1 //===- ModuleTranslation.cpp - MLIR to LLVM conversion --------------------===//
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 implements the translation between an MLIR LLVM dialect module and
10 // the corresponding LLVMIR module. It only handles core LLVM IR operations.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "mlir/Target/LLVMIR/ModuleTranslation.h"
15 
16 #include "DebugTranslation.h"
17 #include "mlir/Dialect/LLVMIR/LLVMDialect.h"
18 #include "mlir/Dialect/OpenMP/OpenMPDialect.h"
19 #include "mlir/IR/Attributes.h"
20 #include "mlir/IR/Module.h"
21 #include "mlir/IR/StandardTypes.h"
22 #include "mlir/Support/LLVM.h"
23 
24 #include "llvm/ADT/SetVector.h"
25 #include "llvm/Frontend/OpenMP/OMPIRBuilder.h"
26 #include "llvm/IR/BasicBlock.h"
27 #include "llvm/IR/Constants.h"
28 #include "llvm/IR/DerivedTypes.h"
29 #include "llvm/IR/IRBuilder.h"
30 #include "llvm/IR/LLVMContext.h"
31 #include "llvm/IR/Module.h"
32 #include "llvm/Transforms/Utils/Cloning.h"
33 
34 using namespace mlir;
35 using namespace mlir::LLVM;
36 using namespace mlir::LLVM::detail;
37 
38 #include "mlir/Dialect/LLVMIR/LLVMConversionEnumsToLLVM.inc"
39 
40 /// Builds a constant of a sequential LLVM type `type`, potentially containing
41 /// other sequential types recursively, from the individual constant values
42 /// provided in `constants`. `shape` contains the number of elements in nested
43 /// sequential types. Reports errors at `loc` and returns nullptr on error.
44 static llvm::Constant *
45 buildSequentialConstant(ArrayRef<llvm::Constant *> &constants,
46                         ArrayRef<int64_t> shape, llvm::Type *type,
47                         Location loc) {
48   if (shape.empty()) {
49     llvm::Constant *result = constants.front();
50     constants = constants.drop_front();
51     return result;
52   }
53 
54   if (!isa<llvm::SequentialType>(type)) {
55     emitError(loc) << "expected sequential LLVM types wrapping a scalar";
56     return nullptr;
57   }
58 
59   llvm::Type *elementType = type->getSequentialElementType();
60   SmallVector<llvm::Constant *, 8> nested;
61   nested.reserve(shape.front());
62   for (int64_t i = 0; i < shape.front(); ++i) {
63     nested.push_back(buildSequentialConstant(constants, shape.drop_front(),
64                                              elementType, loc));
65     if (!nested.back())
66       return nullptr;
67   }
68 
69   if (shape.size() == 1 && type->isVectorTy())
70     return llvm::ConstantVector::get(nested);
71   return llvm::ConstantArray::get(
72       llvm::ArrayType::get(elementType, shape.front()), nested);
73 }
74 
75 /// Returns the first non-sequential type nested in sequential types.
76 static llvm::Type *getInnermostElementType(llvm::Type *type) {
77   while (isa<llvm::SequentialType>(type))
78     type = type->getSequentialElementType();
79   return type;
80 }
81 
82 /// Create an LLVM IR constant of `llvmType` from the MLIR attribute `attr`.
83 /// This currently supports integer, floating point, splat and dense element
84 /// attributes and combinations thereof.  In case of error, report it to `loc`
85 /// and return nullptr.
