lib/CodeGen/LoopGenerators.cpp

   1 //===------ LoopGenerators.cpp -  IR helper to create loops ---------------===//
   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 contains functions to create scalar loops and orchestrate the
  10 // creation of parallel loops as LLVM-IR.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "polly/CodeGen/LoopGenerators.h"
  15 #include "polly/Options.h"
  16 #include "polly/ScopDetection.h"
  17 #include "llvm/Analysis/LoopInfo.h"
  18 #include "llvm/IR/DataLayout.h"
  19 #include "llvm/IR/Dominators.h"
  20 #include "llvm/IR/Module.h"
  21 #include "llvm/Support/CommandLine.h"
  22 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  23
  24 using namespace llvm;
  25 using namespace polly;
  26
  27 int polly::PollyNumThreads;
  28 OMPGeneralSchedulingType polly::PollyScheduling;
  29 int polly::PollyChunkSize;
  30
  31 static cl::opt<int, true>
  32     XPollyNumThreads("polly-num-threads",
  33                      cl::desc("Number of threads to use (0 = auto)"),
  34                      cl::Hidden, cl::location(polly::PollyNumThreads),
  35                      cl::init(0), cl::cat(PollyCategory));
  36
  37 static cl::opt<OMPGeneralSchedulingType, true> XPollyScheduling(
  38     "polly-scheduling",
  39     cl::desc("Scheduling type of parallel OpenMP for loops"),
  40     cl::values(clEnumValN(OMPGeneralSchedulingType::StaticChunked, "static",
  41                           "Static scheduling"),
  42                clEnumValN(OMPGeneralSchedulingType::Dynamic, "dynamic",
  43                           "Dynamic scheduling"),
  44                clEnumValN(OMPGeneralSchedulingType::Guided, "guided",
  45                           "Guided scheduling"),
  46                clEnumValN(OMPGeneralSchedulingType::Runtime, "runtime",
  47                           "Runtime determined (OMP_SCHEDULE)")),
  48     cl::Hidden, cl::location(polly::PollyScheduling),
  49     cl::init(OMPGeneralSchedulingType::Runtime), cl::Optional,
  50     cl::cat(PollyCategory));
  51
  52 static cl::opt<int, true>
  53     XPollyChunkSize("polly-scheduling-chunksize",
  54                     cl::desc("Chunksize to use by the OpenMP runtime calls"),
  55                     cl::Hidden, cl::location(polly::PollyChunkSize),
  56                     cl::init(0), cl::Optional, cl::cat(PollyCategory));
  57
  58 // We generate a loop of either of the following structures:
  59 //
  60 //              BeforeBB                      BeforeBB
  61 //                 |                             |
  62 //                 v                             v
  63 //              GuardBB                      PreHeaderBB
  64 //              /      |                         |   _____
  65 //     __  PreHeaderBB  |                        v  \/    |
  66 //    /  \    /         |                     HeaderBB  latch
  67 // latch  HeaderBB      |                        |\       |
  68 //    \  /    \         /                        | \------/
  69 //     <       \       /                         |
  70 //              \     /                          v
  71 //              ExitBB                         ExitBB
  72 //
  73 // depending on whether or not we know that it is executed at least once. If
  74 // not, GuardBB checks if the loop is executed at least once. If this is the
  75 // case we branch to PreHeaderBB and subsequently to the HeaderBB, which
  76 // contains the loop iv 'polly.indvar', the incremented loop iv
  77 // 'polly.indvar_next' as well as the condition to check if we execute another
  78 // iteration of the loop. After the loop has finished, we branch to ExitBB.
  79 // We expect the type of UB, LB, UB+Stride to be large enough for values that
  80 // UB may take throughout the execution of the loop, including the computation
  81 // of indvar + Stride before the final abort.
