Bug Summary

File:llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
Warning:line 198, column 28
Called C++ object pointer is null

Annotated Source Code

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clang -cc1 -cc1 -triple x86_64-pc-linux-gnu -analyze -disable-free -disable-llvm-verifier -discard-value-names -main-file-name LoopRotationUtils.cpp -analyzer-store=region -analyzer-opt-analyze-nested-blocks -analyzer-checker=core -analyzer-checker=apiModeling -analyzer-checker=unix -analyzer-checker=deadcode -analyzer-checker=cplusplus -analyzer-checker=security.insecureAPI.UncheckedReturn -analyzer-checker=security.insecureAPI.getpw -analyzer-checker=security.insecureAPI.gets -analyzer-checker=security.insecureAPI.mktemp -analyzer-checker=security.insecureAPI.mkstemp -analyzer-checker=security.insecureAPI.vfork -analyzer-checker=nullability.NullPassedToNonnull -analyzer-checker=nullability.NullReturnedFromNonnull -analyzer-output plist -w -setup-static-analyzer -analyzer-config-compatibility-mode=true -mrelocation-model pic -pic-level 2 -mframe-pointer=none -fmath-errno -fno-rounding-math -mconstructor-aliases -munwind-tables -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -ffunction-sections -fdata-sections -fcoverage-compilation-dir=/build/llvm-toolchain-snapshot-14~++20210903100615+fd66b44ec19e/build-llvm/lib/Transforms/Utils -resource-dir /usr/lib/llvm-14/lib/clang/14.0.0 -D _GNU_SOURCE -D __STDC_CONSTANT_MACROS -D __STDC_FORMAT_MACROS -D __STDC_LIMIT_MACROS -I /build/llvm-toolchain-snapshot-14~++20210903100615+fd66b44ec19e/build-llvm/lib/Transforms/Utils -I /build/llvm-toolchain-snapshot-14~++20210903100615+fd66b44ec19e/llvm/lib/Transforms/Utils -I /build/llvm-toolchain-snapshot-14~++20210903100615+fd66b44ec19e/build-llvm/include -I /build/llvm-toolchain-snapshot-14~++20210903100615+fd66b44ec19e/llvm/include -D NDEBUG -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/x86_64-linux-gnu/c++/10 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/backward -internal-isystem /usr/lib/llvm-14/lib/clang/14.0.0/include -internal-isystem /usr/local/include -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/10/../../../../x86_64-linux-gnu/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -O2 -Wno-unused-parameter -Wwrite-strings -Wno-missing-field-initializers -Wno-long-long -Wno-maybe-uninitialized -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wno-comment -std=c++14 -fdeprecated-macro -fdebug-compilation-dir=/build/llvm-toolchain-snapshot-14~++20210903100615+fd66b44ec19e/build-llvm/lib/Transforms/Utils -fdebug-prefix-map=/build/llvm-toolchain-snapshot-14~++20210903100615+fd66b44ec19e=. -ferror-limit 19 -fvisibility-inlines-hidden -stack-protector 2 -fgnuc-version=4.2.1 -vectorize-loops -vectorize-slp -analyzer-output=html -analyzer-config stable-report-filename=true -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o /tmp/scan-build-2021-09-04-040900-46481-1 -x c++ /build/llvm-toolchain-snapshot-14~++20210903100615+fd66b44ec19e/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
1//===----------------- LoopRotationUtils.cpp -----------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file provides utilities to convert a loop into a loop with bottom test.
10//
11//===----------------------------------------------------------------------===//
12
13#include "llvm/Transforms/Utils/LoopRotationUtils.h"
14#include "llvm/ADT/Statistic.h"
15#include "llvm/Analysis/AssumptionCache.h"
16#include "llvm/Analysis/BasicAliasAnalysis.h"
17#include "llvm/Analysis/CodeMetrics.h"
18#include "llvm/Analysis/DomTreeUpdater.h"
19#include "llvm/Analysis/GlobalsModRef.h"
20#include "llvm/Analysis/InstructionSimplify.h"
21#include "llvm/Analysis/LoopPass.h"
22#include "llvm/Analysis/MemorySSA.h"
23#include "llvm/Analysis/MemorySSAUpdater.h"
24#include "llvm/Analysis/ScalarEvolution.h"
25#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
26#include "llvm/Analysis/TargetTransformInfo.h"
27#include "llvm/Analysis/ValueTracking.h"
28#include "llvm/IR/CFG.h"
29#include "llvm/IR/DebugInfo.h"
30#include "llvm/IR/Dominators.h"
31#include "llvm/IR/Function.h"
32#include "llvm/IR/IntrinsicInst.h"
33#include "llvm/IR/Module.h"
34#include "llvm/Support/CommandLine.h"
35#include "llvm/Support/Debug.h"
36#include "llvm/Support/raw_ostream.h"
37#include "llvm/Transforms/Utils/BasicBlockUtils.h"
38#include "llvm/Transforms/Utils/Cloning.h"
39#include "llvm/Transforms/Utils/Local.h"
40#include "llvm/Transforms/Utils/LoopUtils.h"
41#include "llvm/Transforms/Utils/SSAUpdater.h"
42#include "llvm/Transforms/Utils/ValueMapper.h"
43using namespace llvm;
44
