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1 //===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass implements a simple loop unroller.  It works best when loops have
11 // been canonicalized by the -indvars pass, allowing it to determine the trip
12 // counts of loops easily.
13 //===----------------------------------------------------------------------===//
14 
15 #include "llvm/Transforms/Scalar.h"
16 #include "llvm/Analysis/CodeMetrics.h"
17 #include "llvm/Analysis/LoopPass.h"
18 #include "llvm/Analysis/ScalarEvolution.h"
19 #include "llvm/Analysis/TargetTransformInfo.h"
20 #include "llvm/IR/DataLayout.h"
21 #include "llvm/IR/DiagnosticInfo.h"
22 #include "llvm/IR/Dominators.h"
23 #include "llvm/IR/IntrinsicInst.h"
24 #include "llvm/IR/Metadata.h"
25 #include "llvm/Support/CommandLine.h"
26 #include "llvm/Support/Debug.h"
27 #include "llvm/Support/raw_ostream.h"
28 #include "llvm/Transforms/Utils/UnrollLoop.h"
29 #include <climits>
30 
31 using namespace llvm;
32 
33 #define DEBUG_TYPE "loop-unroll"
34 
35 static cl::opt<unsigned>
36 UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
37   cl::desc("The cut-off point for automatic loop unrolling"));
38 
39 static cl::opt<unsigned>
40 UnrollCount("unroll-count", cl::init(0), cl::Hidden,
41   cl::desc("Use this unroll count for all loops including those with "
42            "unroll_count pragma values, for testing purposes"));
43 
44 static cl::opt<bool>
45 UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
46   cl::desc("Allows loops to be partially unrolled until "
47            "-unroll-threshold loop size is reached."));
48 
49 static cl::opt<bool>
50 UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden,
51   cl::desc("Unroll loops with run-time trip counts"));
52 
53 static cl::opt<unsigned>
54 PragmaUnrollThreshold("pragma-unroll-threshold", cl::init(16 * 1024), cl::Hidden,
55   cl::desc("Unrolled size limit for loops with an unroll(enable) or "
56            "unroll_count pragma."));
57 
58 namespace {
59   class LoopUnroll : public LoopPass {
60   public:
61     static char ID; // Pass ID, replacement for typeid
LoopUnroll(int T=-1,int C=-1,int P=-1,int R=-1)62     LoopUnroll(int T = -1, int C = -1, int P = -1, int R = -1) : LoopPass(ID) {
63       CurrentThreshold = (T == -1) ? UnrollThreshold : unsigned(T);
64       CurrentCount = (C == -1) ? UnrollCount : unsigned(C);
65       CurrentAllowPartial = (P == -1) ? UnrollAllowPartial : (bool)P;
66       CurrentRuntime = (R == -1) ? UnrollRuntime : (bool)R;
67 
68       UserThreshold = (T != -1) || (UnrollThreshold.getNumOccurrences() > 0);
69       UserAllowPartial = (P != -1) ||
70                          (UnrollAllowPartial.getNumOccurrences() > 0);
71       UserRuntime = (R != -1) || (UnrollRuntime.getNumOccurrences() > 0);
72       UserCount = (C != -1) || (UnrollCount.getNumOccurrences() > 0);
73 
74       initializeLoopUnrollPass(*PassRegistry::getPassRegistry());
75     }
76 
77     /// A magic value for use with the Threshold parameter to indicate
78     /// that the loop unroll should be performed regardless of how much
79     /// code expansion would result.
80     static const unsigned NoThreshold = UINT_MAX;
81 
82     // Threshold to use when optsize is specified (and there is no
83     // explicit -unroll-threshold).
84     static const unsigned OptSizeUnrollThreshold = 50;
85 
86     // Default unroll count for loops with run-time trip count if
87     // -unroll-count is not set
88     static const unsigned UnrollRuntimeCount = 8;
89 
90     unsigned CurrentCount;
91     unsigned CurrentThreshold;
92     bool     CurrentAllowPartial;
93     bool     CurrentRuntime;
94     bool     UserCount;            // CurrentCount is user-specified.
95     bool     UserThreshold;        // CurrentThreshold is user-specified.
