llvm.org GIT mirror llvm / 0392724 lib / Target / AMDGPU / AMDGPURewriteOutArguments.cpp
0392724

Tree @0392724 (Download .tar.gz)

AMDGPURewriteOutArguments.cpp @0392724raw · history · blame

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
//===- AMDGPURewriteOutArgumentsPass.cpp - Create struct returns ----------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
/// \file This pass attempts to replace out argument usage with a return of a
/// struct.
///
/// We can support returning a lot of values directly in registers, but
/// idiomatic C code frequently uses a pointer argument to return a second value
/// rather than returning a struct by value. GPU stack access is also quite
/// painful, so we want to avoid that if possible. Passing a stack object
/// pointer to a function also requires an additional address expansion code
/// sequence to convert the pointer to be relative to the kernel's scratch wave
/// offset register since the callee doesn't know what stack frame the incoming
/// pointer is relative to.
///
/// The goal is to try rewriting code that looks like this:
///
///  int foo(int a, int b, int* out) {
///     *out = bar();
///     return a + b;
/// }
///
/// into something like this:
///
///  std::pair<int, int> foo(int a, int b) {
///     return std::make_pair(a + b, bar());
/// }
///
/// Typically the incoming pointer is a simple alloca for a temporary variable
/// to use the API, which if replaced with a struct return will be easily SROA'd
/// out when the stub function we create is inlined
///
/// This pass introduces the struct return, but leaves the unused pointer
/// arguments and introduces a new stub function calling the struct returning
/// body. DeadArgumentElimination should be run after this to clean these up.
//
//===----------------------------------------------------------------------===//

#include "AMDGPU.h"
#include "Utils/AMDGPUBaseInfo.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <utility>

#define DEBUG_TYPE "amdgpu-rewrite-out-arguments"

using namespace llvm;

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 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));

STATISTIC(NumOutArgumentsReplaced,
          "Number out arguments moved to struct return values");
STATISTIC(NumOutArgumentFunctionsReplaced,
          "Number of functions with out arguments moved to struct return values");

namespace {

class AMDGPURewriteOutArguments : public FunctionPass {
private:
  const DataLayout *DL = nullptr;
  MemoryDependenceResults *MDA = nullptr;

  bool checkArgumentUses(Value &Arg) const;
  bool isOutArgumentCandidate(Argument &Arg) const;

#ifndef NDEBUG
  bool isVec3ToVec4Shuffle(Type *Ty0, Type* Ty1) const;
#endif

public:
  static char ID;

  AMDGPURewriteOutArguments() : FunctionPass(ID) {}

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<MemoryDependenceWrapperPass>();
    FunctionPass::getAnalysisUsage(AU);
  }

  bool doInitialization(Module &M) override;
  bool runOnFunction(Function &F) override;
};

} // end anonymous namespace

INITIALIZE_PASS_BEGIN(AMDGPURewriteOutArguments, DEBUG_TYPE,
                      "AMDGPU Rewrite Out Arguments", false, false)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
INITIALIZE_PASS_END(AMDGPURewriteOutArguments, DEBUG_TYPE,
                    "AMDGPU Rewrite Out Arguments", false, false)

char AMDGPURewriteOutArguments::ID = 0;

bool AMDGPURewriteOutArguments::checkArgumentUses(Value &Arg) const {
  const int MaxUses = 10;
  int UseCount = 0;

  for (Use &U : Arg.uses()) {
    StoreInst *SI = dyn_cast<StoreInst>(U.getUser());
    if (UseCount > MaxUses)
      return false;

    if (!SI) {
      auto *BCI = dyn_cast<BitCastInst>(U.getUser());
      if (!BCI || !BCI->hasOneUse())
        return false;

      // We don't handle multiple stores currently, so stores to aggregate
      // pointers aren't worth the trouble since they are canonically split up.
      Type *DestEltTy = BCI->getType()->getPointerElementType();
      if (DestEltTy->isAggregateType())
        return false;

      // We could handle these if we had a convenient way to bitcast between
      // them.
      Type *SrcEltTy = Arg.getType()->getPointerElementType();
      if (SrcEltTy->isArrayTy())
        return false;

      // Special case handle structs with single members. It is useful to handle
      // some casts between structs and non-structs, but we can't bitcast
      // directly between them.  directly bitcast between them.  Blender uses
      // some casts that look like { <3 x float> }* to <4 x float>*
      if ((SrcEltTy->isStructTy() && (SrcEltTy->getStructNumElements() != 1)))
        return false;