45#define DEBUG_TYPE"loop-rotate" "loop-rotate"
46
47STATISTIC(NumNotRotatedDueToHeaderSize,static llvm::Statistic NumNotRotatedDueToHeaderSize = {"loop-rotate"
, "NumNotRotatedDueToHeaderSize", "Number of loops not rotated due to the header size"
}
48 "Number of loops not rotated due to the header size")static llvm::Statistic NumNotRotatedDueToHeaderSize = {"loop-rotate"
, "NumNotRotatedDueToHeaderSize", "Number of loops not rotated due to the header size"
}
;
49STATISTIC(NumInstrsHoisted,static llvm::Statistic NumInstrsHoisted = {"loop-rotate", "NumInstrsHoisted"
, "Number of instructions hoisted into loop preheader"}
50 "Number of instructions hoisted into loop preheader")static llvm::Statistic NumInstrsHoisted = {"loop-rotate", "NumInstrsHoisted"
, "Number of instructions hoisted into loop preheader"}
;
51STATISTIC(NumInstrsDuplicated,static llvm::Statistic NumInstrsDuplicated = {"loop-rotate", "NumInstrsDuplicated"
, "Number of instructions cloned into loop preheader"}
52 "Number of instructions cloned into loop preheader")static llvm::Statistic NumInstrsDuplicated = {"loop-rotate", "NumInstrsDuplicated"
, "Number of instructions cloned into loop preheader"}
;
53STATISTIC(NumRotated, "Number of loops rotated")static llvm::Statistic NumRotated = {"loop-rotate", "NumRotated"
, "Number of loops rotated"}
;
54
55static cl::opt<bool>
56 MultiRotate("loop-rotate-multi", cl::init(false), cl::Hidden,
57 cl::desc("Allow loop rotation multiple times in order to reach "
58 "a better latch exit"));
59
60namespace {
61/// A simple loop rotation transformation.
62class LoopRotate {
63 const unsigned MaxHeaderSize;
64 LoopInfo *LI;
65 const TargetTransformInfo *TTI;
66 AssumptionCache *AC;
67 DominatorTree *DT;
68 ScalarEvolution *SE;
69 MemorySSAUpdater *MSSAU;
70 const SimplifyQuery &SQ;
71 bool RotationOnly;
72 bool IsUtilMode;
73 bool PrepareForLTO;
74
75public:
76 LoopRotate(unsigned MaxHeaderSize, LoopInfo *LI,
77 const TargetTransformInfo *TTI, AssumptionCache *AC,
78 DominatorTree *DT, ScalarEvolution *SE, MemorySSAUpdater *MSSAU,
79 const SimplifyQuery &SQ, bool RotationOnly, bool IsUtilMode,
80 bool PrepareForLTO)
81 : MaxHeaderSize(MaxHeaderSize), LI(LI), TTI(TTI), AC(AC), DT(DT), SE(SE),
82 MSSAU(MSSAU), SQ(SQ), RotationOnly(RotationOnly),
83 IsUtilMode(IsUtilMode), PrepareForLTO(PrepareForLTO) {}
84 bool processLoop(Loop *L);
85
86private:
87 bool rotateLoop(Loop *L, bool SimplifiedLatch);
88 bool simplifyLoopLatch(Loop *L);
89};
90} // end anonymous namespace
91
92/// Insert (K, V) pair into the ValueToValueMap, and verify the key did not
93/// previously exist in the map, and the value was inserted.
94static void InsertNewValueIntoMap(ValueToValueMapTy &VM, Value *K, Value *V) {
95 bool Inserted = VM.insert({K, V}).second;
96 assert(Inserted)(static_cast<void> (0));
97 (void)Inserted;
98}
99/// RewriteUsesOfClonedInstructions - We just cloned the instructions from the
100/// old header into the preheader. If there were uses of the values produced by
101/// these instruction that were outside of the loop, we have to insert PHI nodes
102/// to merge the two values. Do this now.
103static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader,
104 BasicBlock *OrigPreheader,
105 ValueToValueMapTy &ValueMap,
106 SmallVectorImpl<PHINode*> *InsertedPHIs) {
107 // Remove PHI node entries that are no longer live.
108 BasicBlock::iterator I, E = OrigHeader->end();
109 for (I = OrigHeader->begin(); PHINode *PN = dyn_cast<PHINode>(I); ++I)
110 PN->removeIncomingValue(PN->getBasicBlockIndex(OrigPreheader));
111
112 // Now fix up users of the instructions in OrigHeader, inserting PHI nodes
113 // as necessary.
114 SSAUpdater SSA(InsertedPHIs);
115 for (I = OrigHeader->begin(); I != E; ++I) {
116 Value *OrigHeaderVal = &*I;
117
118 // If there are no uses of the value (e.g. because it returns void), there
119 // is nothing to rewrite.
120 if (OrigHeaderVal->use_empty())
121 continue;
122
123 Value *OrigPreHeaderVal = ValueMap.lookup(OrigHeaderVal);
124
125 // The value now exits in two versions: the initial value in the preheader
126 // and the loop "next" value in the original header.
127 SSA.Initialize(OrigHeaderVal->getType(), OrigHeaderVal->getName());
128 SSA.AddAvailableValue(OrigHeader, OrigHeaderVal);
129 SSA.AddAvailableValue(OrigPreheader, OrigPreHeaderVal);
130
131 // Visit each use of the OrigHeader instruction.
132 for (Value::use_iterator UI = OrigHeaderVal->use_begin(),
133 UE = OrigHeaderVal->use_end();
134 UI != UE;) {
135 // Grab the use before incrementing the iterator.
136 Use &U = *UI;
137
138 // Increment the iterator before removing the use from the list.