96     bool     UserAllowPartial;     // CurrentAllowPartial is user-specified.
97     bool     UserRuntime;          // CurrentRuntime is user-specified.
98 
99     bool runOnLoop(Loop *L, LPPassManager &LPM) override;
100 
101     /// This transformation requires natural loop information & requires that
102     /// loop preheaders be inserted into the CFG...
103     ///
getAnalysisUsage(AnalysisUsage & AU) const104     void getAnalysisUsage(AnalysisUsage &AU) const override {
105       AU.addRequired<LoopInfo>();
106       AU.addPreserved<LoopInfo>();
107       AU.addRequiredID(LoopSimplifyID);
108       AU.addPreservedID(LoopSimplifyID);
109       AU.addRequiredID(LCSSAID);
110       AU.addPreservedID(LCSSAID);
111       AU.addRequired<ScalarEvolution>();
112       AU.addPreserved<ScalarEvolution>();
113       AU.addRequired<TargetTransformInfo>();
114       // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
115       // If loop unroll does not preserve dom info then LCSSA pass on next
116       // loop will receive invalid dom info.
117       // For now, recreate dom info, if loop is unrolled.
118       AU.addPreserved<DominatorTreeWrapperPass>();
119     }
120 
121     // Fill in the UnrollingPreferences parameter with values from the
122     // TargetTransformationInfo.
getUnrollingPreferences(Loop * L,const TargetTransformInfo & TTI,TargetTransformInfo::UnrollingPreferences & UP)123     void getUnrollingPreferences(Loop *L, const TargetTransformInfo &TTI,
124                                  TargetTransformInfo::UnrollingPreferences &UP) {
125       UP.Threshold = CurrentThreshold;
126       UP.OptSizeThreshold = OptSizeUnrollThreshold;
127       UP.PartialThreshold = CurrentThreshold;
128       UP.PartialOptSizeThreshold = OptSizeUnrollThreshold;
129       UP.Count = CurrentCount;
130       UP.MaxCount = UINT_MAX;
131       UP.Partial = CurrentAllowPartial;
132       UP.Runtime = CurrentRuntime;
133       TTI.getUnrollingPreferences(L, UP);
134     }
135 
136     // Select and return an unroll count based on parameters from
137     // user, unroll preferences, unroll pragmas, or a heuristic.
138     // SetExplicitly is set to true if the unroll count is is set by
139     // the user or a pragma rather than selected heuristically.
140     unsigned
141     selectUnrollCount(const Loop *L, unsigned TripCount, bool HasEnablePragma,
142                       unsigned PragmaCount,
143                       const TargetTransformInfo::UnrollingPreferences &UP,
144                       bool &SetExplicitly);
145 
146 
147     // Select threshold values used to limit unrolling based on a
148     // total unrolled size.  Parameters Threshold and PartialThreshold
149     // are set to the maximum unrolled size for fully and partially
150     // unrolled loops respectively.
selectThresholds(const Loop * L,bool HasPragma,const TargetTransformInfo::UnrollingPreferences & UP,unsigned & Threshold,unsigned & PartialThreshold)151     void selectThresholds(const Loop *L, bool HasPragma,
152                           const TargetTransformInfo::UnrollingPreferences &UP,
153                           unsigned &Threshold, unsigned &PartialThreshold) {
154       // Determine the current unrolling threshold.  While this is
155       // normally set from UnrollThreshold, it is overridden to a
156       // smaller value if the current function is marked as
157       // optimize-for-size, and the unroll threshold was not user
158       // specified.
159       Threshold = UserThreshold ? CurrentThreshold : UP.Threshold;
160       PartialThreshold = UserThreshold ? CurrentThreshold : UP.PartialThreshold;
161       if (!UserThreshold &&
162           L->getHeader()->getParent()->getAttributes().
163               hasAttribute(AttributeSet::FunctionIndex,
164                            Attribute::OptimizeForSize)) {
165         Threshold = UP.OptSizeThreshold;
166         PartialThreshold = UP.PartialOptSizeThreshold;
167       }
168       if (HasPragma) {
169         // If the loop has an unrolling pragma, we want to be more
170         // aggressive with unrolling limits.  Set thresholds to at
171         // least the PragmaTheshold value which is larger than the
172         // default limits.
