llvm-project/llvm/lib/Transforms/Scalar/LoopUnroll.cpp

144 lines
4.7 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/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Transforms/Utils/UnrollLoop.h"
#include <climits>
using namespace llvm;
static cl::opt<unsigned>
UnrollThreshold("unroll-threshold", cl::init(100), 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"));
namespace {
class VISIBILITY_HIDDEN LoopUnroll : public LoopPass {
public:
static char ID; // Pass ID, replacement for typeid
LoopUnroll() : LoopPass((intptr_t)&ID) {}
/// 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;
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.addRequiredID(LoopSimplifyID);
AU.addRequiredID(LCSSAID);
AU.addRequired<LoopInfo>();
AU.addPreservedID(LCSSAID);
AU.addPreserved<LoopInfo>();
}
};
}
char LoopUnroll::ID = 0;
static RegisterPass<LoopUnroll> X("loop-unroll", "Unroll loops");
LoopPass *llvm::createLoopUnrollPass() { return new LoopUnroll(); }
/// ApproximateLoopSize - Approximate the size of the loop.
static unsigned ApproximateLoopSize(const Loop *L) {
unsigned Size = 0;
for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i) {
BasicBlock *BB = L->getBlocks()[i];
Instruction *Term = BB->getTerminator();
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
if (isa<PHINode>(I) && BB == L->getHeader()) {
// Ignore PHI nodes in the header.
} else if (I->hasOneUse() && I->use_back() == Term) {
// Ignore instructions only used by the loop terminator.
} else if (isa<DbgInfoIntrinsic>(I)) {
// Ignore debug instructions
} else if (isa<CallInst>(I)) {
// Estimate size overhead introduced by call instructions which
// is higher than other instructions. Here 3 and 10 are magic
// numbers that help one isolated test case from PR2067 without
// negatively impacting measured benchmarks.
if (isa<IntrinsicInst>(I))
Size = Size + 3;
else
Size = Size + 10;
} else {
++Size;
}
// TODO: Ignore expressions derived from PHI and constants if inval of phi
// is a constant, or if operation is associative. This will get induction
// variables.
}
}
return Size;
}
bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
assert(L->isLCSSAForm());
LoopInfo *LI = &getAnalysis<LoopInfo>();
BasicBlock *Header = L->getHeader();
DOUT << "Loop Unroll: F[" << Header->getParent()->getName()
<< "] Loop %" << Header->getName() << "\n";
// Find trip count
unsigned TripCount = L->getSmallConstantTripCount();
unsigned Count = UnrollCount;
// Automatically select an unroll count.
if (Count == 0) {
// Conservative heuristic: if we know the trip count, see if we can
// completely unroll (subject to the threshold, checked below); otherwise
// don't unroll.
if (TripCount != 0) {
Count = TripCount;
} else {
return false;
}
}
// Enforce the threshold.
if (UnrollThreshold != NoThreshold) {
unsigned LoopSize = ApproximateLoopSize(L);
DOUT << " Loop Size = " << LoopSize << "\n";
uint64_t Size = (uint64_t)LoopSize*Count;
if (TripCount != 1 && Size > UnrollThreshold) {
DOUT << " TOO LARGE TO UNROLL: "
<< Size << ">" << UnrollThreshold << "\n";
return false;
}
}
// Unroll the loop.
if (!UnrollLoop(L, Count, LI, &LPM))
return false;
return true;
}