139 ++UI;
140
141 // SSAUpdater can't handle a non-PHI use in the same block as an
142 // earlier def. We can easily handle those cases manually.
143 Instruction *UserInst = cast<Instruction>(U.getUser());
144 if (!isa<PHINode>(UserInst)) {
145 BasicBlock *UserBB = UserInst->getParent();
146
147 // The original users in the OrigHeader are already using the
148 // original definitions.
149 if (UserBB == OrigHeader)
150 continue;
151
152 // Users in the OrigPreHeader need to use the value to which the
153 // original definitions are mapped.
154 if (UserBB == OrigPreheader) {
155 U = OrigPreHeaderVal;
156 continue;
157 }
158 }
159
160 // Anything else can be handled by SSAUpdater.
161 SSA.RewriteUse(U);
162 }
163
164 // Replace MetadataAsValue(ValueAsMetadata(OrigHeaderVal)) uses in debug
165 // intrinsics.
166 SmallVector<DbgValueInst *, 1> DbgValues;
167 llvm::findDbgValues(DbgValues, OrigHeaderVal);
168 for (auto &DbgValue : DbgValues) {
169 // The original users in the OrigHeader are already using the original
170 // definitions.
171 BasicBlock *UserBB = DbgValue->getParent();
172 if (UserBB == OrigHeader)
173 continue;
174
175 // Users in the OrigPreHeader need to use the value to which the
176 // original definitions are mapped and anything else can be handled by
177 // the SSAUpdater. To avoid adding PHINodes, check if the value is
178 // available in UserBB, if not substitute undef.
179 Value *NewVal;
180 if (UserBB == OrigPreheader)
181 NewVal = OrigPreHeaderVal;
182 else if (SSA.HasValueForBlock(UserBB))
183 NewVal = SSA.GetValueInMiddleOfBlock(UserBB);
184 else
185 NewVal = UndefValue::get(OrigHeaderVal->getType());
186 DbgValue->replaceVariableLocationOp(OrigHeaderVal, NewVal);
187 }
188 }
189}
190
191// Assuming both header and latch are exiting, look for a phi which is only
192// used outside the loop (via a LCSSA phi) in the exit from the header.
193// This means that rotating the loop can remove the phi.
194static bool profitableToRotateLoopExitingLatch(Loop *L) {
195 BasicBlock *Header = L->getHeader();
196 BranchInst *BI = dyn_cast<BranchInst>(Header->getTerminator());
12
Assuming the object is not a 'BranchInst'
13
'BI' initialized to a null pointer value
197 assert(BI && BI->isConditional() && "need header with conditional exit")(static_cast<void> (0));
198 BasicBlock *HeaderExit = BI->getSuccessor(0);
14
Called C++ object pointer is null
199 if (L->contains(HeaderExit))
200 HeaderExit = BI->getSuccessor(1);
201
202 for (auto &Phi : Header->phis()) {
203 // Look for uses of this phi in the loop/via exits other than the header.
204 if (llvm::any_of(Phi.users(), [HeaderExit](const User *U) {
205 return cast<Instruction>(U)->getParent() != HeaderExit;
206 }))
207 continue;
208 return true;
209 }
210 return false;
211}
212
213// Check that latch exit is deoptimizing (which means - very unlikely to happen)
214// and there is another exit from the loop which is non-deoptimizing.
215// If we rotate latch to that exit our loop has a better chance of being fully
216// canonical.
217//
218// It can give false positives in some rare cases.
219static bool canRotateDeoptimizingLatchExit(Loop *L) {
220 BasicBlock *Latch = L->getLoopLatch();
221 assert(Latch && "need latch")(static_cast<void> (0));
222 BranchInst *BI = dyn_cast<BranchInst>(Latch->getTerminator());
223 // Need normal exiting latch.
224 if (!BI || !BI->isConditional())
225 return false;
226
227 BasicBlock *Exit = BI->getSuccessor(1);
228 if (L->contains(Exit))
229 Exit = BI->getSuccessor(0);
230
231 // Latch exit is non-deoptimizing, no need to rotate.
232 if (!Exit->getPostdominatingDeoptimizeCall())
233 return false;
234
235 SmallVector<BasicBlock *, 4> Exits;
236 L->getUniqueExitBlocks(Exits);
237 if (!Exits.empty()) {
238 // There is at least one non-deoptimizing exit.
239 //
240 // Note, that BasicBlock::getPostdominatingDeoptimizeCall is not exact,
241 // as it can conservatively return false for deoptimizing exits with
242 // complex enough control flow down to deoptimize call.
243 //
244 // That means here we can report success for a case where
245 // all exits are deoptimizing but one of them has complex enough
246 // control flow (e.g. with loops).
247 //
248 // That should be a very rare case and false positives for this function
249 // have compile-time effect only.
250 return any_of(Exits, [](const BasicBlock *BB) {
251 return !BB->getPostdominatingDeoptimizeCall();
252 });
253 }
254 return false;
255}
256
257/// Rotate loop LP. Return true if the loop is rotated.
258///
259/// \param SimplifiedLatch is true if the latch was just folded into the final
260/// loop exit. In this case we may want to rotate even though the new latch is
261/// now an exiting branch. This rotation would have happened had the latch not
262/// been simplified. However, if SimplifiedLatch is false, then we avoid
263/// rotating loops in which the latch exits to avoid excessive or endless
264/// rotation. LoopRotate should be repeatable and converge to a canonical
265/// form. This property is satisfied because simplifying the loop latch can only
266/// happen once across multiple invocations of the LoopRotate pass.