86 llvm::Constant *ModuleTranslation::getLLVMConstant(llvm::Type *llvmType,
87                                                    Attribute attr,
88                                                    Location loc) {
89   if (!attr)
90     return llvm::UndefValue::get(llvmType);
91   if (llvmType->isStructTy()) {
92     emitError(loc, "struct types are not supported in constants");
93     return nullptr;
94   }
95   if (auto intAttr = attr.dyn_cast<IntegerAttr>())
96     return llvm::ConstantInt::get(llvmType, intAttr.getValue());
97   if (auto floatAttr = attr.dyn_cast<FloatAttr>())
98     return llvm::ConstantFP::get(llvmType, floatAttr.getValue());
99   if (auto funcAttr = attr.dyn_cast<FlatSymbolRefAttr>())
100     return functionMapping.lookup(funcAttr.getValue());
101   if (auto splatAttr = attr.dyn_cast<SplatElementsAttr>()) {
102     auto *sequentialType = cast<llvm::SequentialType>(llvmType);
103     auto elementType = sequentialType->getElementType();
104     uint64_t numElements = sequentialType->getNumElements();
105     // Splat value is a scalar. Extract it only if the element type is not
106     // another sequence type. The recursion terminates because each step removes
107     // one outer sequential type.
108     llvm::Constant *child = getLLVMConstant(
109         elementType,
110         isa<llvm::SequentialType>(elementType) ? splatAttr
111                                                : splatAttr.getSplatValue(),
112         loc);
113     if (!child)
114       return nullptr;
115     if (llvmType->isVectorTy())
116       return llvm::ConstantVector::getSplat(
117           llvm::ElementCount(numElements, /*Scalable=*/false), child);
118     if (llvmType->isArrayTy()) {
119       auto arrayType = llvm::ArrayType::get(elementType, numElements);
120       SmallVector<llvm::Constant *, 8> constants(numElements, child);
121       return llvm::ConstantArray::get(arrayType, constants);
122     }
123   }
124 
125   if (auto elementsAttr = attr.dyn_cast<ElementsAttr>()) {
126     assert(elementsAttr.getType().hasStaticShape());
127     assert(elementsAttr.getNumElements() != 0 &&
128            "unexpected empty elements attribute");
129     assert(!elementsAttr.getType().getShape().empty() &&
130            "unexpected empty elements attribute shape");
131 
132     SmallVector<llvm::Constant *, 8> constants;
133     constants.reserve(elementsAttr.getNumElements());
134     llvm::Type *innermostType = getInnermostElementType(llvmType);
135     for (auto n : elementsAttr.getValues<Attribute>()) {
136       constants.push_back(getLLVMConstant(innermostType, n, loc));
137       if (!constants.back())
138         return nullptr;
139     }
140     ArrayRef<llvm::Constant *> constantsRef = constants;
141     llvm::Constant *result = buildSequentialConstant(
142         constantsRef, elementsAttr.getType().getShape(), llvmType, loc);
143     assert(constantsRef.empty() && "did not consume all elemental constants");
144     return result;
145   }
146 
147   if (auto stringAttr = attr.dyn_cast<StringAttr>()) {
148     return llvm::ConstantDataArray::get(
149         llvmModule->getContext(), ArrayRef<char>{stringAttr.getValue().data(),
150                                                  stringAttr.getValue().size()});
151   }
152   emitError(loc, "unsupported constant value");
153   return nullptr;
154 }
155 
156 /// Convert MLIR integer comparison predicate to LLVM IR comparison predicate.
