPEMI;
      unsigned MaxDisp;
      CPUser(MachineInstr *mi, MachineInstr *cpemi, unsigned maxdisp)
        : MI(mi), CPEMI(cpemi), MaxDisp(maxdisp) {}
    };
    
    /// CPUsers - Keep track of all of the machine instructions that use various
    /// constant pools and their max displacement.
    std::vector<CPUser> CPUsers;
    
    /// CPEntry - One per constant pool entry, keeping the machine instruction
    /// pointer, the constpool index, and the number of CPUser's which
    /// reference this entry.
    struct CPEntry {
      MachineInstr *CPEMI;
      unsigned CPI;
      unsigned RefCount;
      CPEntry(MachineInstr *cpemi, unsigned cp