267///
268/// If -loop-rotate-multi is enabled we can do multiple rotations in one go
269/// so to reach a suitable (non-deoptimizing) exit.
270bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) {
271 // If the loop has only one block then there is not much to rotate.
272 if (L->getBlocks().size() == 1)
1
Assuming the condition is false
2
Taking false branch
273 return false;
274
275 bool Rotated = false;
276 do {
277 BasicBlock *OrigHeader = L->getHeader();
278 BasicBlock *OrigLatch = L->getLoopLatch();
279
280 BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
3
Assuming the object is a 'BranchInst'
281 if (!BI
3.1
'BI' is non-null
|| BI->isUnconditional())
4
Taking false branch
282 return Rotated;
283
284 // If the loop header is not one of the loop exiting blocks then
285 // either this loop is already rotated or it is not
286 // suitable for loop rotation transformations.
287 if (!L->isLoopExiting(OrigHeader))
5
Assuming the condition is false
6
Taking false branch
288 return Rotated;
289
290 // If the loop latch already contains a branch that leaves the loop then the
291 // loop is already rotated.
292 if (!OrigLatch)
7
Assuming 'OrigLatch' is non-null
293 return Rotated;
294
295 // Rotate if either the loop latch does *not* exit the loop, or if the loop
296 // latch was just simplified. Or if we think it will be profitable.
297 if (L->isLoopExiting(OrigLatch) && !SimplifiedLatch && IsUtilMode == false &&
8
Assuming the condition is true
9
Assuming 'SimplifiedLatch' is false
10
Assuming the condition is true
298 !profitableToRotateLoopExitingLatch(L) &&
11
Calling 'profitableToRotateLoopExitingLatch'
299 !canRotateDeoptimizingLatchExit(L))
300 return Rotated;
301
302 // Check size of original header and reject loop if it is very big or we can't
303 // duplicate blocks inside it.
304 {
305 SmallPtrSet<const Value *, 32> EphValues;
306 CodeMetrics::collectEphemeralValues(L, AC, EphValues);
307
308 CodeMetrics Metrics;
309 Metrics.analyzeBasicBlock(OrigHeader, *TTI, EphValues, PrepareForLTO);
310 if (Metrics.notDuplicatable) {
311 LLVM_DEBUG(do { } while (false)
312 dbgs() << "LoopRotation: NOT rotating - contains non-duplicatable"do { } while (false)
313 << " instructions: ";do { } while (false)
314 L->dump())do { } while (false);
315 return Rotated;
316 }
317 if (Metrics.convergent) {
318 LLVM_DEBUG(dbgs() << "LoopRotation: NOT rotating - contains convergent "do { } while (false)
319 "instructions: ";do { } while (false)
320 L->dump())do { } while (false);
321 return Rotated;
322 }
323 if (Metrics.NumInsts > MaxHeaderSize) {
324 LLVM_DEBUG(dbgs() << "LoopRotation: NOT rotating - contains "do { } while (false)
325 << Metrics.NumInstsdo { } while (false)
326 << " instructions, which is more than the threshold ("do { } while (false)
327 << MaxHeaderSize << " instructions): ";do { } while (false)
328 L->dump())do { } while (false);
329 ++NumNotRotatedDueToHeaderSize;
330 return Rotated;
331 }
332
333 // When preparing for LTO, avoid rotating loops with calls that could be
334 // inlined during the LTO stage.
335 if (PrepareForLTO && Metrics.NumInlineCandidates > 0)
336 return Rotated;
337 }
338
339 // Now, this loop is suitable for rotation.
340 BasicBlock *OrigPreheader = L->getLoopPreheader();
341
342 // If the loop could not be converted to canonical form, it must have an
343 // indirectbr in it, just give up.
344 if (!OrigPreheader || !L->hasDedicatedExits())
345 return Rotated;
346
347 // Anything ScalarEvolution may know about this loop or the PHI nodes
348 // in its header will soon be invalidated. We should also invalidate
349 // all outer loops because insertion and deletion of blocks that happens
350 // during the rotation may violate invariants related to backedge taken
351 // infos in them.
352 if (SE)
353 SE->forgetTopmostLoop(L);
354
355 LLVM_DEBUG(dbgs() << "LoopRotation: rotating "; L->dump())do { } while (false);
356 if (MSSAU && VerifyMemorySSA)
357 MSSAU->getMemorySSA()->verifyMemorySSA();
358
359 // Find new Loop header. NewHeader is a Header's one and only successor
360 // that is inside loop. Header's other successor is outside the
361 // loop. Otherwise loop is not suitable for rotation.
362 BasicBlock *Exit = BI->getSuccessor(0);
363 BasicBlock *NewHeader = BI->getSuccessor(1);
364 if (L->contains(Exit))
365 std::swap(Exit, NewHeader);
366 assert(NewHeader && "Unable to determine new loop header")(static_cast<void> (0));
367 assert(L->contains(NewHeader) && !L->contains(Exit) &&(static_cast<void> (0))
368 "Unable to determine loop header and exit blocks")(static_cast<void> (0));
369
370 // This code assumes that the new header has exactly one predecessor.
371 // Remove any single-entry PHI nodes in it.
372 assert(NewHeader->getSinglePredecessor() &&(static_cast<void> (0))
373 "New header doesn't have one pred!")(static_cast<void> (0));
374 FoldSingleEntryPHINodes(NewHeader);
375
376 // Begin by walking OrigHeader and populating ValueMap with an entry for
377 // each Instruction.
