forked from OSchip/llvm-project
515 lines
20 KiB
C++
515 lines
20 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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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Analysis/AssumptionTracker.h"
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#include "llvm/Analysis/CodeMetrics.h"
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#include "llvm/Analysis/FunctionTargetTransformInfo.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/DiagnosticInfo.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/IR/Metadata.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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#define DEBUG_TYPE "loop-unroll"
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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 including those with "
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"unroll_count pragma values, 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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static cl::opt<unsigned>
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PragmaUnrollThreshold("pragma-unroll-threshold", cl::init(16 * 1024), cl::Hidden,
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cl::desc("Unrolled size limit for loops with an unroll(full) or "
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"unroll_count pragma."));
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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<AssumptionTracker>();
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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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AU.addRequired<FunctionTargetTransformInfo>();
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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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// Fill in the UnrollingPreferences parameter with values from the
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// TargetTransformationInfo.
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void getUnrollingPreferences(Loop *L, const FunctionTargetTransformInfo &FTTI,
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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.PartialThreshold = CurrentThreshold;
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UP.PartialOptSizeThreshold = OptSizeUnrollThreshold;
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UP.Count = CurrentCount;
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UP.MaxCount = UINT_MAX;
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UP.Partial = CurrentAllowPartial;
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UP.Runtime = CurrentRuntime;
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FTTI.getUnrollingPreferences(L, UP);
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}
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// Select and return an unroll count based on parameters from
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// user, unroll preferences, unroll pragmas, or a heuristic.
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// SetExplicitly is set to true if the unroll count is is set by
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// the user or a pragma rather than selected heuristically.
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unsigned
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selectUnrollCount(const Loop *L, unsigned TripCount, bool PragmaFullUnroll,
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unsigned PragmaCount,
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const TargetTransformInfo::UnrollingPreferences &UP,
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bool &SetExplicitly);
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// Select threshold values used to limit unrolling based on a
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// total unrolled size. Parameters Threshold and PartialThreshold
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// are set to the maximum unrolled size for fully and partially
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// unrolled loops respectively.
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void selectThresholds(const Loop *L, bool HasPragma,
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const TargetTransformInfo::UnrollingPreferences &UP,
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unsigned &Threshold, unsigned &PartialThreshold) {
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// Determine the current unrolling threshold. While this is
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// normally set from UnrollThreshold, it is overridden to a
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// smaller value if the current function is marked as
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// optimize-for-size, and the unroll threshold was not user
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// specified.
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Threshold = UserThreshold ? CurrentThreshold : UP.Threshold;
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PartialThreshold = UserThreshold ? CurrentThreshold : UP.PartialThreshold;
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if (!UserThreshold &&
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L->getHeader()->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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PartialThreshold = UP.PartialOptSizeThreshold;
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}
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if (HasPragma) {
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// If the loop has an unrolling pragma, we want to be more
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// aggressive with unrolling limits. Set thresholds to at
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// least the PragmaTheshold value which is larger than the
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// default limits.
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if (Threshold != NoThreshold)
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Threshold = std::max<unsigned>(Threshold, PragmaUnrollThreshold);
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if (PartialThreshold != NoThreshold)
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PartialThreshold =
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std::max<unsigned>(PartialThreshold, PragmaUnrollThreshold);
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}
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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(AssumptionTracker)
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INITIALIZE_PASS_DEPENDENCY(FunctionTargetTransformInfo)
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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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Pass *llvm::createSimpleLoopUnrollPass() {
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return llvm::createLoopUnrollPass(-1, -1, 0, 0);
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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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AssumptionTracker *AT) {
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SmallPtrSet<const Value *, 32> EphValues;
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CodeMetrics::collectEphemeralValues(L, AT, EphValues);
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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, EphValues);
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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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// Returns the loop hint metadata node with the given name (for example,
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// "llvm.loop.unroll.count"). If no such metadata node exists, then nullptr is
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// returned.
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static const MDNode *GetUnrollMetadata(const Loop *L, StringRef Name) {
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MDNode *LoopID = L->getLoopID();
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if (!LoopID)
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return nullptr;
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// First operand should refer to the loop id itself.
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assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
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assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
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for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
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const MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
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if (!MD)
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continue;
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const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
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if (!S)
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continue;
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if (Name.equals(S->getString()))
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return MD;
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}
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return nullptr;
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}
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// Returns true if the loop has an unroll(full) pragma.