157 static llvm::CmpInst::Predicate getLLVMCmpPredicate(ICmpPredicate p) {
158   switch (p) {
159   case LLVM::ICmpPredicate::eq:
160     return llvm::CmpInst::Predicate::ICMP_EQ;
161   case LLVM::ICmpPredicate::ne:
162     return llvm::CmpInst::Predicate::ICMP_NE;
163   case LLVM::ICmpPredicate::slt:
164     return llvm::CmpInst::Predicate::ICMP_SLT;
165   case LLVM::ICmpPredicate::sle:
166     return llvm::CmpInst::Predicate::ICMP_SLE;
167   case LLVM::ICmpPredicate::sgt:
168     return llvm::CmpInst::Predicate::ICMP_SGT;
169   case LLVM::ICmpPredicate::sge:
170     return llvm::CmpInst::Predicate::ICMP_SGE;
171   case LLVM::ICmpPredicate::ult:
172     return llvm::CmpInst::Predicate::ICMP_ULT;
173   case LLVM::ICmpPredicate::ule:
174     return llvm::CmpInst::Predicate::ICMP_ULE;
175   case LLVM::ICmpPredicate::ugt:
176     return llvm::CmpInst::Predicate::ICMP_UGT;
177   case LLVM::ICmpPredicate::uge:
178     return llvm::CmpInst::Predicate::ICMP_UGE;
179   }
180   llvm_unreachable("incorrect comparison predicate");
181 }
182 
183 static llvm::CmpInst::Predicate getLLVMCmpPredicate(FCmpPredicate p) {
184   switch (p) {
185   case LLVM::FCmpPredicate::_false:
186     return llvm::CmpInst::Predicate::FCMP_FALSE;
187   case LLVM::FCmpPredicate::oeq:
188     return llvm::CmpInst::Predicate::FCMP_OEQ;
189   case LLVM::FCmpPredicate::ogt:
190     return llvm::CmpInst::Predicate::FCMP_OGT;
191   case LLVM::FCmpPredicate::oge:
192     return llvm::CmpInst::Predicate::FCMP_OGE;
193   case LLVM::FCmpPredicate::olt:
194     return llvm::CmpInst::Predicate::FCMP_OLT;
195   case LLVM::FCmpPredicate::ole:
196     return llvm::CmpInst::Predicate::FCMP_OLE;
197   case LLVM::FCmpPredicate::one:
198     return llvm::CmpInst::Predicate::FCMP_ONE;
199   case LLVM::FCmpPredicate::ord:
200     return llvm::CmpInst::Predicate::FCMP_ORD;
201   case LLVM::FCmpPredicate::ueq:
202     return llvm::CmpInst::Predicate::FCMP_UEQ;
203   case LLVM::FCmpPredicate::ugt:
204     return llvm::CmpInst::Predicate::FCMP_UGT;
205   case LLVM::FCmpPredicate::uge:
206     return llvm::CmpInst::Predicate::FCMP_UGE;
207   case LLVM::FCmpPredicate::ult:
208     return llvm::CmpInst::Predicate::FCMP_ULT;
209   case LLVM::FCmpPredicate::ule:
210     return llvm::CmpInst::Predicate::FCMP_ULE;
211   case LLVM::FCmpPredicate::une:
212     return llvm::CmpInst::Predicate::FCMP_UNE;
213   case LLVM::FCmpPredicate::uno:
214     return llvm::CmpInst::Predicate::FCMP_UNO;
215   case LLVM::FCmpPredicate::_true:
216     return llvm::CmpInst::Predicate::FCMP_TRUE;
217   }
218   llvm_unreachable("incorrect comparison predicate");
219 }
220 
221 static llvm::AtomicRMWInst::BinOp getLLVMAtomicBinOp(AtomicBinOp op) {
222   switch (op) {
223   case LLVM::AtomicBinOp::xchg:
224     return llvm::AtomicRMWInst::BinOp::Xchg;
225   case LLVM::AtomicBinOp::add:
226     return llvm::AtomicRMWInst::BinOp::Add;
227   case LLVM::AtomicBinOp::sub:
228     return llvm::AtomicRMWInst::BinOp::Sub;
229   case LLVM::AtomicBinOp::_and:
230     return llvm::AtomicRMWInst::BinOp::And;
231   case LLVM::AtomicBinOp::nand:
232     return llvm::AtomicRMWInst::BinOp::Nand;
233   case LLVM::AtomicBinOp::_or:
234     return llvm::AtomicRMWInst::BinOp::Or;
235   case LLVM::AtomicBinOp::_xor:
236     return llvm::AtomicRMWInst::BinOp::Xor;
237   case LLVM::AtomicBinOp::max:
238     return llvm::AtomicRMWInst::BinOp::Max;
239   case LLVM::AtomicBinOp::min:
240     return llvm::AtomicRMWInst::BinOp::Min;
241   case LLVM::AtomicBinOp::umax:
242     return llvm::AtomicRMWInst::BinOp::UMax;
243   case LLVM::AtomicBinOp::umin:
244     return llvm::AtomicRMWInst::BinOp::UMin;
245   case LLVM::AtomicBinOp::fadd:
246     return llvm::AtomicRMWInst::BinOp::FAdd;