173         if (Threshold != NoThreshold)
174           Threshold = std::max<unsigned>(Threshold, PragmaUnrollThreshold);
175         if (PartialThreshold != NoThreshold)
176           PartialThreshold =
177               std::max<unsigned>(PartialThreshold, PragmaUnrollThreshold);
178       }
179     }
180   };
181 }
182 
183 char LoopUnroll::ID = 0;
184 INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)185 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
186 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
187 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
188 INITIALIZE_PASS_DEPENDENCY(LCSSA)
189 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
190 INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
191 
192 Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial,
193                                  int Runtime) {
194   return new LoopUnroll(Threshold, Count, AllowPartial, Runtime);
195 }
196 
createSimpleLoopUnrollPass()197 Pass *llvm::createSimpleLoopUnrollPass() {
198   return llvm::createLoopUnrollPass(-1, -1, 0, 0);
199 }
200 
201 /// ApproximateLoopSize - Approximate the size of the loop.
ApproximateLoopSize(const Loop * L,unsigned & NumCalls,bool & NotDuplicatable,const TargetTransformInfo & TTI)202 static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
203                                     bool &NotDuplicatable,
204                                     const TargetTransformInfo &TTI) {
205   CodeMetrics Metrics;
206   for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
207        I != E; ++I)
208     Metrics.analyzeBasicBlock(*I, TTI);
209   NumCalls = Metrics.NumInlineCandidates;
210   NotDuplicatable = Metrics.notDuplicatable;
211 
212   unsigned LoopSize = Metrics.NumInsts;
213 
214   // Don't allow an estimate of size zero.  This would allows unrolling of loops
215   // with huge iteration counts, which is a compile time problem even if it's
216   // not a problem for code quality.
217   if (LoopSize == 0) LoopSize = 1;
218 
219   return LoopSize;
220 }
221 
222 // Returns the value associated with the given metadata node name (for
223 // example, "llvm.loop.unroll.count").  If no such named metadata node
224 // exists, then nullptr is returned.
GetUnrollMetadataValue(const Loop * L,StringRef Name)225 static const ConstantInt *GetUnrollMetadataValue(const Loop *L,
226                                                  StringRef Name) {
227   MDNode *LoopID = L->getLoopID();
228   if (!LoopID) return nullptr;
229 
230   // First operand should refer to the loop id itself.
231   assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
232   assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
233 
234   for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
235     const MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
236     if (!MD) continue;
237 
238     const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
239     if (!S) continue;
240 
241     if (Name.equals(S->getString())) {
242       assert(MD->getNumOperands() == 2 &&
243              "Unroll hint metadata should have two operands.");
244       return cast<ConstantInt>(MD->getOperand(1));
245     }
246   }
247   return nullptr;
248 }
249 
250 // Returns true if the loop has an unroll(enable) pragma.
HasUnrollEnablePragma(const Loop * L)251 static bool HasUnrollEnablePragma(const Loop *L) {
252   const ConstantInt *EnableValue =
253       GetUnrollMetadataValue(L, "llvm.loop.unroll.enable");
254   return (EnableValue && EnableValue->getZExtValue());
255 }
256 
257 // Returns true if the loop has an unroll(disable) pragma.
HasUnrollDisablePragma(const Loop * L)258 static bool HasUnrollDisablePragma(const Loop *L) {
259   const ConstantInt *EnableValue =
260       GetUnrollMetadataValue(L, "llvm.loop.unroll.enable");
261   return (EnableValue && !EnableValue->getZExtValue());
262 }
263 
264 // If loop has an unroll_count pragma return the (necessarily
265 // positive) value from the pragma.  Otherwise return 0.
