1 //===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements some loop unrolling utilities. It does not define any
11 // actual pass or policy, but provides a single function to perform loop
14 // It works best when loops have been canonicalized by the -indvars pass,
15 // allowing it to determine the trip counts of loops easily.
17 // The process of unrolling can produce extraneous basic blocks linked with
18 // unconditional branches. This will be corrected in the future.
19 //===----------------------------------------------------------------------===//
21 #define DEBUG_TYPE "loop-unroll"
22 #include "llvm/Transforms/Utils/UnrollLoop.h"
23 #include "llvm/BasicBlock.h"
24 #include "llvm/ADT/Statistic.h"
25 #include "llvm/Analysis/ConstantFolding.h"
26 #include "llvm/Analysis/LoopPass.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Support/raw_ostream.h"
29 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
30 #include "llvm/Transforms/Utils/Cloning.h"
31 #include "llvm/Transforms/Utils/Local.h"
35 // TODO: Should these be here or in LoopUnroll?
36 STATISTIC(NumCompletelyUnrolled
, "Number of loops completely unrolled");
37 STATISTIC(NumUnrolled
, "Number of loops unrolled (completely or otherwise)");
39 /// RemapInstruction - Convert the instruction operands from referencing the
40 /// current values into those specified by ValueMap.
41 static inline void RemapInstruction(Instruction
*I
,
42 DenseMap
<const Value
*, Value
*> &ValueMap
) {
43 for (unsigned op
= 0, E
= I
->getNumOperands(); op
!= E
; ++op
) {
44 Value
*Op
= I
->getOperand(op
);
45 DenseMap
<const Value
*, Value
*>::iterator It
= ValueMap
.find(Op
);
46 if (It
!= ValueMap
.end())
47 I
->setOperand(op
, It
->second
);
51 /// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it
52 /// only has one predecessor, and that predecessor only has one successor.
53 /// The LoopInfo Analysis that is passed will be kept consistent.
54 /// Returns the new combined block.
55 static BasicBlock
*FoldBlockIntoPredecessor(BasicBlock
*BB
, LoopInfo
* LI
) {
56 // Merge basic blocks into their predecessor if there is only one distinct
57 // pred, and if there is only one distinct successor of the predecessor, and
58 // if there are no PHI nodes.
59 BasicBlock
*OnlyPred
= BB
->getSinglePredecessor();
60 if (!OnlyPred
) return 0;
62 if (OnlyPred
->getTerminator()->getNumSuccessors() != 1)
65 DEBUG(dbgs() << "Merging: " << *BB
<< "into: " << *OnlyPred
);
67 // Resolve any PHI nodes at the start of the block. They are all
68 // guaranteed to have exactly one entry if they exist, unless there are
69 // multiple duplicate (but guaranteed to be equal) entries for the
70 // incoming edges. This occurs when there are multiple edges from
71 // OnlyPred to OnlySucc.
72 FoldSingleEntryPHINodes(BB
);
74 // Delete the unconditional branch from the predecessor...
75 OnlyPred
->getInstList().pop_back();
77 // Move all definitions in the successor to the predecessor...
78 OnlyPred
->getInstList().splice(OnlyPred
->end(), BB
->getInstList());
80 // Make all PHI nodes that referred to BB now refer to Pred as their
82 BB
->replaceAllUsesWith(OnlyPred
);
84 std::string OldName
= BB
->getName();
86 // Erase basic block from the function...
88 BB
->eraseFromParent();
90 // Inherit predecessor's name if it exists...
91 if (!OldName
.empty() && !OnlyPred
->hasName())
92 OnlyPred
->setName(OldName
);
97 /// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
98 /// if unrolling was succesful, or false if the loop was unmodified. Unrolling
99 /// can only fail when the loop's latch block is not terminated by a conditional
100 /// branch instruction. However, if the trip count (and multiple) are not known,
101 /// loop unrolling will mostly produce more code that is no faster.
