forked from OSchip/llvm-project
265 lines
10 KiB
C++
265 lines
10 KiB
C++
//===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass implements a simple loop unroller. It works best when loops have
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// been canonicalized by the -indvars pass, allowing it to determine the trip
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// counts of loops easily.
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "loop-unroll"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Analysis/CodeMetrics.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Analysis/TargetTransformInfo.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Utils/UnrollLoop.h"
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#include <climits>
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using namespace llvm;
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static cl::opt<unsigned>
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UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
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cl::desc("The cut-off point for automatic loop unrolling"));
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static cl::opt<unsigned>
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UnrollCount("unroll-count", cl::init(0), cl::Hidden,
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cl::desc("Use this unroll count for all loops, for testing purposes"));
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static cl::opt<bool>
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UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
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cl::desc("Allows loops to be partially unrolled until "
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"-unroll-threshold loop size is reached."));
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static cl::opt<bool>
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UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden,
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cl::desc("Unroll loops with run-time trip counts"));
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namespace {
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class LoopUnroll : public LoopPass {
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public:
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static char ID; // Pass ID, replacement for typeid
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LoopUnroll(int T = -1, int C = -1, int P = -1, int R = -1) : LoopPass(ID) {
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CurrentThreshold = (T == -1) ? UnrollThreshold : unsigned(T);
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CurrentCount = (C == -1) ? UnrollCount : unsigned(C);
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CurrentAllowPartial = (P == -1) ? UnrollAllowPartial : (bool)P;
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CurrentRuntime = (R == -1) ? UnrollRuntime : (bool)R;
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UserThreshold = (T != -1) || (UnrollThreshold.getNumOccurrences() > 0);
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UserAllowPartial = (P != -1) ||
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(UnrollAllowPartial.getNumOccurrences() > 0);
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UserRuntime = (R != -1) || (UnrollRuntime.getNumOccurrences() > 0);
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UserCount = (C != -1) || (UnrollCount.getNumOccurrences() > 0);
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initializeLoopUnrollPass(*PassRegistry::getPassRegistry());
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}
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/// A magic value for use with the Threshold parameter to indicate
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/// that the loop unroll should be performed regardless of how much
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/// code expansion would result.
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static const unsigned NoThreshold = UINT_MAX;
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// Threshold to use when optsize is specified (and there is no
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// explicit -unroll-threshold).
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static const unsigned OptSizeUnrollThreshold = 50;
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// Default unroll count for loops with run-time trip count if
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// -unroll-count is not set
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static const unsigned UnrollRuntimeCount = 8;
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unsigned CurrentCount;
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unsigned CurrentThreshold;
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bool CurrentAllowPartial;
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bool CurrentRuntime;
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bool UserCount; // CurrentCount is user-specified.
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bool UserThreshold; // CurrentThreshold is user-specified.
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bool UserAllowPartial; // CurrentAllowPartial is user-specified.
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bool UserRuntime; // CurrentRuntime is user-specified.
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bool runOnLoop(Loop *L, LPPassManager &LPM) override;
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/// This transformation requires natural loop information & requires that
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/// loop preheaders be inserted into the CFG...
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///
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<LoopInfo>();
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AU.addPreserved<LoopInfo>();
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AU.addRequiredID(LoopSimplifyID);
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AU.addPreservedID(LoopSimplifyID);
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AU.addRequiredID(LCSSAID);
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AU.addPreservedID(LCSSAID);
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AU.addRequired<ScalarEvolution>();
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AU.addPreserved<ScalarEvolution>();
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AU.addRequired<TargetTransformInfo>();
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// FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
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// If loop unroll does not preserve dom info then LCSSA pass on next
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// loop will receive invalid dom info.
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// For now, recreate dom info, if loop is unrolled.
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AU.addPreserved<DominatorTreeWrapperPass>();
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}
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};
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}
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char LoopUnroll::ID = 0;
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INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
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INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
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INITIALIZE_PASS_DEPENDENCY(LoopInfo)
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INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
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INITIALIZE_PASS_DEPENDENCY(LCSSA)
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INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
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INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
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Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial,
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int Runtime) {
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return new LoopUnroll(Threshold, Count, AllowPartial, Runtime);
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}
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/// ApproximateLoopSize - Approximate the size of the loop.
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static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
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bool &NotDuplicatable,
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const TargetTransformInfo &TTI) {
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CodeMetrics Metrics;
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for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
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I != E; ++I)
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Metrics.analyzeBasicBlock(*I, TTI);
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NumCalls = Metrics.NumInlineCandidates;
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NotDuplicatable = Metrics.notDuplicatable;
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unsigned LoopSize = Metrics.NumInsts;
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// Don't allow an estimate of size zero. This would allows unrolling of loops
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// with huge iteration counts, which is a compile time problem even if it's
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// not a problem for code quality.