247   case LLVM::AtomicBinOp::fsub:
248     return llvm::AtomicRMWInst::BinOp::FSub;
249   }
250   llvm_unreachable("incorrect atomic binary operator");
251 }
252 
253 static llvm::AtomicOrdering getLLVMAtomicOrdering(AtomicOrdering ordering) {
254   switch (ordering) {
255   case LLVM::AtomicOrdering::not_atomic:
256     return llvm::AtomicOrdering::NotAtomic;
257   case LLVM::AtomicOrdering::unordered:
258     return llvm::AtomicOrdering::Unordered;
259   case LLVM::AtomicOrdering::monotonic:
260     return llvm::AtomicOrdering::Monotonic;
261   case LLVM::AtomicOrdering::acquire:
262     return llvm::AtomicOrdering::Acquire;
263   case LLVM::AtomicOrdering::release:
264     return llvm::AtomicOrdering::Release;
265   case LLVM::AtomicOrdering::acq_rel:
266     return llvm::AtomicOrdering::AcquireRelease;
267   case LLVM::AtomicOrdering::seq_cst:
268     return llvm::AtomicOrdering::SequentiallyConsistent;
269   }
270   llvm_unreachable("incorrect atomic ordering");
271 }
272 
273 ModuleTranslation::ModuleTranslation(Operation *module,
274                                      std::unique_ptr<llvm::Module> llvmModule)
275     : mlirModule(module), llvmModule(std::move(llvmModule)),
276       debugTranslation(
277           std::make_unique<DebugTranslation>(module, *this->llvmModule)),
278       ompDialect(
279           module->getContext()->getRegisteredDialect<omp::OpenMPDialect>()) {
280   assert(satisfiesLLVMModule(mlirModule) &&
281          "mlirModule should honor LLVM's module semantics.");
282 }
283 ModuleTranslation::~ModuleTranslation() {}
284 
285 /// Given a single MLIR operation, create the corresponding LLVM IR operation
286 /// using the `builder`.  LLVM IR Builder does not have a generic interface so
287 /// this has to be a long chain of `if`s calling different functions with a
288 /// different number of arguments.
289 LogicalResult ModuleTranslation::convertOperation(Operation &opInst,
290                                                   llvm::IRBuilder<> &builder) {
291   auto extractPosition = [](ArrayAttr attr) {
292     SmallVector<unsigned, 4> position;
293     position.reserve(attr.size());
294     for (Attribute v : attr)
295       position.push_back(v.cast<IntegerAttr>().getValue().getZExtValue());
296     return position;
297   };
298 
299 #include "mlir/Dialect/LLVMIR/LLVMConversions.inc"
300 
301   // Emit function calls.  If the "callee" attribute is present, this is a
302   // direct function call and we also need to look up the remapped function
303   // itself.  Otherwise, this is an indirect call and the callee is the first
304   // operand, look it up as a normal value.  Return the llvm::Value representing
305   // the function result, which may be of llvm::VoidTy type.
306   auto convertCall = [this, &builder](Operation &op) -> llvm::Value * {
307     auto operands = lookupValues(op.getOperands());
308     ArrayRef<llvm::Value *> operandsRef(operands);
309     if (auto attr = op.getAttrOfType<FlatSymbolRefAttr>("callee")) {
310       return builder.CreateCall(functionMapping.lookup(attr.getValue()),
311                                 operandsRef);
312     } else {
313       return builder.CreateCall(operandsRef.front(), operandsRef.drop_front());
314     }
315   };
316 
317   // Emit calls.  If the called function has a result, remap the corresponding
318   // value.  Note that LLVM IR dialect CallOp has either 0 or 1 result.
319   if (isa<LLVM::CallOp>(opInst)) {
320     llvm::Value *result = convertCall(opInst);
321     if (opInst.getNumResults() != 0) {
322       valueMapping[opInst.getResult(0)] = result;
323       return success();
324     }
325     // Check that LLVM call returns void for 0-result functions.