103 /// The LoopInfo Analysis that is passed will be kept consistent.
105 /// If a LoopPassManager is passed in, and the loop is fully removed, it will be
106 /// removed from the LoopPassManager as well. LPM can also be NULL.
107 bool llvm::UnrollLoop(Loop
*L
, unsigned Count
, LoopInfo
* LI
, LPPassManager
* LPM
) {
108 BasicBlock
*Preheader
= L
->getLoopPreheader();
110 DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n");
114 BasicBlock
*LatchBlock
= L
->getLoopLatch();
116 DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n");
120 BasicBlock
*Header
= L
->getHeader();
121 BranchInst
*BI
= dyn_cast
<BranchInst
>(LatchBlock
->getTerminator());
123 if (!BI
|| BI
->isUnconditional()) {
124 // The loop-rotate pass can be helpful to avoid this in many cases.
126 " Can't unroll; loop not terminated by a conditional branch.\n");
131 unsigned TripCount
= L
->getSmallConstantTripCount();
132 // Find trip multiple if count is not available
133 unsigned TripMultiple
= 1;
135 TripMultiple
= L
->getSmallConstantTripMultiple();
138 DEBUG(dbgs() << " Trip Count = " << TripCount
<< "\n");
139 if (TripMultiple
!= 1)
140 DEBUG(dbgs() << " Trip Multiple = " << TripMultiple
<< "\n");
142 // Effectively "DCE" unrolled iterations that are beyond the tripcount
143 // and will never be executed.
144 if (TripCount
!= 0 && Count
> TripCount
)
148 assert(TripMultiple
> 0);
149 assert(TripCount
== 0 || TripCount
% TripMultiple
== 0);
151 // Are we eliminating the loop control altogether?
152 bool CompletelyUnroll
= Count
== TripCount
;
154 // If we know the trip count, we know the multiple...
155 unsigned BreakoutTrip
= 0;
156 if (TripCount
!= 0) {
157 BreakoutTrip
= TripCount
% Count
;
160 // Figure out what multiple to use.
161 BreakoutTrip
= TripMultiple
=
162 (unsigned)GreatestCommonDivisor64(Count
, TripMultiple
);
165 if (CompletelyUnroll
) {
166 DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header
->getName()
167 << " with trip count " << TripCount
<< "!\n");
169 DEBUG(dbgs() << "UNROLLING loop %" << Header
->getName()
171 if (TripMultiple
== 0 || BreakoutTrip
!= TripMultiple
) {
172 DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip
);
173 } else if (TripMultiple
!= 1) {
174 DEBUG(dbgs() << " with " << TripMultiple
<< " trips per branch");
176 DEBUG(dbgs() << "!\n");
179 std::vector
<BasicBlock
*> LoopBlocks
= L
->getBlocks();
181 bool ContinueOnTrue
= L
->contains(BI
->getSuccessor(0));
182 BasicBlock
*LoopExit
= BI
->getSuccessor(ContinueOnTrue
);
184 // For the first iteration of the loop, we should use the precloned values for
185 // PHI nodes. Insert associations now.
186 typedef DenseMap
<const Value
*, Value
*> ValueMapTy
;
187 ValueMapTy LastValueMap
;
188 std::vector
<PHINode
*> OrigPHINode
;
189 for (BasicBlock::iterator I
= Header
->begin(); isa
<PHINode
>(I
); ++I
) {
190 PHINode
*PN
= cast
<PHINode
>(I
);
191 OrigPHINode
.push_back(PN
);
193 dyn_cast
<Instruction
>(PN
->getIncomingValueForBlock(LatchBlock
)))
198 std::vector
<BasicBlock
*> Headers
;
199 std::vector
<BasicBlock
*> Latches
;
200 Headers
.push_back(Header
);
201 Latches
.push_back(LatchBlock
);
203 for (unsigned It
= 1; It
!= Count
; ++It
) {
204 std::vector
<BasicBlock
*> NewBlocks
;
206 for (std::vector
<BasicBlock
*>::iterator BB
= LoopBlocks
.begin(),
207 E
= LoopBlocks
.end(); BB
!= E
; ++BB
) {
209 BasicBlock
*New
= CloneBasicBlock(*BB
, ValueMap
, "." + Twine(It
));
210 Header
->getParent()->getBasicBlockList().push_back(New
);
212 // Loop over all of the PHI nodes in the block, changing them to use the
213 // incoming values from the previous block.