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if (LoopSize == 0) LoopSize = 1;
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return LoopSize;
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}
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bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
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if (skipOptnoneFunction(L))
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return false;
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LoopInfo *LI = &getAnalysis<LoopInfo>();
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ScalarEvolution *SE = &getAnalysis<ScalarEvolution>();
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const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
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BasicBlock *Header = L->getHeader();
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DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
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<< "] Loop %" << Header->getName() << "\n");
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(void)Header;
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TargetTransformInfo::UnrollingPreferences UP;
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UP.Threshold = CurrentThreshold;
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UP.OptSizeThreshold = OptSizeUnrollThreshold;
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UP.Count = CurrentCount;
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UP.Partial = CurrentAllowPartial;
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UP.Runtime = CurrentRuntime;
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TTI.getUnrollingPreferences(L, UP);
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// Determine the current unrolling threshold. While this is normally set
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// from UnrollThreshold, it is overridden to a smaller value if the current
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// function is marked as optimize-for-size, and the unroll threshold was
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// not user specified.
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unsigned Threshold = UserThreshold ? CurrentThreshold : UP.Threshold;
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if (!UserThreshold &&
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Header->getParent()->getAttributes().
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hasAttribute(AttributeSet::FunctionIndex,
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Attribute::OptimizeForSize))
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Threshold = UP.OptSizeThreshold;
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// Find trip count and trip multiple if count is not available
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unsigned TripCount = 0;
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unsigned TripMultiple = 1;
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// Find "latch trip count". UnrollLoop assumes that control cannot exit
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// via the loop latch on any iteration prior to TripCount. The loop may exit
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// early via an earlier branch.
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BasicBlock *LatchBlock = L->getLoopLatch();
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if (LatchBlock) {
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TripCount = SE->getSmallConstantTripCount(L, LatchBlock);
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TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock);
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}
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bool Runtime = UserRuntime ? CurrentRuntime : UP.Runtime;
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// Use a default unroll-count if the user doesn't specify a value
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// and the trip count is a run-time value. The default is different
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// for run-time or compile-time trip count loops.
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unsigned Count = UserCount ? CurrentCount : UP.Count;
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if (Runtime && Count == 0 && TripCount == 0)
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Count = UnrollRuntimeCount;
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if (Count == 0) {
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// Conservative heuristic: if we know the trip count, see if we can
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// completely unroll (subject to the threshold, checked below); otherwise
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// try to find greatest modulo of the trip count which is still under
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// threshold value.
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if (TripCount == 0)
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return false;
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Count = TripCount;
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}
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// Enforce the threshold.
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if (Threshold != NoThreshold) {
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unsigned NumInlineCandidates;
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bool notDuplicatable;
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unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates,
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notDuplicatable, TTI);
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DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n");
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if (notDuplicatable) {
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DEBUG(dbgs() << " Not unrolling loop which contains non-duplicatable"
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<< " instructions.\n");
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return false;
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}
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if (NumInlineCandidates != 0) {
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DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n");
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return false;
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}
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uint64_t Size = (uint64_t)LoopSize*Count;
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if (TripCount != 1 && Size > Threshold) {
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DEBUG(dbgs() << " Too large to fully unroll with count: " << Count
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<< " because size: " << Size << ">" << Threshold << "\n");
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bool AllowPartial = UserAllowPartial ? CurrentAllowPartial : UP.Partial;
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if (!AllowPartial && !(Runtime && TripCount == 0)) {
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DEBUG(dbgs() << " will not try to unroll partially because "
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<< "-unroll-allow-partial not given\n");
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return false;
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}
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if (TripCount) {
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// Reduce unroll count to be modulo of TripCount for partial unrolling
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Count = Threshold / LoopSize;
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while (Count != 0 && TripCount%Count != 0)
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Count--;
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}
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else if (Runtime) {
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// Reduce unroll count to be a lower power-of-two value
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while (Count != 0 && Size > Threshold) {
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Count >>= 1;
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Size = LoopSize*Count;
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}
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}
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if (Count < 2) {
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DEBUG(dbgs() << " could not unroll partially\n");
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return false;
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}
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DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n");
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}
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}
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// Unroll the loop.
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if (!UnrollLoop(L, Count, TripCount, Runtime, TripMultiple, LI, this, &LPM))
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return false;
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return true;
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}
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