378 BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
379 ValueToValueMapTy ValueMap, ValueMapMSSA;
380
381 // For PHI nodes, the value available in OldPreHeader is just the
382 // incoming value from OldPreHeader.
383 for (; PHINode *PN = dyn_cast<PHINode>(I); ++I)
384 InsertNewValueIntoMap(ValueMap, PN,
385 PN->getIncomingValueForBlock(OrigPreheader));
386
387 // For the rest of the instructions, either hoist to the OrigPreheader if
388 // possible or create a clone in the OldPreHeader if not.
389 Instruction *LoopEntryBranch = OrigPreheader->getTerminator();
390
391 // Record all debug intrinsics preceding LoopEntryBranch to avoid
392 // duplication.
393 using DbgIntrinsicHash =
394 std::pair<std::pair<hash_code, DILocalVariable *>, DIExpression *>;
395 auto makeHash = [](DbgVariableIntrinsic *D) -> DbgIntrinsicHash {
396 auto VarLocOps = D->location_ops();
397 return {{hash_combine_range(VarLocOps.begin(), VarLocOps.end()),
398 D->getVariable()},
399 D->getExpression()};
400 };
401 SmallDenseSet<DbgIntrinsicHash, 8> DbgIntrinsics;
402 for (auto I = std::next(OrigPreheader->rbegin()), E = OrigPreheader->rend();
403 I != E; ++I) {
404 if (auto *DII = dyn_cast<DbgVariableIntrinsic>(&*I))
405 DbgIntrinsics.insert(makeHash(DII));
406 else
407 break;
408 }
409
410 // Remember the local noalias scope declarations in the header. After the
411 // rotation, they must be duplicated and the scope must be cloned. This
412 // avoids unwanted interaction across iterations.
413 SmallVector<NoAliasScopeDeclInst *, 6> NoAliasDeclInstructions;
414 for (Instruction &I : *OrigHeader)
415 if (auto *Decl = dyn_cast<NoAliasScopeDeclInst>(&I))
416 NoAliasDeclInstructions.push_back(Decl);
417
418 while (I != E) {
419 Instruction *Inst = &*I++;
420
421 // If the instruction's operands are invariant and it doesn't read or write
422 // memory, then it is safe to hoist. Doing this doesn't change the order of
423 // execution in the preheader, but does prevent the instruction from
424 // executing in each iteration of the loop. This means it is safe to hoist
425 // something that might trap, but isn't safe to hoist something that reads
426 // memory (without proving that the loop doesn't write).
427 if (L->hasLoopInvariantOperands(Inst) && !Inst->mayReadFromMemory() &&
428 !Inst->mayWriteToMemory() && !Inst->isTerminator() &&
429 !isa<DbgInfoIntrinsic>(Inst) && !isa<AllocaInst>(Inst)) {
430 Inst->moveBefore(LoopEntryBranch);
431 ++NumInstrsHoisted;
432 continue;
433 }
434
435 // Otherwise, create a duplicate of the instruction.
436 Instruction *C = Inst->clone();
437 ++NumInstrsDuplicated;
438
439 // Eagerly remap the operands of the instruction.
440 RemapInstruction(C, ValueMap,
441 RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
442
443 // Avoid inserting the same intrinsic twice.
444 if (auto *DII = dyn_cast<DbgVariableIntrinsic>(C))
445 if (DbgIntrinsics.count(makeHash(DII))) {
446 C->deleteValue();
447 continue;
448 }
449
450 // With the operands remapped, see if the instruction constant folds or is
451 // otherwise simplifyable. This commonly occurs because the entry from PHI
452 // nodes allows icmps and other instructions to fold.
453 Value *V = SimplifyInstruction(C, SQ);
454 if (V && LI->replacementPreservesLCSSAForm(C, V)) {
455 // If so, then delete the temporary instruction and stick the folded value
456 // in the map.
457 InsertNewValueIntoMap(ValueMap, Inst, V);
458 if (!C->mayHaveSideEffects()) {
459 C->deleteValue();
460 C = nullptr;
461 }
462 } else {
463 InsertNewValueIntoMap(ValueMap, Inst, C);
464 }
465 if (C) {
466 // Otherwise, stick the new instruction into the new block!
467 C->setName(Inst->getName());
468 C->insertBefore(LoopEntryBranch);
469
470 if (auto *II = dyn_cast<AssumeInst>(C))
471 AC->registerAssumption(II);
472 // MemorySSA cares whether the cloned instruction was inserted or not, and
473 // not whether it can be remapped to a simplified value.
474 if (MSSAU)
475 InsertNewValueIntoMap(ValueMapMSSA, Inst, C);
476 }
477 }
478
479 if (!NoAliasDeclInstructions.empty()) {
480 // There are noalias scope declarations:
481 // (general):
482 // Original: OrigPre { OrigHeader NewHeader ... Latch }
483 // after: (OrigPre+OrigHeader') { NewHeader ... Latch OrigHeader }
484 //
485 // with D: llvm.experimental.noalias.scope.decl,
486 // U: !noalias or !alias.scope depending on D
487 // ... { D U1 U2 } can transform into:
488 // (0) : ... { D U1 U2 } // no relevant rotation for this part
489 // (1) : ... D' { U1 U2 D } // D is part of OrigHeader
490 // (2) : ... D' U1' { U2 D U1 } // D, U1 are part of OrigHeader
491 //
492 // We now want to transform:
493 // (1) -> : ... D' { D U1 U2 D'' }
494 // (2) -> : ... D' U1' { D U2 D'' U1'' }
495 // D: original llvm.experimental.noalias.scope.decl
496 // D', U1': duplicate with replaced scopes
497 // D'', U1'': different duplicate with replaced scopes
498 // This ensures a safe fallback to 'may_alias' introduced by the rotate,
499 // as U1'' and U1' scopes will not be compatible wrt to the local restrict
500
501 // Clone the llvm.experimental.noalias.decl again for the NewHeader.
