LLVM 19.0.0git
AMDGPURewriteOutArguments.cpp
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1//===- AMDGPURewriteOutArgumentsPass.cpp - Create struct returns ----------===//
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/// \file This pass attempts to replace out argument usage with a return of a
10/// struct.
11///
12/// We can support returning a lot of values directly in registers, but
13/// idiomatic C code frequently uses a pointer argument to return a second value
14/// rather than returning a struct by value. GPU stack access is also quite
15/// painful, so we want to avoid that if possible. Passing a stack object
16/// pointer to a function also requires an additional address expansion code
17/// sequence to convert the pointer to be relative to the kernel's scratch wave
18/// offset register since the callee doesn't know what stack frame the incoming
19/// pointer is relative to.
20///
21/// The goal is to try rewriting code that looks like this:
22///
23/// int foo(int a, int b, int* out) {
24/// *out = bar();
25/// return a + b;
26/// }
27///
28/// into something like this:
29///
30/// std::pair<int, int> foo(int a, int b) {
31/// return std::pair(a + b, bar());
32/// }
33///
34/// Typically the incoming pointer is a simple alloca for a temporary variable
35/// to use the API, which if replaced with a struct return will be easily SROA'd
36/// out when the stub function we create is inlined
37///
38/// This pass introduces the struct return, but leaves the unused pointer
39/// arguments and introduces a new stub function calling the struct returning
40/// body. DeadArgumentElimination should be run after this to clean these up.
41//
42//===----------------------------------------------------------------------===//
43
44#include "AMDGPU.h"
46#include "llvm/ADT/Statistic.h"
49#include "llvm/IR/IRBuilder.h"
52#include "llvm/Pass.h"
54#include "llvm/Support/Debug.h"
56
57#define DEBUG_TYPE "amdgpu-rewrite-out-arguments"
58
59using namespace llvm;
60
62 "amdgpu-any-address-space-out-arguments",
63 cl::desc("Replace pointer out arguments with "
64 "struct returns for non-private address space"),
66 cl::init(false));
67
69 "amdgpu-max-return-arg-num-regs",
70 cl::desc("Approximately limit number of return registers for replacing out arguments"),
72 cl::init(16));
73
74STATISTIC(NumOutArgumentsReplaced,
75 "Number out arguments moved to struct return values");
76STATISTIC(NumOutArgumentFunctionsReplaced,
77 "Number of functions with out arguments moved to struct return values");
78
79namespace {
80
81class AMDGPURewriteOutArguments : public FunctionPass {
82private:
83 const DataLayout *DL = nullptr;
84 MemoryDependenceResults *MDA = nullptr;
85
86 Type *getStoredType(Value &Arg) const;
87 Type *getOutArgumentType(Argument &Arg) const;
88
89public:
90 static char ID;
91
92 AMDGPURewriteOutArguments() : FunctionPass(ID) {}
93
94 void getAnalysisUsage(AnalysisUsage &AU) const override {
97 }
98
99 bool doInitialization(Module &M) override;
100 bool runOnFunction(Function &F) override;
101};
102
103} // end anonymous namespace
104
105INITIALIZE_PASS_BEGIN(AMDGPURewriteOutArguments, DEBUG_TYPE,
106 "AMDGPU Rewrite Out Arguments", false, false)
108INITIALIZE_PASS_END(AMDGPURewriteOutArguments, DEBUG_TYPE,
109 "AMDGPU Rewrite Out Arguments", false, false)
110
111char AMDGPURewriteOutArguments::ID = 0;
112
113Type *AMDGPURewriteOutArguments::getStoredType(Value &Arg) const {
114 const int MaxUses = 10;
115 int UseCount = 0;
116
117 SmallVector<Use *> Worklist;
118 for (Use &U : Arg.uses())
119 Worklist.push_back(&U);
120
121 Type *StoredType = nullptr;
122 while (!Worklist.empty()) {
123 Use *U = Worklist.pop_back_val();
124
125 if (auto *BCI = dyn_cast<BitCastInst>(U->getUser())) {
126 for (Use &U : BCI->uses())
127 Worklist.push_back(&U);
128 continue;
129 }
130
131 if (auto *SI = dyn_cast<StoreInst>(U->getUser())) {
132 if (UseCount++ > MaxUses)
133 return nullptr;
134
135 if (!SI->isSimple() ||
136 U->getOperandNo() != StoreInst::getPointerOperandIndex())
137 return nullptr;
138
139 if (StoredType && StoredType != SI->getValueOperand()->getType())
140 return nullptr; // More than one type.