215 for (unsigned i
= 0, e
= OrigPHINode
.size(); i
!= e
; ++i
) {
216 PHINode
*NewPHI
= cast
<PHINode
>(ValueMap
[OrigPHINode
[i
]]);
217 Value
*InVal
= NewPHI
->getIncomingValueForBlock(LatchBlock
);
218 if (Instruction
*InValI
= dyn_cast
<Instruction
>(InVal
))
219 if (It
> 1 && L
->contains(InValI
))
220 InVal
= LastValueMap
[InValI
];
221 ValueMap
[OrigPHINode
[i
]] = InVal
;
222 New
->getInstList().erase(NewPHI
);
225 // Update our running map of newest clones
226 LastValueMap
[*BB
] = New
;
227 for (ValueMapTy::iterator VI
= ValueMap
.begin(), VE
= ValueMap
.end();
229 LastValueMap
[VI
->first
] = VI
->second
;
231 L
->addBasicBlockToLoop(New
, LI
->getBase());
233 // Add phi entries for newly created values to all exit blocks except
234 // the successor of the latch block. The successor of the exit block will
235 // be updated specially after unrolling all the way.
236 if (*BB
!= LatchBlock
)
237 for (Value::use_iterator UI
= (*BB
)->use_begin(), UE
= (*BB
)->use_end();
239 Instruction
*UseInst
= cast
<Instruction
>(*UI
);
241 if (isa
<PHINode
>(UseInst
) && !L
->contains(UseInst
)) {
242 PHINode
*phi
= cast
<PHINode
>(UseInst
);
243 Value
*Incoming
= phi
->getIncomingValueForBlock(*BB
);
244 phi
->addIncoming(Incoming
, New
);
248 // Keep track of new headers and latches as we create them, so that
249 // we can insert the proper branches later.
251 Headers
.push_back(New
);
252 if (*BB
== LatchBlock
) {
253 Latches
.push_back(New
);
255 // Also, clear out the new latch's back edge so that it doesn't look
256 // like a new loop, so that it's amenable to being merged with adjacent
258 TerminatorInst
*Term
= New
->getTerminator();
259 assert(L
->contains(Term
->getSuccessor(!ContinueOnTrue
)));
260 assert(Term
->getSuccessor(ContinueOnTrue
) == LoopExit
);
261 Term
->setSuccessor(!ContinueOnTrue
, NULL
);
264 NewBlocks
.push_back(New
);
267 // Remap all instructions in the most recent iteration
268 for (unsigned i
= 0; i
< NewBlocks
.size(); ++i
)
269 for (BasicBlock::iterator I
= NewBlocks
[i
]->begin(),
270 E
= NewBlocks
[i
]->end(); I
!= E
; ++I
)
271 RemapInstruction(I
, LastValueMap
);
274 // The latch block exits the loop. If there are any PHI nodes in the
275 // successor blocks, update them to use the appropriate values computed as the
276 // last iteration of the loop.
278 SmallPtrSet
<PHINode
*, 8> Users
;
279 for (Value::use_iterator UI
= LatchBlock
->use_begin(),
280 UE
= LatchBlock
->use_end(); UI
!= UE
; ++UI
)
281 if (PHINode
*phi
= dyn_cast
<PHINode
>(*UI
))
284 BasicBlock
*LastIterationBB
= cast
<BasicBlock
>(LastValueMap
[LatchBlock
]);
285 for (SmallPtrSet
<PHINode
*,8>::iterator SI
= Users
.begin(), SE
= Users
.end();
288 Value
*InVal
= PN
->removeIncomingValue(LatchBlock
, false);
289 // If this value was defined in the loop, take the value defined by the
290 // last iteration of the loop.