502 Instruction *NewHeaderInsertionPoint = &(*NewHeader->getFirstNonPHI());
503 for (NoAliasScopeDeclInst *NAD : NoAliasDeclInstructions) {
504 LLVM_DEBUG(dbgs() << " Cloning llvm.experimental.noalias.scope.decl:"do { } while (false)
505 << *NAD << "\n")do { } while (false);
506 Instruction *NewNAD = NAD->clone();
507 NewNAD->insertBefore(NewHeaderInsertionPoint);
508 }
509
510 // Scopes must now be duplicated, once for OrigHeader and once for
511 // OrigPreHeader'.
512 {
513 auto &Context = NewHeader->getContext();
514
515 SmallVector<MDNode *, 8> NoAliasDeclScopes;
516 for (NoAliasScopeDeclInst *NAD : NoAliasDeclInstructions)
517 NoAliasDeclScopes.push_back(NAD->getScopeList());
518
519 LLVM_DEBUG(dbgs() << " Updating OrigHeader scopes\n")do { } while (false);
520 cloneAndAdaptNoAliasScopes(NoAliasDeclScopes, {OrigHeader}, Context,
521 "h.rot");
522 LLVM_DEBUG(OrigHeader->dump())do { } while (false);
523
524 // Keep the compile time impact low by only adapting the inserted block
525 // of instructions in the OrigPreHeader. This might result in slightly
526 // more aliasing between these instructions and those that were already
527 // present, but it will be much faster when the original PreHeader is
528 // large.
529 LLVM_DEBUG(dbgs() << " Updating part of OrigPreheader scopes\n")do { } while (false);
530 auto *FirstDecl =
531 cast<Instruction>(ValueMap[*NoAliasDeclInstructions.begin()]);
532 auto *LastInst = &OrigPreheader->back();
533 cloneAndAdaptNoAliasScopes(NoAliasDeclScopes, FirstDecl, LastInst,
534 Context, "pre.rot");
535 LLVM_DEBUG(OrigPreheader->dump())do { } while (false);
536
537 LLVM_DEBUG(dbgs() << " Updated NewHeader:\n")do { } while (false);
538 LLVM_DEBUG(NewHeader->dump())do { } while (false);
539 }
540 }
541
542 // Along with all the other instructions, we just cloned OrigHeader's
543 // terminator into OrigPreHeader. Fix up the PHI nodes in each of OrigHeader's
544 // successors by duplicating their incoming values for OrigHeader.
545 for (BasicBlock *SuccBB : successors(OrigHeader))
546 for (BasicBlock::iterator BI = SuccBB->begin();
547 PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
548 PN->addIncoming(PN->getIncomingValueForBlock(OrigHeader), OrigPreheader);
549
550 // Now that OrigPreHeader has a clone of OrigHeader's terminator, remove
551 // OrigPreHeader's old terminator (the original branch into the loop), and
552 // remove the corresponding incoming values from the PHI nodes in OrigHeader.
553 LoopEntryBranch->eraseFromParent();
554
555 // Update MemorySSA before the rewrite call below changes the 1:1
556 // instruction:cloned_instruction_or_value mapping.
557 if (MSSAU) {
558 InsertNewValueIntoMap(ValueMapMSSA, OrigHeader, OrigPreheader);
559 MSSAU->updateForClonedBlockIntoPred(OrigHeader, OrigPreheader,
560 ValueMapMSSA);
561 }
562
563 SmallVector<PHINode*, 2> InsertedPHIs;
564 // If there were any uses of instructions in the duplicated block outside the
565 // loop, update them, inserting PHI nodes as required
566 RewriteUsesOfClonedInstructions(OrigHeader, OrigPreheader, ValueMap,
567 &InsertedPHIs);
568
569 // Attach dbg.value intrinsics to the new phis if that phi uses a value that
570 // previously had debug metadata attached. This keeps the debug info
571 // up-to-date in the loop body.
572 if (!InsertedPHIs.empty())
573 insertDebugValuesForPHIs(OrigHeader, InsertedPHIs);
574
575 // NewHeader is now the header of the loop.
576 L->moveToHeader(NewHeader);
577 assert(L->getHeader() == NewHeader && "Latch block is our new header")(static_cast<void> (0));
578
579 // Inform DT about changes to the CFG.
580 if (DT) {
581 // The OrigPreheader branches to the NewHeader and Exit now. Then, inform
582 // the DT about the removed edge to the OrigHeader (that got removed).