141 StoredType = SI->getValueOperand()->getType();
142 continue;
143 }
144
145 // Unsupported user.
146 return nullptr;
147 }
148
149 return StoredType;
150}
151
152Type *AMDGPURewriteOutArguments::getOutArgumentType(Argument &Arg) const {
153 const unsigned MaxOutArgSizeBytes = 4 * MaxNumRetRegs;
154 PointerType *ArgTy = dyn_cast<PointerType>(Arg.getType());
155
156 // TODO: It might be useful for any out arguments, not just privates.
157 if (!ArgTy || (ArgTy->getAddressSpace() != DL->getAllocaAddrSpace() &&
158 !AnyAddressSpace) ||
159 Arg.hasByValAttr() || Arg.hasStructRetAttr()) {
160 return nullptr;
161 }
162
163 Type *StoredType = getStoredType(Arg);
164 if (!StoredType || DL->getTypeStoreSize(StoredType) > MaxOutArgSizeBytes)
165 return nullptr;
166
167 return StoredType;
168}
169
170bool AMDGPURewriteOutArguments::doInitialization(Module &M) {
171 DL = &M.getDataLayout();
172 return false;
173}
174
175bool AMDGPURewriteOutArguments::runOnFunction(Function &F) {
176 if (skipFunction(F))
177 return false;
178
179 // TODO: Could probably handle variadic functions.
180 if (F.isVarArg() || F.hasStructRetAttr() ||
181 AMDGPU::isEntryFunctionCC(F.getCallingConv()))
182 return false;
183
184 MDA = &getAnalysis<MemoryDependenceWrapperPass>().getMemDep();
185
186 unsigned ReturnNumRegs = 0;
187 SmallDenseMap<int, Type *, 4> OutArgIndexes;
188 SmallVector<Type *, 4> ReturnTypes;
189 Type *RetTy = F.getReturnType();
190 if (!RetTy->isVoidTy()) {
191 ReturnNumRegs = DL->getTypeStoreSize(RetTy) / 4;
192
193 if (ReturnNumRegs >= MaxNumRetRegs)
194 return false;
195
196 ReturnTypes.push_back(RetTy);
197 }
198
200 for (Argument &Arg : F.args()) {
201 if (Type *Ty = getOutArgumentType(Arg)) {
202 LLVM_DEBUG(dbgs() << "Found possible out argument " << Arg
203 << " in function " << F.getName() << '\n');
204 OutArgs.push_back({&Arg, Ty});
205 }
206 }
207
208 if (OutArgs.empty())
209 return false;
210
211 using ReplacementVec = SmallVector<std::pair<Argument *, Value *>, 4>;
212
214
216 for (BasicBlock &BB : F) {
217 if (ReturnInst *RI = dyn_cast<ReturnInst>(&BB.back()))
218 Returns.push_back(RI);
219 }
220
221 if (Returns.empty())
222 return false;
223
224 bool Changing;
225
226 do {
227 Changing = false;
228
229 // Keep retrying if we are able to successfully eliminate an argument. This
230 // helps with cases with multiple arguments which may alias, such as in a
231 // sincos implementation. If we have 2 stores to arguments, on the first
232 // attempt the MDA query will succeed for the second store but not the
233 // first. On the second iteration we've removed that out clobbering argument
234 // (by effectively moving it into another function) and will find the second
235 // argument is OK to move.
236 for (const auto &Pair : OutArgs) {
237 bool ThisReplaceable = true;
239
240 Argument *OutArg = Pair.first;
241 Type *ArgTy = Pair.second;
242
243 // Skip this argument if converting it will push us over the register
244 // count to return limit.
245
246 // TODO: This is an approximation. When legalized this could be more. We
247 // can ask TLI for exactly how many.
248 unsigned ArgNumRegs = DL->getTypeStoreSize(ArgTy) / 4;
249 if (ArgNumRegs + ReturnNumRegs > MaxNumRetRegs)
250 continue;
251
252 // An argument is convertible only if all exit blocks are able to replace
253 // it.