291 if (Instruction
*InValI
= dyn_cast
<Instruction
>(InVal
)) {
292 if (L
->contains(InValI
))
293 InVal
= LastValueMap
[InVal
];
295 PN
->addIncoming(InVal
, LastIterationBB
);
299 // Now, if we're doing complete unrolling, loop over the PHI nodes in the
300 // original block, setting them to their incoming values.
301 if (CompletelyUnroll
) {
302 BasicBlock
*Preheader
= L
->getLoopPreheader();
303 for (unsigned i
= 0, e
= OrigPHINode
.size(); i
!= e
; ++i
) {
304 PHINode
*PN
= OrigPHINode
[i
];
305 PN
->replaceAllUsesWith(PN
->getIncomingValueForBlock(Preheader
));
306 Header
->getInstList().erase(PN
);
310 // Now that all the basic blocks for the unrolled iterations are in place,
311 // set up the branches to connect them.
312 for (unsigned i
= 0, e
= Latches
.size(); i
!= e
; ++i
) {
313 // The original branch was replicated in each unrolled iteration.
314 BranchInst
*Term
= cast
<BranchInst
>(Latches
[i
]->getTerminator());
316 // The branch destination.
317 unsigned j
= (i
+ 1) % e
;
318 BasicBlock
*Dest
= Headers
[j
];
319 bool NeedConditional
= true;
321 // For a complete unroll, make the last iteration end with a branch
322 // to the exit block.
323 if (CompletelyUnroll
&& j
== 0) {
325 NeedConditional
= false;
328 // If we know the trip count or a multiple of it, we can safely use an
329 // unconditional branch for some iterations.
330 if (j
!= BreakoutTrip
&& (TripMultiple
== 0 || j
% TripMultiple
!= 0)) {
331 NeedConditional
= false;
334 if (NeedConditional
) {
335 // Update the conditional branch's successor for the following
337 Term
->setSuccessor(!ContinueOnTrue
, Dest
);
339 Term
->setUnconditionalDest(Dest
);
340 // Merge adjacent basic blocks, if possible.
341 if (BasicBlock
*Fold
= FoldBlockIntoPredecessor(Dest
, LI
)) {
342 std::replace(Latches
.begin(), Latches
.end(), Dest
, Fold
);
343 std::replace(Headers
.begin(), Headers
.end(), Dest
, Fold
);
348 // At this point, the code is well formed. We now do a quick sweep over the
349 // inserted code, doing constant propagation and dead code elimination as we
351 const std::vector
<BasicBlock
*> &NewLoopBlocks
= L
->getBlocks();
352 for (std::vector
<BasicBlock
*>::const_iterator BB
= NewLoopBlocks
.begin(),
353 BBE
= NewLoopBlocks
.end(); BB
!= BBE
; ++BB
)
354 for (BasicBlock::iterator I
= (*BB
)->begin(), E
= (*BB
)->end(); I
!= E
; ) {
355 Instruction
*Inst
= I
++;
357 if (isInstructionTriviallyDead(Inst
))
358 (*BB
)->getInstList().erase(Inst
);
359 else if (Constant
*C
= ConstantFoldInstruction(Inst
)) {
360 Inst
->replaceAllUsesWith(C
);
361 (*BB
)->getInstList().erase(Inst
);
365 NumCompletelyUnrolled
+= CompletelyUnroll
;
367 // Remove the loop from the LoopPassManager if it's completely removed.
368 if (CompletelyUnroll
&& LPM
!= NULL
)
369 LPM
->deleteLoopFromQueue(L
);