583 SmallVector<DominatorTree::UpdateType, 3> Updates;
584 Updates.push_back({DominatorTree::Insert, OrigPreheader, Exit});
585 Updates.push_back({DominatorTree::Insert, OrigPreheader, NewHeader});
586 Updates.push_back({DominatorTree::Delete, OrigPreheader, OrigHeader});
587
588 if (MSSAU) {
589 MSSAU->applyUpdates(Updates, *DT, /*UpdateDT=*/true);
590 if (VerifyMemorySSA)
591 MSSAU->getMemorySSA()->verifyMemorySSA();
592 } else {
593 DT->applyUpdates(Updates);
594 }
595 }
596
597 // At this point, we've finished our major CFG changes. As part of cloning
598 // the loop into the preheader we've simplified instructions and the
599 // duplicated conditional branch may now be branching on a constant. If it is
600 // branching on a constant and if that constant means that we enter the loop,
601 // then we fold away the cond branch to an uncond branch. This simplifies the
602 // loop in cases important for nested loops, and it also means we don't have
603 // to split as many edges.
604 BranchInst *PHBI = cast<BranchInst>(OrigPreheader->getTerminator());
605 assert(PHBI->isConditional() && "Should be clone of BI condbr!")(static_cast<void> (0));
606 if (!isa<ConstantInt>(PHBI->getCondition()) ||
607 PHBI->getSuccessor(cast<ConstantInt>(PHBI->getCondition())->isZero()) !=
608 NewHeader) {
609 // The conditional branch can't be folded, handle the general case.
610 // Split edges as necessary to preserve LoopSimplify form.
611
612 // Right now OrigPreHeader has two successors, NewHeader and ExitBlock, and
613 // thus is not a preheader anymore.
614 // Split the edge to form a real preheader.
615 BasicBlock *NewPH = SplitCriticalEdge(
616 OrigPreheader, NewHeader,
617 CriticalEdgeSplittingOptions(DT, LI, MSSAU).setPreserveLCSSA());
618 NewPH->setName(NewHeader->getName() + ".lr.ph");
619
620 // Preserve canonical loop form, which means that 'Exit' should have only
621 // one predecessor. Note that Exit could be an exit block for multiple
622 // nested loops, causing both of the edges to now be critical and need to
623 // be split.
624 SmallVector<BasicBlock *, 4> ExitPreds(pred_begin(Exit), pred_end(Exit));
625 bool SplitLatchEdge = false;
626 for (BasicBlock *ExitPred : ExitPreds) {
627 // We only need to split loop exit edges.
628 Loop *PredLoop = LI->getLoopFor(ExitPred);
629 if (!PredLoop || PredLoop->contains(Exit) ||
630 ExitPred->getTerminator()->isIndirectTerminator())
631 continue;
632 SplitLatchEdge |= L->getLoopLatch() == ExitPred;
633 BasicBlock *ExitSplit = SplitCriticalEdge(
634 ExitPred, Exit,
635 CriticalEdgeSplittingOptions(DT, LI, MSSAU).setPreserveLCSSA());
636 ExitSplit->moveBefore(Exit);
637 }
638 assert(SplitLatchEdge &&(static_cast<void> (0))
639 "Despite splitting all preds, failed to split latch exit?")(static_cast<void> (0));
640 (void)SplitLatchEdge;
641 } else {
642 // We can fold the conditional branch in the preheader, this makes things
643 // simpler. The first step is to remove the extra edge to the Exit block.
644 Exit->removePredecessor(OrigPreheader, true /*preserve LCSSA*/);
645 BranchInst *NewBI = BranchInst::Create(NewHeader, PHBI);
646 NewBI->setDebugLoc(PHBI->getDebugLoc());
647 PHBI->eraseFromParent();
648
649 // With our CFG finalized, update DomTree if it is available.
650 if (DT) DT->deleteEdge(OrigPreheader, Exit);
651
652 // Update MSSA too, if available.
653 if (MSSAU)
654 MSSAU->removeEdge(OrigPreheader, Exit);
655 }
656
657 assert(L->getLoopPreheader() && "Invalid loop preheader after loop rotation")(static_cast<void> (0));
658 assert(L->getLoopLatch() && "Invalid loop latch after loop rotation")(static_cast<void> (0));
659
660 if (MSSAU && VerifyMemorySSA)
661 MSSAU->getMemorySSA()->verifyMemorySSA();
662
663 // Now that the CFG and DomTree are in a consistent state again, try to merge
664 // the OrigHeader block into OrigLatch. This will succeed if they are
665 // connected by an unconditional branch. This is just a cleanup so the
666 // emitted code isn't too gross in this common case.
667 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
668 BasicBlock *PredBB = OrigHeader->getUniquePredecessor();
669 bool DidMerge = MergeBlockIntoPredecessor(OrigHeader, &DTU, LI, MSSAU);
670 if (DidMerge)
671 RemoveRedundantDbgInstrs(PredBB);
672
673 if (MSSAU && VerifyMemorySSA)
674 MSSAU->getMemorySSA()->verifyMemorySSA();
675
676 LLVM_DEBUG(dbgs() << "LoopRotation: into "; L->dump())do { } while (false);
677
678 ++NumRotated;
679
680 Rotated = true;
681 SimplifiedLatch = false;
682
683 // Check that new latch is a deoptimizing exit and then repeat rotation if possible.
684 // Deoptimizing latch exit is not a generally typical case, so we just loop over.
685 // TODO: if it becomes a performance bottleneck extend rotation algorithm
686 // to handle multiple rotations in one go.
687 } while (MultiRotate && canRotateDeoptimizingLatchExit(L));
688
689
690 return true;
691}
692
693/// Determine whether the instructions in this range may be safely and cheaply
694/// speculated. This is not an important enough situation to develop complex
695/// heuristics. We handle a single arithmetic instruction along with any type
696/// conversions.