254 for (ReturnInst *RI : Returns) {
255 BasicBlock *BB = RI->getParent();
256
257 MemDepResult Q = MDA->getPointerDependencyFrom(
258 MemoryLocation::getBeforeOrAfter(OutArg), true, BB->end(), BB, RI);
259 StoreInst *SI = nullptr;
260 if (Q.isDef())
261 SI = dyn_cast<StoreInst>(Q.getInst());
262
263 if (SI) {
264 LLVM_DEBUG(dbgs() << "Found out argument store: " << *SI << '\n');
265 ReplaceableStores.emplace_back(RI, SI);
266 } else {
267 ThisReplaceable = false;
268 break;
269 }
270 }
271
272 if (!ThisReplaceable)
273 continue; // Try the next argument candidate.
274
275 for (std::pair<ReturnInst *, StoreInst *> Store : ReplaceableStores) {
276 Value *ReplVal = Store.second->getValueOperand();
277
278 auto &ValVec = Replacements[Store.first];
279 if (llvm::any_of(ValVec,
280 [OutArg](const std::pair<Argument *, Value *> &Entry) {
281 return Entry.first == OutArg;
282 })) {
284 << "Saw multiple out arg stores" << *OutArg << '\n');
285 // It is possible to see stores to the same argument multiple times,
286 // but we expect these would have been optimized out already.
287 ThisReplaceable = false;
288 break;
289 }
290
291 ValVec.emplace_back(OutArg, ReplVal);
292 Store.second->eraseFromParent();
293 }
294
295 if (ThisReplaceable) {
296 ReturnTypes.push_back(ArgTy);
297 OutArgIndexes.insert({OutArg->getArgNo(), ArgTy});
298 ++NumOutArgumentsReplaced;
299 Changing = true;
300 }
301 }
302 } while (Changing);
303
304 if (Replacements.empty())
305 return false;
306
307 LLVMContext &Ctx = F.getParent()->getContext();
308 StructType *NewRetTy = StructType::create(Ctx, ReturnTypes, F.getName());
309
310 FunctionType *NewFuncTy = FunctionType::get(NewRetTy,
311 F.getFunctionType()->params(),
312 F.isVarArg());
313
314 LLVM_DEBUG(dbgs() << "Computed new return type: " << *NewRetTy << '\n');
315
316 Function *NewFunc = Function::Create(NewFuncTy, Function::PrivateLinkage,
317 F.getName() + ".body");
318 F.getParent()->getFunctionList().insert(F.getIterator(), NewFunc);
319 NewFunc->copyAttributesFrom(&F);
320 NewFunc->setComdat(F.getComdat());
321
322 // We want to preserve the function and param attributes, but need to strip
323 // off any return attributes, e.g. zeroext doesn't make sense with a struct.
324 NewFunc->stealArgumentListFrom(F);
325
326 AttributeMask RetAttrs;
327 RetAttrs.addAttribute(Attribute::SExt);
328 RetAttrs.addAttribute(Attribute::ZExt);
329 RetAttrs.addAttribute(Attribute::NoAlias);
330 NewFunc->removeRetAttrs(RetAttrs);
331 // TODO: How to preserve metadata?
332
333 NewFunc->setIsNewDbgInfoFormat(F.IsNewDbgInfoFormat);
334
335 // Move the body of the function into the new rewritten function, and replace
336 // this function with a stub.
337 NewFunc->splice(NewFunc->begin(), &F);
338
339 for (std::pair<ReturnInst *, ReplacementVec> &Replacement : Replacements) {
340 ReturnInst *RI = Replacement.first;
341 IRBuilder<> B(RI);
342 B.SetCurrentDebugLocation(RI->getDebugLoc());
343
344 int RetIdx = 0;
345 Value *NewRetVal = PoisonValue::get(NewRetTy);
346
347 Value *RetVal = RI->getReturnValue();
348 if (RetVal)
349 NewRetVal = B.CreateInsertValue(NewRetVal, RetVal, RetIdx++);
350
351 for (std::pair<Argument *, Value *> ReturnPoint : Replacement.second)
352 NewRetVal = B.CreateInsertValue(NewRetVal, ReturnPoint.second, RetIdx++);
353
354 if (RetVal)
355 RI->setOperand(0, NewRetVal);
356 else {
357 B.CreateRet(NewRetVal);
358 RI->eraseFromParent();
359 }
360 }
361
362 SmallVector<Value *, 16> StubCallArgs;
363 for (Argument &Arg : F.args()) {
364 if (OutArgIndexes.count(Arg.getArgNo())) {
365 // It's easier to preserve the type of the argument list. We rely on
366 // DeadArgumentElimination to take care of these.