  82 Value *polly::createLoop(Value *LB, Value *UB, Value *Stride,
  83                          PollyIRBuilder &Builder, LoopInfo &LI,
  84                          DominatorTree &DT, BasicBlock *&ExitBB,
  85                          ICmpInst::Predicate Predicate,
  86                          ScopAnnotator *Annotator, bool Parallel, bool UseGuard,
  87                          bool LoopVectDisabled) {
  88   Function *F = Builder.GetInsertBlock()->getParent();
  89   LLVMContext &Context = F->getContext();
  90
  91   assert(LB->getType() == UB->getType() && "Types of loop bounds do not match");
  92   IntegerType *LoopIVType = dyn_cast<IntegerType>(UB->getType());
  93   assert(LoopIVType && "UB is not integer?");
  94
  95   BasicBlock *BeforeBB = Builder.GetInsertBlock();
  96   BasicBlock *GuardBB =
  97       UseGuard ? BasicBlock::Create(Context, "polly.loop_if", F) : nullptr;
  98   BasicBlock *HeaderBB = BasicBlock::Create(Context, "polly.loop_header", F);
  99   BasicBlock *PreHeaderBB =
 100       BasicBlock::Create(Context, "polly.loop_preheader", F);
 101
 102   // Update LoopInfo
 103   Loop *OuterLoop = LI.getLoopFor(BeforeBB);
 104   Loop *NewLoop = LI.AllocateLoop();
 105
 106   if (OuterLoop)
 107     OuterLoop->addChildLoop(NewLoop);
 108   else
 109     LI.addTopLevelLoop(NewLoop);
 110
 111   if (OuterLoop) {
 112     if (GuardBB)
 113       OuterLoop->addBasicBlockToLoop(GuardBB, LI);
 114     OuterLoop->addBasicBlockToLoop(PreHeaderBB, LI);
 115   }
 116
 117   NewLoop->addBasicBlockToLoop(HeaderBB, LI);
 118
 119   // Notify the annotator (if present) that we have a new loop, but only
 120   // after the header block is set.
 121   if (Annotator)
 122     Annotator->pushLoop(NewLoop, Parallel);
 123
 124   // ExitBB
 125   ExitBB = SplitBlock(BeforeBB, &*Builder.GetInsertPoint(), &DT, &LI);
 126   ExitBB->setName("polly.loop_exit");
 127
 128   // BeforeBB
 129   if (GuardBB) {
 130     BeforeBB->getTerminator()->setSuccessor(0, GuardBB);
 131     DT.addNewBlock(GuardBB, BeforeBB);
 132
 133     // GuardBB
 134     Builder.SetInsertPoint(GuardBB);
 135     Value *LoopGuard;
 136     LoopGuard = Builder.CreateICmp(Predicate, LB, UB);
 137     LoopGuard->setName("polly.loop_guard");
 138     Builder.CreateCondBr(LoopGuard, PreHeaderBB, ExitBB);
 139     DT.addNewBlock(PreHeaderBB, GuardBB);
 140   } else {
 141     BeforeBB->getTerminator()->setSuccessor(0, PreHeaderBB);
 142     DT.addNewBlock(PreHeaderBB, BeforeBB);
 143   }
 144
 145   // PreHeaderBB
 146   Builder.SetInsertPoint(PreHeaderBB);
 147   Builder.CreateBr(HeaderBB);
 148
 149   // HeaderBB
 150   DT.addNewBlock(HeaderBB, PreHeaderBB);
 151   Builder.SetInsertPoint(HeaderBB);
 152   PHINode *IV = Builder.CreatePHI(LoopIVType, 2, "polly.indvar");
 153   IV->addIncoming(LB, PreHeaderBB);
 154   Stride = Builder.CreateZExtOrBitCast(Stride, LoopIVType);
 155   Value *IncrementedIV = Builder.CreateNSWAdd(IV, Stride, "polly.indvar_next");
 156   Value *LoopCondition =
 157       Builder.CreateICmp(Predicate, IncrementedIV, UB, "polly.loop_cond");
 158
 159   // Create the loop latch and annotate it as such.
 160   BranchInst *B = Builder.CreateCondBr(LoopCondition, HeaderBB, ExitBB);
 161   if (Annotator)
 162     Annotator->annotateLoopLatch(B, NewLoop, Parallel, LoopVectDisabled);
 163
 164   IV->addIncoming(IncrementedIV, HeaderBB);
 165   if (GuardBB)
 166     DT.changeImmediateDominator(ExitBB, GuardBB);
 167   else
 168     DT.changeImmediateDominator(ExitBB, HeaderBB);
 169
 170   // The loop body should be added here.