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static bool HasUnrollFullPragma(const Loop *L) {
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return GetUnrollMetadata(L, "llvm.loop.unroll.full");
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}
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// Returns true if the loop has an unroll(disable) pragma.
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static bool HasUnrollDisablePragma(const Loop *L) {
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return GetUnrollMetadata(L, "llvm.loop.unroll.disable");
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}
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// If loop has an unroll_count pragma return the (necessarily
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// positive) value from the pragma. Otherwise return 0.
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static unsigned UnrollCountPragmaValue(const Loop *L) {
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const MDNode *MD = GetUnrollMetadata(L, "llvm.loop.unroll.count");
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if (MD) {
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assert(MD->getNumOperands() == 2 &&
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"Unroll count hint metadata should have two operands.");
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unsigned Count = cast<ConstantInt>(MD->getOperand(1))->getZExtValue();
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assert(Count >= 1 && "Unroll count must be positive.");
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return Count;
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}
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return 0;
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}
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// Remove existing unroll metadata and add unroll disable metadata to
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// indicate the loop has already been unrolled. This prevents a loop
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// from being unrolled more than is directed by a pragma if the loop
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// unrolling pass is run more than once (which it generally is).
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static void SetLoopAlreadyUnrolled(Loop *L) {
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MDNode *LoopID = L->getLoopID();
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if (!LoopID) return;
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// First remove any existing loop unrolling metadata.
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SmallVector<Value *, 4> Vals;
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// Reserve first location for self reference to the LoopID metadata node.
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Vals.push_back(nullptr);
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for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
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bool IsUnrollMetadata = false;
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MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
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if (MD) {
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const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
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IsUnrollMetadata = S && S->getString().startswith("llvm.loop.unroll.");
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}
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if (!IsUnrollMetadata) Vals.push_back(LoopID->getOperand(i));
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}
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// Add unroll(disable) metadata to disable future unrolling.
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LLVMContext &Context = L->getHeader()->getContext();
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SmallVector<Value *, 1> DisableOperands;
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DisableOperands.push_back(MDString::get(Context, "llvm.loop.unroll.disable"));
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MDNode *DisableNode = MDNode::get(Context, DisableOperands);
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Vals.push_back(DisableNode);
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MDNode *NewLoopID = MDNode::get(Context, Vals);
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// Set operand 0 to refer to the loop id itself.
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NewLoopID->replaceOperandWith(0, NewLoopID);
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L->setLoopID(NewLoopID);
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}
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unsigned LoopUnroll::selectUnrollCount(
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const Loop *L, unsigned TripCount, bool PragmaFullUnroll,
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unsigned PragmaCount, const TargetTransformInfo::UnrollingPreferences &UP,
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bool &SetExplicitly) {
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SetExplicitly = true;
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// User-specified count (either as a command-line option or
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// constructor parameter) has highest precedence.
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unsigned Count = UserCount ? CurrentCount : 0;
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// If there is no user-specified count, unroll pragmas have the next
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// highest precendence.
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if (Count == 0) {
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if (PragmaCount) {
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Count = PragmaCount;
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} else if (PragmaFullUnroll) {
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Count = TripCount;
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}
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}
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if (Count == 0)
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Count = UP.Count;
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if (Count == 0) {
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SetExplicitly = false;
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if (TripCount == 0)
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// Runtime trip count.
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Count = UnrollRuntimeCount;
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else
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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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Count = TripCount;
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}
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if (TripCount && Count > TripCount)
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return TripCount;
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return Count;
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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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const FunctionTargetTransformInfo &FTTI =
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getAnalysis<FunctionTargetTransformInfo>();
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AssumptionTracker *AT = &getAnalysis<AssumptionTracker>();
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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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if (HasUnrollDisablePragma(L)) {
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return false;
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}
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bool PragmaFullUnroll = HasUnrollFullPragma(L);
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unsigned PragmaCount = UnrollCountPragmaValue(L);
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bool HasPragma = PragmaFullUnroll || PragmaCount > 0;
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TargetTransformInfo::UnrollingPreferences UP;
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getUnrollingPreferences(L, FTTI, UP);
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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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// Select an initial unroll count. This may be reduced later based
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// on size thresholds.