367 StubCallArgs.push_back(PoisonValue::get(Arg.getType()));
368 } else {
369 StubCallArgs.push_back(&Arg);
370 }
371 }
372
373 BasicBlock *StubBB = BasicBlock::Create(Ctx, "", &F);
374 IRBuilder<> B(StubBB);
375 CallInst *StubCall = B.CreateCall(NewFunc, StubCallArgs);
376
377 int RetIdx = RetTy->isVoidTy() ? 0 : 1;
378 for (Argument &Arg : F.args()) {
379 if (!OutArgIndexes.count(Arg.getArgNo()))
380 continue;
381
382 Type *EltTy = OutArgIndexes[Arg.getArgNo()];
383 const auto Align =
384 DL->getValueOrABITypeAlignment(Arg.getParamAlign(), EltTy);
385
386 Value *Val = B.CreateExtractValue(StubCall, RetIdx++);
387 B.CreateAlignedStore(Val, &Arg, Align);
388 }
389
390 if (!RetTy->isVoidTy()) {
391 B.CreateRet(B.CreateExtractValue(StubCall, 0));
392 } else {
393 B.CreateRetVoid();
394 }
395
396 // The function is now a stub we want to inline.
397 F.addFnAttr(Attribute::AlwaysInline);
398
399 ++NumOutArgumentFunctionsReplaced;
400 return true;
401}
402
404 return new AMDGPURewriteOutArguments();
405}
MachineBasicBlock MachineBasicBlock::iterator DebugLoc DL
AMDGPU Rewrite Out Arguments
static cl::opt< unsigned > MaxNumRetRegs("amdgpu-max-return-arg-num-regs", cl::desc("Approximately limit number of return registers for replacing out arguments"), cl::Hidden, cl::init(16))
#define DEBUG_TYPE
static cl::opt< bool > AnyAddressSpace("amdgpu-any-address-space-out-arguments", cl::desc("Replace pointer out arguments with " "struct returns for non-private address space"), cl::Hidden, cl::init(false))
static GCRegistry::Add< OcamlGC > B("ocaml", "ocaml 3.10-compatible GC")
return RetTy
#define LLVM_DEBUG(X)
Definition: Debug.h:101
#define F(x, y, z)
Definition: MD5.cpp:55
#define INITIALIZE_PASS_DEPENDENCY(depName)
Definition: PassSupport.h:55
#define INITIALIZE_PASS_END(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:59
#define INITIALIZE_PASS_BEGIN(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:52
This file defines the 'Statistic' class, which is designed to be an easy way to expose various metric...
#define STATISTIC(VARNAME, DESC)
Definition: Statistic.h:167
Represent the analysis usage information of a pass.
AnalysisUsage & addRequired()
This class represents an incoming formal argument to a Function.
Definition: Argument.h:31
bool hasByValAttr() const
Return true if this argument has the byval attribute.
Definition: Function.cpp:139
unsigned getArgNo() const
Return the index of this formal argument in its containing function.
Definition: Argument.h:49
MaybeAlign getParamAlign() const
If this is a byval or inalloca argument, return its alignment.
Definition: Function.cpp:221
bool hasStructRetAttr() const
Return true if this argument has the sret attribute.
Definition: Function.cpp:293
AttributeMask & addAttribute(Attribute::AttrKind Val)
Add an attribute to the mask.
Definition: AttributeMask.h:44
LLVM Basic Block Representation.
Definition: BasicBlock.h:60
iterator end()
Definition: BasicBlock.h:443
static BasicBlock * Create(LLVMContext &Context, const Twine &Name="", Function *Parent=nullptr, BasicBlock *InsertBefore=nullptr)
Creates a new BasicBlock.
Definition: BasicBlock.h:199
const Function * getParent() const
Return the enclosing method, or null if none.
Definition: BasicBlock.h:206
This class represents a function call, abstracting a target machine's calling convention.
A parsed version of the target data layout string in and methods for querying it.
Definition: DataLayout.h:110
bool empty() const
Definition: DenseMap.h:98
size_type count(const_arg_type_t< KeyT > Val) const
Return 1 if the specified key is in the map, 0 otherwise.