326     return success(result->getType()->isVoidTy());
327   }
328 
329   if (auto invOp = dyn_cast<LLVM::InvokeOp>(opInst)) {
330     auto operands = lookupValues(opInst.getOperands());
331     ArrayRef<llvm::Value *> operandsRef(operands);
332     if (auto attr = opInst.getAttrOfType<FlatSymbolRefAttr>("callee"))
333       builder.CreateInvoke(functionMapping.lookup(attr.getValue()),
334                            blockMapping[invOp.getSuccessor(0)],
335                            blockMapping[invOp.getSuccessor(1)], operandsRef);
336     else
337       builder.CreateInvoke(
338           operandsRef.front(), blockMapping[invOp.getSuccessor(0)],
339           blockMapping[invOp.getSuccessor(1)], operandsRef.drop_front());
340     return success();
341   }
342 
343   if (auto lpOp = dyn_cast<LLVM::LandingpadOp>(opInst)) {
344     llvm::Type *ty = lpOp.getType().dyn_cast<LLVMType>().getUnderlyingType();
345     llvm::LandingPadInst *lpi =
346         builder.CreateLandingPad(ty, lpOp.getNumOperands());
347 
348     // Add clauses
349     for (auto operand : lookupValues(lpOp.getOperands())) {
350       // All operands should be constant - checked by verifier
351       if (auto constOperand = dyn_cast<llvm::Constant>(operand))
352         lpi->addClause(constOperand);
353     }
354     return success();
355   }
356 
357   // Emit branches.  We need to look up the remapped blocks and ignore the block
358   // arguments that were transformed into PHI nodes.
359   if (auto brOp = dyn_cast<LLVM::BrOp>(opInst)) {
360     builder.CreateBr(blockMapping[brOp.getSuccessor()]);
361     return success();
362   }
363   if (auto condbrOp = dyn_cast<LLVM::CondBrOp>(opInst)) {
364     builder.CreateCondBr(valueMapping.lookup(condbrOp.getOperand(0)),
365                          blockMapping[condbrOp.getSuccessor(0)],
366                          blockMapping[condbrOp.getSuccessor(1)]);
367     return success();
368   }
369 
370   // Emit addressof.  We need to look up the global value referenced by the
371   // operation and store it in the MLIR-to-LLVM value mapping.  This does not
372   // emit any LLVM instruction.
373   if (auto addressOfOp = dyn_cast<LLVM::AddressOfOp>(opInst)) {
374     LLVM::GlobalOp global = addressOfOp.getGlobal();
375     // The verifier should not have allowed this.
376     assert(global && "referencing an undefined global");
377 
378     valueMapping[addressOfOp.getResult()] = globalsMapping.lookup(global);
379     return success();
380   }
381 
382   if (opInst.getDialect() == ompDialect) {
383     if (!ompBuilder) {
384       ompBuilder = std::make_unique<llvm::OpenMPIRBuilder>(*llvmModule);
385       ompBuilder->initialize();
386     }
387 
388     if (isa<omp::BarrierOp>(opInst)) {
389       ompBuilder->CreateBarrier(builder.saveIP(), llvm::omp::OMPD_barrier);
390       return success();
391     }
392     return opInst.emitError("unsupported OpenMP operation: ")
393            << opInst.getName();
394   }
395 
396   return opInst.emitError("unsupported or non-LLVM operation: ")
397          << opInst.getName();
398 }
399 
400 /// Convert block to LLVM IR.  Unless `ignoreArguments` is set, emit PHI nodes
401 /// to define values corresponding to the MLIR block arguments.  These nodes
402 /// are not connected to the source basic blocks, which may not exist yet.
403 LogicalResult ModuleTranslation::convertBlock(Block &bb, bool ignoreArguments) {
404   llvm::IRBuilder<> builder(blockMapping[&bb]);
405   auto *subprogram = builder.GetInsertBlock()->getParent()->getSubprogram();
406 
407   // Before traversing operations, make block arguments available through
408   // value remapping and PHI nodes, but do not add incoming edges for the PHI
409   // nodes just yet: those values may be defined by this or following blocks.
