llvm-6502/lib/Transforms/Scalar/LoopUnrollPass.cpp
Devang Patel 9be72d4394 Initialze ScalarEvalution dependency.
Patch by Pranav Bhandarkar!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142556 91177308-0d34-0410-b5e6-96231b3b80d8
2011-10-19 23:56:07 +00:00

216 lines
7.9 KiB
C++

//===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements a simple loop unroller. It works best when loops have
// been canonicalized by the -indvars pass, allowing it to determine the trip
// counts of loops easily.
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "loop-unroll"
#include "llvm/IntrinsicInst.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/UnrollLoop.h"
#include "llvm/Target/TargetData.h"
#include <climits>
using namespace llvm;
static cl::opt<unsigned>
UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
cl::desc("The cut-off point for automatic loop unrolling"));
static cl::opt<unsigned>
UnrollCount("unroll-count", cl::init(0), cl::Hidden,
cl::desc("Use this unroll count for all loops, for testing purposes"));
static cl::opt<bool>
UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
cl::desc("Allows loops to be partially unrolled until "
"-unroll-threshold loop size is reached."));
// Temporary flag to be removed in 3.0
static cl::opt<bool>
NoSCEVUnroll("disable-unroll-scev", cl::init(false), cl::Hidden,
cl::desc("Use ScalarEvolution to analyze loop trip counts for unrolling"));
namespace {
class LoopUnroll : public LoopPass {
public:
static char ID; // Pass ID, replacement for typeid
LoopUnroll(int T = -1, int C = -1, int P = -1) : LoopPass(ID) {
CurrentThreshold = (T == -1) ? UnrollThreshold : unsigned(T);
CurrentCount = (C == -1) ? UnrollCount : unsigned(C);
CurrentAllowPartial = (P == -1) ? UnrollAllowPartial : (bool)P;
UserThreshold = (T != -1) || (UnrollThreshold.getNumOccurrences() > 0);
initializeLoopUnrollPass(*PassRegistry::getPassRegistry());
}
/// A magic value for use with the Threshold parameter to indicate
/// that the loop unroll should be performed regardless of how much
/// code expansion would result.
static const unsigned NoThreshold = UINT_MAX;
// Threshold to use when optsize is specified (and there is no
// explicit -unroll-threshold).
static const unsigned OptSizeUnrollThreshold = 50;
unsigned CurrentCount;
unsigned CurrentThreshold;
bool CurrentAllowPartial;
bool UserThreshold; // CurrentThreshold is user-specified.
bool runOnLoop(Loop *L, LPPassManager &LPM);
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG...
///
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<LoopInfo>();
AU.addPreserved<LoopInfo>();
AU.addRequiredID(LoopSimplifyID);
AU.addPreservedID(LoopSimplifyID);
AU.addRequiredID(LCSSAID);
AU.addPreservedID(LCSSAID);
AU.addRequired<ScalarEvolution>();
AU.addPreserved<ScalarEvolution>();
// FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
// If loop unroll does not preserve dom info then LCSSA pass on next
// loop will receive invalid dom info.
// For now, recreate dom info, if loop is unrolled.
AU.addPreserved<DominatorTree>();
}
};
}
char LoopUnroll::ID = 0;
INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
INITIALIZE_PASS_DEPENDENCY(LCSSA)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial) {
return new LoopUnroll(Threshold, Count, AllowPartial);
}
/// ApproximateLoopSize - Approximate the size of the loop.
static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
const TargetData *TD) {
CodeMetrics Metrics;
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I)
Metrics.analyzeBasicBlock(*I, TD);
NumCalls = Metrics.NumInlineCandidates;
unsigned LoopSize = Metrics.NumInsts;
// Don't allow an estimate of size zero. This would allows unrolling of loops
// with huge iteration counts, which is a compile time problem even if it's
// not a problem for code quality.
if (LoopSize == 0) LoopSize = 1;
return LoopSize;
}
bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
LoopInfo *LI = &getAnalysis<LoopInfo>();
ScalarEvolution *SE = &getAnalysis<ScalarEvolution>();
BasicBlock *Header = L->getHeader();
DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
<< "] Loop %" << Header->getName() << "\n");
(void)Header;
// Determine the current unrolling threshold. While this is normally set
// from UnrollThreshold, it is overridden to a smaller value if the current
// function is marked as optimize-for-size, and the unroll threshold was
// not user specified.
unsigned Threshold = CurrentThreshold;
if (!UserThreshold &&
Header->getParent()->hasFnAttr(Attribute::OptimizeForSize))
Threshold = OptSizeUnrollThreshold;
// Find trip count and trip multiple if count is not available
unsigned TripCount = 0;
unsigned TripMultiple = 1;
if (!NoSCEVUnroll) {
// Find "latch trip count". UnrollLoop assumes that control cannot exit
// via the loop latch on any iteration prior to TripCount. The loop may exit
// early via an earlier branch.
BasicBlock *LatchBlock = L->getLoopLatch();
if (LatchBlock) {
TripCount = SE->getSmallConstantTripCount(L, LatchBlock);
TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock);
}
}
else {
TripCount = L->getSmallConstantTripCount();
if (TripCount == 0)
TripMultiple = L->getSmallConstantTripMultiple();
}
// Automatically select an unroll count.
unsigned Count = CurrentCount;
if (Count == 0) {
// Conservative heuristic: if we know the trip count, see if we can
// completely unroll (subject to the threshold, checked below); otherwise
// try to find greatest modulo of the trip count which is still under
// threshold value.
if (TripCount == 0)
return false;
Count = TripCount;
}
// Enforce the threshold.
if (Threshold != NoThreshold) {
const TargetData *TD = getAnalysisIfAvailable<TargetData>();
unsigned NumInlineCandidates;
unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates, TD);
DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n");
if (NumInlineCandidates != 0) {
DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n");
return false;
}
uint64_t Size = (uint64_t)LoopSize*Count;
if (TripCount != 1 && Size > Threshold) {
DEBUG(dbgs() << " Too large to fully unroll with count: " << Count
<< " because size: " << Size << ">" << Threshold << "\n");
if (!CurrentAllowPartial) {
DEBUG(dbgs() << " will not try to unroll partially because "
<< "-unroll-allow-partial not given\n");
return false;
}
// Reduce unroll count to be modulo of TripCount for partial unrolling
Count = Threshold / LoopSize;
while (Count != 0 && TripCount%Count != 0) {
Count--;
}
if (Count < 2) {
DEBUG(dbgs() << " could not unroll partially\n");
return false;
}
DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n");
}
}
// Unroll the loop.
if (!UnrollLoop(L, Count, TripCount, TripMultiple, LI, &LPM))
return false;
return true;
}