Definition: DenseMap.h:151
std::pair< iterator, bool > insert(const std::pair< KeyT, ValueT > &KV)
Definition: DenseMap.h:220
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:311
virtual bool runOnFunction(Function &F)=0
runOnFunction - Virtual method overriden by subclasses to do the per-function processing of the pass.
static Function * Create(FunctionType *Ty, LinkageTypes Linkage, unsigned AddrSpace, const Twine &N="", Module *M=nullptr)
Definition: Function.h:163
void splice(Function::iterator ToIt, Function *FromF)
Transfer all blocks from FromF to this function at ToIt.
Definition: Function.h:735
iterator begin()
Definition: Function.h:799
void stealArgumentListFrom(Function &Src)
Steal arguments from another function.
Definition: Function.cpp:522
void removeRetAttrs(const AttributeMask &Attrs)
removes the attributes from the return value list of attributes.
Definition: Function.cpp:661
void setIsNewDbgInfoFormat(bool NewVal)
Definition: Function.cpp:100
void copyAttributesFrom(const Function *Src)
copyAttributesFrom - copy all additional attributes (those not needed to create a Function) from the ...
Definition: Function.cpp:799
void setComdat(Comdat *C)
Definition: Globals.cpp:197
This provides a uniform API for creating instructions and inserting them into a basic block: either a...
Definition: IRBuilder.h:2666
const DebugLoc & getDebugLoc() const
Return the debug location for this node as a DebugLoc.
Definition: Instruction.h:454
InstListType::iterator eraseFromParent()
This method unlinks 'this' from the containing basic block and deletes it.
This is an important class for using LLVM in a threaded context.
Definition: LLVMContext.h:67
A memory dependence query can return one of three different answers.
bool isDef() const
Tests if this MemDepResult represents a query that is an instruction definition dependency.
Instruction * getInst() const
If this is a normal dependency, returns the instruction that is depended on.
Provides a lazy, caching interface for making common memory aliasing information queries,...
A wrapper analysis pass for the legacy pass manager that exposes a MemoryDepnedenceResults instance.
static MemoryLocation getBeforeOrAfter(const Value *Ptr, const AAMDNodes &AATags=AAMDNodes())
Return a location that may access any location before or after Ptr, while remaining within the underl...
A Module instance is used to store all the information related to an LLVM module.
Definition: Module.h:65
virtual void getAnalysisUsage(AnalysisUsage &) const
getAnalysisUsage - This function should be overriden by passes that need analysis information to do t...
Definition: Pass.cpp:98
virtual bool doInitialization(Module &)
doInitialization - Virtual method overridden by subclasses to do any necessary initialization before ...
Definition: Pass.h:119
static PoisonValue * get(Type *T)
Static factory methods - Return an 'poison' object of the specified type.
Definition: Constants.cpp:1827
Return a value (possibly void), from a function.
Value * getReturnValue() const
Convenience accessor. Returns null if there is no return value.
bool empty() const
Definition: SmallVector.h:94
reference emplace_back(ArgTypes &&... Args)
Definition: SmallVector.h:950
void push_back(const T &Elt)
Definition: SmallVector.h:426
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1209
An instruction for storing to memory.
Definition: Instructions.h:317
static unsigned getPointerOperandIndex()
Definition: Instructions.h:419
Class to represent struct types.
Definition: DerivedTypes.h:216
static StructType * create(LLVMContext &Context, StringRef Name)
This creates an identified struct.
Definition: Type.cpp:513
The instances of the Type class are immutable: once they are created, they are never changed.
Definition: Type.h:45
A Use represents the edge between a Value definition and its users.
Definition: Use.h:43
void setOperand(unsigned i, Value *Val)
Definition: User.h:174
LLVM Value Representation.
Definition: Value.h:74
Type * getType() const
All values are typed, get the type of this value.
Definition: Value.h:255
bool isEntryFunctionCC(CallingConv::ID CC)
unsigned ID
LLVM IR allows to use arbitrary numbers as calling convention identifiers.
Definition: CallingConv.h:24
initializer< Ty > init(const Ty &Val)
Definition: CommandLine.h:450
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
FunctionPass * createAMDGPURewriteOutArgumentsPass()
bool any_of(R &&range, UnaryPredicate P)
Provide wrappers to std::any_of which take ranges instead of having to pass begin/end explicitly.
Definition: STLExtras.h:1729
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:163
This struct is a compact representation of a valid (non-zero power of two) alignment.
Definition: Alignment.h:39