UnrollCountPragmaValue(const Loop * L)266 static unsigned UnrollCountPragmaValue(const Loop *L) {
267   const ConstantInt *CountValue =
268       GetUnrollMetadataValue(L, "llvm.loop.unroll.count");
269   if (CountValue) {
270     unsigned Count = CountValue->getZExtValue();
271     assert(Count >= 1 && "Unroll count must be positive.");
272     return Count;
273   }
274   return 0;
275 }
276 
selectUnrollCount(const Loop * L,unsigned TripCount,bool HasEnablePragma,unsigned PragmaCount,const TargetTransformInfo::UnrollingPreferences & UP,bool & SetExplicitly)277 unsigned LoopUnroll::selectUnrollCount(
278     const Loop *L, unsigned TripCount, bool HasEnablePragma,
279     unsigned PragmaCount, const TargetTransformInfo::UnrollingPreferences &UP,
280     bool &SetExplicitly) {
281   SetExplicitly = true;
282 
283   // User-specified count (either as a command-line option or
284   // constructor parameter) has highest precedence.
285   unsigned Count = UserCount ? CurrentCount : 0;
286 
287   // If there is no user-specified count, unroll pragmas have the next
288   // highest precendence.
289   if (Count == 0) {
290     if (PragmaCount) {
291       Count = PragmaCount;
292     } else if (HasEnablePragma) {
293       // unroll(enable) pragma without an unroll_count pragma
294       // indicates to unroll loop fully.
295       Count = TripCount;
296     }
297   }
298 
299   if (Count == 0)
300     Count = UP.Count;
301 
302   if (Count == 0) {
303     SetExplicitly = false;
304     if (TripCount == 0)
305       // Runtime trip count.
306       Count = UnrollRuntimeCount;
307     else
308       // Conservative heuristic: if we know the trip count, see if we can
309       // completely unroll (subject to the threshold, checked below); otherwise
310       // try to find greatest modulo of the trip count which is still under
311       // threshold value.
312       Count = TripCount;
313   }
314   if (TripCount && Count > TripCount)
315     return TripCount;
316   return Count;
317 }
318 
runOnLoop(Loop * L,LPPassManager & LPM)319 bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
320   if (skipOptnoneFunction(L))
321     return false;
322 
323   LoopInfo *LI = &getAnalysis<LoopInfo>();
324   ScalarEvolution *SE = &getAnalysis<ScalarEvolution>();
325   const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
326 
327   BasicBlock *Header = L->getHeader();
328   DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
329         << "] Loop %" << Header->getName() << "\n");
330 
331   if (HasUnrollDisablePragma(L)) {
332     return false;
333   }
334   bool HasEnablePragma = HasUnrollEnablePragma(L);
335   unsigned PragmaCount = UnrollCountPragmaValue(L);
336   bool HasPragma = HasEnablePragma || PragmaCount > 0;
337 
338   TargetTransformInfo::UnrollingPreferences UP;
339   getUnrollingPreferences(L, TTI, UP);
340 
341   // Find trip count and trip multiple if count is not available
342   unsigned TripCount = 0;
343   unsigned TripMultiple = 1;
344   // Find "latch trip count". UnrollLoop assumes that control cannot exit
345   // via the loop latch on any iteration prior to TripCount. The loop may exit
346   // early via an earlier branch.
347   BasicBlock *LatchBlock = L->getLoopLatch();
348   if (LatchBlock) {
349     TripCount = SE->getSmallConstantTripCount(L, LatchBlock);
350     TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock);
351   }
352 
353   // Select an initial unroll count.  This may be reduced later based
354   // on size thresholds.
355   bool CountSetExplicitly;
356   unsigned Count = selectUnrollCount(L, TripCount, HasEnablePragma, PragmaCount,
357                                      UP, CountSetExplicitly);
358 
359   unsigned NumInlineCandidates;
360   bool notDuplicatable;
361   unsigned LoopSize =
362       ApproximateLoopSize(L, NumInlineCandidates, notDuplicatable, TTI);
363   DEBUG(dbgs() << "  Loop Size = " << LoopSize << "\n");
364   uint64_t UnrolledSize = (uint64_t)LoopSize * Count;
365   if (notDuplicatable) {
366     DEBUG(dbgs() << "  Not unrolling loop which contains non-duplicatable"
367                  << " instructions.\n");
368     return false;
369   }
370   if (NumInlineCandidates != 0) {
371     DEBUG(dbgs() << "  Not unrolling loop with inlinable calls.\n");
372     return false;
373   }
374 
375   unsigned Threshold, PartialThreshold;
376   selectThresholds(L, HasPragma, UP, Threshold, PartialThreshold);
377 
378   // Given Count, TripCount and thresholds determine the type of
379   // unrolling which is to be performed.