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bool CountSetExplicitly;
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unsigned Count = selectUnrollCount(L, TripCount, PragmaFullUnroll,
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PragmaCount, UP, CountSetExplicitly);
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unsigned NumInlineCandidates;
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bool notDuplicatable;
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unsigned LoopSize =
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ApproximateLoopSize(L, NumInlineCandidates, notDuplicatable, TTI, AT);
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DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n");
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uint64_t UnrolledSize = (uint64_t)LoopSize * Count;
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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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unsigned Threshold, PartialThreshold;
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selectThresholds(L, HasPragma, UP, Threshold, PartialThreshold);
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// Given Count, TripCount and thresholds determine the type of
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// unrolling which is to be performed.
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enum { Full = 0, Partial = 1, Runtime = 2 };
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int Unrolling;
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if (TripCount && Count == TripCount) {
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if (Threshold != NoThreshold && UnrolledSize > Threshold) {
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DEBUG(dbgs() << " Too large to fully unroll with count: " << Count
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<< " because size: " << UnrolledSize << ">" << Threshold
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<< "\n");
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Unrolling = Partial;
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} else {
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Unrolling = Full;
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}
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} else if (TripCount && Count < TripCount) {
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Unrolling = Partial;
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} else {
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Unrolling = Runtime;
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}
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// Reduce count based on the type of unrolling and the threshold values.
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unsigned OriginalCount = Count;
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bool AllowRuntime = UserRuntime ? CurrentRuntime : UP.Runtime;
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if (Unrolling == Partial) {
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bool AllowPartial = UserAllowPartial ? CurrentAllowPartial : UP.Partial;
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if (!AllowPartial && !CountSetExplicitly) {
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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 (PartialThreshold != NoThreshold && UnrolledSize > PartialThreshold) {
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// Reduce unroll count to be modulo of TripCount for partial unrolling.
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Count = PartialThreshold / 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 (Unrolling == Runtime) {
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if (!AllowRuntime && !CountSetExplicitly) {
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DEBUG(dbgs() << " will not try to unroll loop with runtime trip count "
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<< "-unroll-runtime not given\n");
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return false;
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}
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// Reduce unroll count to be the largest power-of-two factor of
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// the original count which satisfies the threshold limit.
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while (Count != 0 && UnrolledSize > PartialThreshold) {
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Count >>= 1;
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UnrolledSize = LoopSize * Count;
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}
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if (Count > UP.MaxCount)
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Count = UP.MaxCount;
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DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n");
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}
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if (HasPragma) {
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if (PragmaCount != 0)
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// If loop has an unroll count pragma mark loop as unrolled to prevent
|
|
// unrolling beyond that requested by the pragma.
|
|
SetLoopAlreadyUnrolled(L);
|
|
|
|
// Emit optimization remarks if we are unable to unroll the loop
|
|
// as directed by a pragma.
|
|
DebugLoc LoopLoc = L->getStartLoc();
|
|
Function *F = Header->getParent();
|
|
LLVMContext &Ctx = F->getContext();
|
|
if (PragmaFullUnroll && PragmaCount == 0) {
|
|
if (TripCount && Count != TripCount) {
|
|
emitOptimizationRemarkMissed(
|
|
Ctx, DEBUG_TYPE, *F, LoopLoc,
|
|
"Unable to fully unroll loop as directed by unroll(full) pragma "
|
|
"because unrolled size is too large.");
|
|
} else if (!TripCount) {
|
|
emitOptimizationRemarkMissed(
|
|
Ctx, DEBUG_TYPE, *F, LoopLoc,
|
|
"Unable to fully unroll loop as directed by unroll(full) pragma "
|
|
"because loop has a runtime trip count.");
|
|
}
|
|
} else if (PragmaCount > 0 && Count != OriginalCount) {
|
|
emitOptimizationRemarkMissed(
|
|
Ctx, DEBUG_TYPE, *F, LoopLoc,
|
|
"Unable to unroll loop the number of times directed by "
|
|
"unroll_count pragma because unrolled size is too large.");
|
|
}
|
|
}
|
|
|
|
if (Unrolling != Full && Count < 2) {
|
|
// Partial unrolling by 1 is a nop. For full unrolling, a factor
|
|
// of 1 makes sense because loop control can be eliminated.
|
|
return false;
|
|
}
|
|
|
|
// Unroll the loop.
|
|
if (!UnrollLoop(L, Count, TripCount, AllowRuntime, TripMultiple, LI, this,
|
|
&LPM, AT))
|
|
return false;
|
|
|
|
return true;
|
|
}
|