 171   Builder.SetInsertPoint(HeaderBB->getFirstNonPHI());
 172   return IV;
 173 }
 174
 175 Value *ParallelLoopGenerator::createParallelLoop(
 176     Value *LB, Value *UB, Value *Stride, SetVector<Value *> &UsedValues,
 177     ValueMapT &Map, BasicBlock::iterator *LoopBody) {
 178
 179   AllocaInst *Struct = storeValuesIntoStruct(UsedValues);
 180   BasicBlock::iterator BeforeLoop = Builder.GetInsertPoint();
 181
 182   Value *IV;
 183   Function *SubFn;
 184   std::tie(IV, SubFn) = createSubFn(Stride, Struct, UsedValues, Map);
 185   *LoopBody = Builder.GetInsertPoint();
 186   Builder.SetInsertPoint(&*BeforeLoop);
 187
 188   Value *SubFnParam = Builder.CreateBitCast(Struct, Builder.getInt8PtrTy(),
 189                                             "polly.par.userContext");
 190
 191   // Add one as the upper bound provided by OpenMP is a < comparison
 192   // whereas the codegenForSequential function creates a <= comparison.
 193   UB = Builder.CreateAdd(UB, ConstantInt::get(LongType, 1));
 194
 195   // Execute the prepared subfunction in parallel.
 196   deployParallelExecution(SubFn, SubFnParam, LB, UB, Stride);
 197
 198   return IV;
 199 }
 200
 201 Function *ParallelLoopGenerator::createSubFnDefinition() {
 202   Function *F = Builder.GetInsertBlock()->getParent();
 203   Function *SubFn = prepareSubFnDefinition(F);
 204
 205   // Certain backends (e.g., NVPTX) do not support '.'s in function names.
 206   // Hence, we ensure that all '.'s are replaced by '_'s.
 207   std::string FunctionName = SubFn->getName();
 208   std::replace(FunctionName.begin(), FunctionName.end(), '.', '_');
 209   SubFn->setName(FunctionName);
 210
 211   // Do not run any polly pass on the new function.
 212   SubFn->addFnAttr(PollySkipFnAttr);
 213
 214   return SubFn;
 215 }
 216
 217 AllocaInst *
 218 ParallelLoopGenerator::storeValuesIntoStruct(SetVector<Value *> &Values) {
 219   SmallVector<Type *, 8> Members;
 220
 221   for (Value *V : Values)
 222     Members.push_back(V->getType());
 223
 224   const DataLayout &DL = Builder.GetInsertBlock()->getModule()->getDataLayout();
 225
 226   // We do not want to allocate the alloca inside any loop, thus we allocate it
 227   // in the entry block of the function and use annotations to denote the actual
 228   // live span (similar to clang).
 229   BasicBlock &EntryBB = Builder.GetInsertBlock()->getParent()->getEntryBlock();
 230   Instruction *IP = &*EntryBB.getFirstInsertionPt();
 231   StructType *Ty = StructType::get(Builder.getContext(), Members);
 232   AllocaInst *Struct = new AllocaInst(Ty, DL.getAllocaAddrSpace(), nullptr,
 233                                       "polly.par.userContext", IP);
 234
 235   for (unsigned i = 0; i < Values.size(); i++) {
 236     Value *Address = Builder.CreateStructGEP(Ty, Struct, i);
 237     Address->setName("polly.subfn.storeaddr." + Values[i]->getName());
 238     Builder.CreateStore(Values[i], Address);
 239   }
 240
 241   return Struct;
 242 }
 243
 244 void ParallelLoopGenerator::extractValuesFromStruct(
 245     SetVector<Value *> OldValues, Type *Ty, Value *Struct, ValueMapT &Map) {
 246   for (unsigned i = 0; i < OldValues.size(); i++) {
 247     Value *Address = Builder.CreateStructGEP(Ty, Struct, i);
 248     Value *NewValue = Builder.CreateLoad(Address);
 249     NewValue->setName("polly.subfunc.arg." + OldValues[i]->getName());
 250     Map[OldValues[i]] = NewValue;
 251   }
 252 }