380   enum { Full = 0, Partial = 1, Runtime = 2 };
381   int Unrolling;
382   if (TripCount && Count == TripCount) {
383     if (Threshold != NoThreshold && UnrolledSize > Threshold) {
384       DEBUG(dbgs() << "  Too large to fully unroll with count: " << Count
385                    << " because size: " << UnrolledSize << ">" << Threshold
386                    << "\n");
387       Unrolling = Partial;
388     } else {
389       Unrolling = Full;
390     }
391   } else if (TripCount && Count < TripCount) {
392     Unrolling = Partial;
393   } else {
394     Unrolling = Runtime;
395   }
396 
397   // Reduce count based on the type of unrolling and the threshold values.
398   unsigned OriginalCount = Count;
399   bool AllowRuntime = UserRuntime ? CurrentRuntime : UP.Runtime;
400   if (Unrolling == Partial) {
401     bool AllowPartial = UserAllowPartial ? CurrentAllowPartial : UP.Partial;
402     if (!AllowPartial && !CountSetExplicitly) {
403       DEBUG(dbgs() << "  will not try to unroll partially because "
404                    << "-unroll-allow-partial not given\n");
405       return false;
406     }
407     if (PartialThreshold != NoThreshold && UnrolledSize > PartialThreshold) {
408       // Reduce unroll count to be modulo of TripCount for partial unrolling.
409       Count = PartialThreshold / LoopSize;
410       while (Count != 0 && TripCount % Count != 0)
411         Count--;
412     }
413   } else if (Unrolling == Runtime) {
414     if (!AllowRuntime && !CountSetExplicitly) {
415       DEBUG(dbgs() << "  will not try to unroll loop with runtime trip count "
416                    << "-unroll-runtime not given\n");
417       return false;
418     }
419     // Reduce unroll count to be the largest power-of-two factor of
420     // the original count which satisfies the threshold limit.
421     while (Count != 0 && UnrolledSize > PartialThreshold) {
422       Count >>= 1;
423       UnrolledSize = LoopSize * Count;
424     }
425     if (Count > UP.MaxCount)
426       Count = UP.MaxCount;
427     DEBUG(dbgs() << "  partially unrolling with count: " << Count << "\n");
428   }
429 
430   if (HasPragma) {
431     // Emit optimization remarks if we are unable to unroll the loop
432     // as directed by a pragma.
433     DebugLoc LoopLoc = L->getStartLoc();
434     Function *F = Header->getParent();
435     LLVMContext &Ctx = F->getContext();
436     if (HasEnablePragma && PragmaCount == 0) {
437       if (TripCount && Count != TripCount) {
438         emitOptimizationRemarkMissed(
439             Ctx, DEBUG_TYPE, *F, LoopLoc,
440             "Unable to fully unroll loop as directed by unroll(enable) pragma "
441             "because unrolled size is too large.");
442       } else if (!TripCount) {
443         emitOptimizationRemarkMissed(
444             Ctx, DEBUG_TYPE, *F, LoopLoc,
445             "Unable to fully unroll loop as directed by unroll(enable) pragma "
446             "because loop has a runtime trip count.");
447       }
448     } else if (PragmaCount > 0 && Count != OriginalCount) {
449       emitOptimizationRemarkMissed(
450           Ctx, DEBUG_TYPE, *F, LoopLoc,
451           "Unable to unroll loop the number of times directed by "
452           "unroll_count pragma because unrolled size is too large.");
453     }
454   }
455 
456   if (Unrolling != Full && Count < 2) {
457     // Partial unrolling by 1 is a nop.  For full unrolling, a factor
458     // of 1 makes sense because loop control can be eliminated.
459     return false;
460   }
461 
462   // Unroll the loop.
463   if (!UnrollLoop(L, Count, TripCount, AllowRuntime, TripMultiple, LI, this, &LPM))
464     return false;
465 
466   return true;
467 }
468