410   // This step is omitted if "ignoreArguments" is set.  The arguments of the
411   // first block have been already made available through the remapping of
412   // LLVM function arguments.
413   if (!ignoreArguments) {
414     auto predecessors = bb.getPredecessors();
415     unsigned numPredecessors =
416         std::distance(predecessors.begin(), predecessors.end());
417     for (auto arg : bb.getArguments()) {
418       auto wrappedType = arg.getType().dyn_cast<LLVM::LLVMType>();
419       if (!wrappedType)
420         return emitError(bb.front().getLoc(),
421                          "block argument does not have an LLVM type");
422       llvm::Type *type = wrappedType.getUnderlyingType();
423       llvm::PHINode *phi = builder.CreatePHI(type, numPredecessors);
424       valueMapping[arg] = phi;
425     }
426   }
427 
428   // Traverse operations.
429   for (auto &op : bb) {
430     // Set the current debug location within the builder.
431     builder.SetCurrentDebugLocation(
432         debugTranslation->translateLoc(op.getLoc(), subprogram));
433 
434     if (failed(convertOperation(op, builder)))
435       return failure();
436   }
437 
438   return success();
439 }
440 
441 /// Create named global variables that correspond to llvm.mlir.global
442 /// definitions.
443 LogicalResult ModuleTranslation::convertGlobals() {
444   for (auto op : getModuleBody(mlirModule).getOps<LLVM::GlobalOp>()) {
445     llvm::Type *type = op.getType().getUnderlyingType();
446     llvm::Constant *cst = llvm::UndefValue::get(type);
447     if (op.getValueOrNull()) {
448       // String attributes are treated separately because they cannot appear as
449       // in-function constants and are thus not supported by getLLVMConstant.
450       if (auto strAttr = op.getValueOrNull().dyn_cast_or_null<StringAttr>()) {
451         cst = llvm::ConstantDataArray::getString(
452             llvmModule->getContext(), strAttr.getValue(), /*AddNull=*/false);
453         type = cst->getType();
454       } else if (!(cst = getLLVMConstant(type, op.getValueOrNull(),
455                                          op.getLoc()))) {
456         return failure();
457       }
458     } else if (Block *initializer = op.getInitializerBlock()) {
459       llvm::IRBuilder<> builder(llvmModule->getContext());
460       for (auto &op : initializer->without_terminator()) {
461         if (failed(convertOperation(op, builder)) ||
462             !isa<llvm::Constant>(valueMapping.lookup(op.getResult(0))))
463           return emitError(op.getLoc(), "unemittable constant value");
464       }
465       ReturnOp ret = cast<ReturnOp>(initializer->getTerminator());
466       cst = cast<llvm::Constant>(valueMapping.lookup(ret.getOperand(0)));
467     }
468 
469     auto linkage = convertLinkageToLLVM(op.linkage());
470     bool anyExternalLinkage =
471         (linkage == llvm::GlobalVariable::ExternalLinkage ||
472          linkage == llvm::GlobalVariable::ExternalWeakLinkage);
473     auto addrSpace = op.addr_space().getLimitedValue();
474     auto *var = new llvm::GlobalVariable(
475         *llvmModule, type, op.constant(), linkage,
476         anyExternalLinkage ? nullptr : cst, op.sym_name(),
477         /*InsertBefore=*/nullptr, llvm::GlobalValue::NotThreadLocal, addrSpace);
478 
479     globalsMapping.try_emplace(op, var);
480   }
481 
482   return success();
483 }
484 
485 /// Get the SSA value passed to the current block from the terminator operation
486 /// of its predecessor.
487 static Value getPHISourceValue(Block *current, Block *pred,
488                                unsigned numArguments, unsigned index) {
489   auto &terminator = *pred->getTerminator();
490   if (isa<LLVM::BrOp>(terminator)) {
491     return terminator.getOperand(index);
492   }
493 
494   // For conditional branches, we need to check if the current block is reached
495   // through the "true" or the "false" branch and take the relevant operands.