697static bool shouldSpeculateInstrs(BasicBlock::iterator Begin,
698 BasicBlock::iterator End, Loop *L) {
699 bool seenIncrement = false;
700 bool MultiExitLoop = false;
701
702 if (!L->getExitingBlock())
703 MultiExitLoop = true;
704
705 for (BasicBlock::iterator I = Begin; I != End; ++I) {
706
707 if (!isSafeToSpeculativelyExecute(&*I))
708 return false;
709
710 if (isa<DbgInfoIntrinsic>(I))
711 continue;
712
713 switch (I->getOpcode()) {
714 default:
715 return false;
716 case Instruction::GetElementPtr:
717 // GEPs are cheap if all indices are constant.
718 if (!cast<GEPOperator>(I)->hasAllConstantIndices())
719 return false;
720 // fall-thru to increment case
721 LLVM_FALLTHROUGH[[gnu::fallthrough]];
722 case Instruction::Add:
723 case Instruction::Sub:
724 case Instruction::And:
725 case Instruction::Or:
726 case Instruction::Xor:
727 case Instruction::Shl:
728 case Instruction::LShr:
729 case Instruction::AShr: {
730 Value *IVOpnd =
731 !isa<Constant>(I->getOperand(0))
732 ? I->getOperand(0)
733 : !isa<Constant>(I->getOperand(1)) ? I->getOperand(1) : nullptr;
734 if (!IVOpnd)
735 return false;
736
737 // If increment operand is used outside of the loop, this speculation
738 // could cause extra live range interference.
739 if (MultiExitLoop) {
740 for (User *UseI : IVOpnd->users()) {
741 auto *UserInst = cast<Instruction>(UseI);
742 if (!L->contains(UserInst))
743 return false;
744 }
745 }
746
747 if (seenIncrement)
748 return false;
749 seenIncrement = true;
750 break;
751 }
752 case Instruction::Trunc:
753 case Instruction::ZExt:
754 case Instruction::SExt:
755 // ignore type conversions
756 break;
757 }
758 }
759 return true;
760}
761
762/// Fold the loop tail into the loop exit by speculating the loop tail
763/// instructions. Typically, this is a single post-increment. In the case of a
764/// simple 2-block loop, hoisting the increment can be much better than
765/// duplicating the entire loop header. In the case of loops with early exits,
766/// rotation will not work anyway, but simplifyLoopLatch will put the loop in
767/// canonical form so downstream passes can handle it.
768///
769/// I don't believe this invalidates SCEV.
770bool LoopRotate::simplifyLoopLatch(Loop *L) {
771 BasicBlock *Latch = L->getLoopLatch();
772 if (!Latch || Latch->hasAddressTaken())
773 return false;
774
775 BranchInst *Jmp = dyn_cast<BranchInst>(Latch->getTerminator());
776 if (!Jmp || !Jmp->isUnconditional())
777 return false;
778
779 BasicBlock *LastExit = Latch->getSinglePredecessor();
780 if (!LastExit || !L->isLoopExiting(LastExit))
781 return false;
782
783 BranchInst *BI = dyn_cast<BranchInst>(LastExit->getTerminator());
784 if (!BI)
785 return false;
786
787 if (!shouldSpeculateInstrs(Latch->begin(), Jmp->getIterator(), L))
788 return false;
789
790 LLVM_DEBUG(dbgs() << "Folding loop latch " << Latch->getName() << " into "do { } while (false)
791 << LastExit->getName() << "\n")do { } while (false);
792
793 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
794 MergeBlockIntoPredecessor(Latch, &DTU, LI, MSSAU, nullptr,
795 /*PredecessorWithTwoSuccessors=*/true);
796
797 if (MSSAU && VerifyMemorySSA)
798 MSSAU->getMemorySSA()->verifyMemorySSA();
799
800 return true;
801}
802
803/// Rotate \c L, and return true if any modification was made.
804bool LoopRotate::processLoop(Loop *L) {
805 // Save the loop metadata.
806 MDNode *LoopMD = L->getLoopID();
807
808 bool SimplifiedLatch = false;
809
810 // Simplify the loop latch before attempting to rotate the header
811 // upward. Rotation may not be needed if the loop tail can be folded into the
812 // loop exit.
813 if (!RotationOnly)
814 SimplifiedLatch = simplifyLoopLatch(L);
815
816 bool MadeChange = rotateLoop(L, SimplifiedLatch);
817 assert((!MadeChange || L->isLoopExiting(L->getLoopLatch())) &&(static_cast<void> (0))
818 "Loop latch should be exiting after loop-rotate.")(static_cast<void> (0));
819
820 // Restore the loop metadata.
821 // NB! We presume LoopRotation DOESN'T ADD its own metadata.
822 if ((MadeChange || SimplifiedLatch) && LoopMD)
823 L->setLoopID(LoopMD);
824
825 return MadeChange || SimplifiedLatch;
826}
827
828
829/// The utility to convert a loop into a loop with bottom test.
830bool llvm::LoopRotation(Loop *L, LoopInfo *LI, const TargetTransformInfo *TTI,
831 AssumptionCache *AC, DominatorTree *DT,
832 ScalarEvolution *SE, MemorySSAUpdater *MSSAU,
833 const SimplifyQuery &SQ, bool RotationOnly = true,
834 unsigned Threshold = unsigned(-1),
835 bool IsUtilMode = true, bool PrepareForLTO) {
836 LoopRotate LR(Threshold, LI, TTI, AC, DT, SE, MSSAU, SQ, RotationOnly,
837 IsUtilMode, PrepareForLTO);
838 return LR.processLoop(L);
839}