path: root/lib/Transforms/Scalar/LoopUnswitch.cpp

//===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass transforms loops that contain branches on loop-invariant conditions
// to have multiple loops.  For example, it turns the left into the right code:
//
//  for (...)                  if (lic)
//    A                          for (...)
//    if (lic)                     A; B; C
//      B                      else
//    C                          for (...)
//                                 A; C
//
// This can increase the size of the code exponentially (doubling it every time
// a loop is unswitched) so we only unswitch if the resultant code will be
// smaller than a threshold.
//
// This pass expects LICM to be run before it to hoist invariant conditions out
// of the loop, to make the unswitching opportunity obvious.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "loop-unswitch"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <map>
#include <set>
using namespace llvm;

STATISTIC(NumBranches, "Number of branches unswitched");
STATISTIC(NumSwitches, "Number of switches unswitched");
STATISTIC(NumSelects , "Number of selects unswitched");
STATISTIC(NumTrivial , "Number of unswitches that are trivial");
STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
STATISTIC(TotalInsts,  "Total number of instructions analyzed");

// The specific value of 100 here was chosen based only on intuition and a
// few specific examples.
static cl::opt<unsigned>
Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
          cl::init(100), cl::Hidden);

namespace {

  class LUAnalysisCache {

    typedef DenseMap<const SwitchInst*, SmallPtrSet<const Value *, 8> >
      UnswitchedValsMap;

    typedef UnswitchedValsMap::iterator UnswitchedValsIt;

    struct LoopProperties {
      unsigned CanBeUnswitchedCount;
      unsigned SizeEstimation;
      UnswitchedValsMap UnswitchedVals;
    };

    // Here we use std::map instead of DenseMap, since we need to keep valid
    // LoopProperties pointer for current loop for better performance.
    typedef std::map<const Loop*, LoopProperties> LoopPropsMap;
    typedef LoopPropsMap::iterator LoopPropsMapIt;

    LoopPropsMap LoopsProperties;
    UnswitchedValsMap* CurLoopInstructions;
    LoopProperties* CurrentLoopProperties;

    // Max size of code we can produce on remained iterations.
    unsigned MaxSize;

    public:

      LUAnalysisCache() :