496   auto condBranchOp = dyn_cast<LLVM::CondBrOp>(terminator);
497   assert(condBranchOp &&
498          "only branch operations can be terminators of a block that "
499          "has successors");
500   assert((condBranchOp.getSuccessor(0) != condBranchOp.getSuccessor(1)) &&
501          "successors with arguments in LLVM conditional branches must be "
502          "different blocks");
503 
504   return condBranchOp.getSuccessor(0) == current
505              ? condBranchOp.trueDestOperands()[index]
506              : condBranchOp.falseDestOperands()[index];
507 }
508 
509 void ModuleTranslation::connectPHINodes(LLVMFuncOp func) {
510   // Skip the first block, it cannot be branched to and its arguments correspond
511   // to the arguments of the LLVM function.
512   for (auto it = std::next(func.begin()), eit = func.end(); it != eit; ++it) {
513     Block *bb = &*it;
514     llvm::BasicBlock *llvmBB = blockMapping.lookup(bb);
515     auto phis = llvmBB->phis();
516     auto numArguments = bb->getNumArguments();
517     assert(numArguments == std::distance(phis.begin(), phis.end()));
518     for (auto &numberedPhiNode : llvm::enumerate(phis)) {
519       auto &phiNode = numberedPhiNode.value();
520       unsigned index = numberedPhiNode.index();
521       for (auto *pred : bb->getPredecessors()) {
522         phiNode.addIncoming(valueMapping.lookup(getPHISourceValue(
523                                 bb, pred, numArguments, index)),
524                             blockMapping.lookup(pred));
525       }
526     }
527   }
528 }
529 
530 // TODO(mlir-team): implement an iterative version
531 static void topologicalSortImpl(llvm::SetVector<Block *> &blocks, Block *b) {
532   blocks.insert(b);
533   for (Block *bb : b->getSuccessors()) {
534     if (blocks.count(bb) == 0)
535       topologicalSortImpl(blocks, bb);
536   }
537 }
538 
539 /// Sort function blocks topologically.
540 static llvm::SetVector<Block *> topologicalSort(LLVMFuncOp f) {
541   // For each blocks that has not been visited yet (i.e. that has no
542   // predecessors), add it to the list and traverse its successors in DFS
543   // preorder.
544   llvm::SetVector<Block *> blocks;
545   for (Block &b : f.getBlocks()) {
546     if (blocks.count(&b) == 0)
547       topologicalSortImpl(blocks, &b);
548   }
549   assert(blocks.size() == f.getBlocks().size() && "some blocks are not sorted");
550 
551   return blocks;
552 }
553 
554 LogicalResult ModuleTranslation::convertOneFunction(LLVMFuncOp func) {
555   // Clear the block and value mappings, they are only relevant within one
556   // function.
557   blockMapping.clear();
558   valueMapping.clear();
559   llvm::Function *llvmFunc = functionMapping.lookup(func.getName());
560 
561   // Translate the debug information for this function.
562   debugTranslation->translate(func, *llvmFunc);
563 
564   // Add function arguments to the value remapping table.
565   // If there was noalias info then we decorate each argument accordingly.
566   unsigned int argIdx = 0;
567   for (auto kvp : llvm::zip(func.getArguments(), llvmFunc->args())) {
568     llvm::Argument &llvmArg = std::get<1>(kvp);
569     BlockArgument mlirArg = std::get<0>(kvp);
570 
571     if (auto attr = func.getArgAttrOfType<BoolAttr>(argIdx, "llvm.noalias")) {
572       // NB: Attribute already verified to be boolean, so check if we can indeed
573       // attach the attribute to this argument, based on its type.
574       auto argTy = mlirArg.getType().dyn_cast<LLVM::LLVMType>();
575       if (!argTy.getUnderlyingType()->isPointerTy())
576         return func.emitError(
577             "llvm.noalias attribute attached to LLVM non-pointer argument");
578       if (attr.getValue())
579         llvmArg.addAttr(llvm::Attribute::AttrKind::NoAlias);
580     }
581     valueMapping[mlirArg] = &llvmArg;
582     argIdx++;
583   }
584 
585   // First, create all blocks so we can jump to them.
586   llvm::LLVMContext &llvmContext = llvmFunc->getContext();
587   for (auto &bb : func) {
588     auto *llvmBB = llvm::BasicBlock::Create(llvmContext);
589     llvmBB->insertInto(llvmFunc);
590     blockMapping[&bb] = llvmBB;
591   }
592 
593   // Then, convert blocks one by one in topological order to ensure defs are
594   // converted before uses.
595   auto blocks = topologicalSort(func);
596   for (auto indexedBB : llvm::enumerate(blocks)) {
597     auto *bb = indexedBB.value();
598     if (failed(convertBlock(*bb, /*ignoreArguments=*/indexedBB.index() == 0)))
599       return failure();
600   }
601 
602   // Finally, after all blocks have been traversed and values mapped, connect
603   // the PHI nodes to the results of preceding blocks.
604   connectPHINodes(func);
605   return success();
606 }
607 
608 LogicalResult ModuleTranslation::checkSupportedModuleOps(Operation *m) {
609   for (Operation &o : getModuleBody(m).getOperations())
610     if (!isa<LLVM::LLVMFuncOp>(&o) && !isa<LLVM::GlobalOp>(&o) &&
611         !o.isKnownTerminator())
612       return o.emitOpError("unsupported module-level operation");
613   return success();
614 }
615 
616 LogicalResult ModuleTranslation::convertFunctions() {
617   // Declare all functions first because there may be function calls that form a
618   // call graph with cycles.
619   for (auto function : getModuleBody(mlirModule).getOps<LLVMFuncOp>()) {
620     llvm::FunctionCallee llvmFuncCst = llvmModule->getOrInsertFunction(
621         function.getName(),
622         cast<llvm::FunctionType>(function.getType().getUnderlyingType()));
623     assert(isa<llvm::Function>(llvmFuncCst.getCallee()));
624     functionMapping[function.getName()] =
625         cast<llvm::Function>(llvmFuncCst.getCallee());
626   }
627 
628   // Convert functions.
629   for (auto function : getModuleBody(mlirModule).getOps<LLVMFuncOp>()) {
630     // Ignore external functions.
631     if (function.isExternal())
632       continue;
633 
634     if (failed(convertOneFunction(function)))
635       return failure();
636   }
637 
638   return success();
639 }
640 
641 /// A helper to look up remapped operands in the value remapping table.`
642 SmallVector<llvm::Value *, 8>
643 ModuleTranslation::lookupValues(ValueRange values) {
644   SmallVector<llvm::Value *, 8> remapped;
645   remapped.reserve(values.size());
646   for (Value v : values)
647     remapped.push_back(valueMapping.lookup(v));
648   return remapped;
649 }
650 
651 std::unique_ptr<llvm::Module>
652 ModuleTranslation::prepareLLVMModule(Operation *m) {
653   auto *dialect = m->getContext()->getRegisteredDialect<LLVM::LLVMDialect>();
654   assert(dialect && "LLVM dialect must be registered");
655 
656   auto llvmModule = llvm::CloneModule(dialect->getLLVMModule());
657   if (!llvmModule)
658     return nullptr;
659 
660   llvm::LLVMContext &llvmContext = llvmModule->getContext();
661   llvm::IRBuilder<> builder(llvmContext);
662 
663   // Inject declarations for `malloc` and `free` functions that can be used in
664   // memref allocation/deallocation coming from standard ops lowering.
665   llvmModule->getOrInsertFunction("malloc", builder.getInt8PtrTy(),
666                                   builder.getInt64Ty());
667   llvmModule->getOrInsertFunction("free", builder.getVoidTy(),
668                                   builder.getInt8PtrTy());
669 
670   return llvmModule;
671 }
672