      // Clang emits OpenCL 3-vector type accesses with a bitcast to the
      // equivalent 4-element vector and accesses that, and we're looking for
      // this pointer cast.
      if (DL->getTypeAllocSize(SrcEltTy) != DL->getTypeAllocSize(DestEltTy))
        return false;

      return checkArgumentUses(*BCI);
    }

    if (!SI->isSimple() ||
        U.getOperandNo() != StoreInst::getPointerOperandIndex())
      return false;

    ++UseCount;
  }

  // Skip unused arguments.
  return UseCount > 0;
}

bool AMDGPURewriteOutArguments::isOutArgumentCandidate(Argument &Arg) const {
  const unsigned MaxOutArgSizeBytes = 4 * MaxNumRetRegs;
  PointerType *ArgTy = dyn_cast<PointerType>(Arg.getType());

  // TODO: It might be useful for any out arguments, not just privates.
  if (!ArgTy || (ArgTy->getAddressSpace() != DL->getAllocaAddrSpace() &&
                 !AnyAddressSpace) ||
      Arg.hasByValAttr() || Arg.hasStructRetAttr() ||
      DL->getTypeStoreSize(ArgTy->getPointerElementType()) > MaxOutArgSizeBytes) {
    return false;
  }

  return checkArgumentUses(Arg);
}

bool AMDGPURewriteOutArguments::doInitialization(Module &M) {
  DL = &M.getDataLayout();
  return false;
}

#ifndef NDEBUG
bool AMDGPURewriteOutArguments::isVec3ToVec4Shuffle(Type *Ty0, Type* Ty1) const {
  VectorType *VT0 = dyn_cast<VectorType>(Ty0);
  VectorType *VT1 = dyn_cast<VectorType>(Ty1);
  if (!VT0 || !VT1)
    return false;

  if (VT0->getNumElements() != 3 ||
      VT1->getNumElements() != 4)
    return false;

  return DL->getTypeSizeInBits(VT0->getElementType()) ==
         DL->getTypeSizeInBits(VT1->getElementType());
}
#endif

bool AMDGPURewriteOutArguments::runOnFunction(Function &F) {
  if (skipFunction(F))
    return false;

  // TODO: Could probably handle variadic functions.
  if (F.isVarArg() || F.hasStructRetAttr() ||
      AMDGPU::isEntryFunctionCC(F.getCallingConv()))
    return false;

  MDA = &getAnalysis<MemoryDependenceWrapperPass>().getMemDep();

  unsigned ReturnNumRegs = 0;
  SmallSet<int, 4> OutArgIndexes;
  SmallVector<Type *, 4> ReturnTypes;
  Type *RetTy = F.getReturnType();
  if (!RetTy->isVoidTy()) {
    ReturnNumRegs = DL->getTypeStoreSize(RetTy) / 4;

    if (ReturnNumRegs >= MaxNumRetRegs)
      return false;

    ReturnTypes.push_back(RetTy);
  }

  SmallVector<Argument *, 4> OutArgs;
  for (Argument &Arg : F.args()) {
    if (isOutArgumentCandidate(Arg)) {
      LLVM_DEBUG(dbgs() << "Found possible out argument " << Arg
                        << " in function " << F.getName() << '\n');
      OutArgs.push_back(&Arg);
    }
  }

  if (OutArgs.empty())
    return false;

  using ReplacementVec = SmallVector<std::pair<Argument *, Value *>, 4>;

  DenseMap<ReturnInst *, ReplacementVec> Replacements;

  SmallVector<ReturnInst *, 4> Returns;
  for (BasicBlock &BB : F) {
    if (ReturnInst *RI = dyn_cast<ReturnInst>(&BB.back()))
      Returns.push_back(RI);
  }

  if (Returns.empty())
    return false;

  bool Changing;

  do {
    Changing = false;

    // Keep retrying if we are able to successfully eliminate an argument. This
    // helps with cases with multiple arguments which may alias, such as in a
    // sincos implemntation. If we have 2 stores to arguments, on the first
    // attempt the MDA query will succeed for the second store but not the
    // first. On the second iteration we've removed that out clobbering argument
    // (by effectively moving it into another function) and will find the second
    // argument is OK to move.
    for (Argument *OutArg : OutArgs) {
      bool ThisReplaceable = true;
      SmallVector<std::pair<ReturnInst *, StoreInst *>, 4> ReplaceableStores;

      Type *ArgTy = OutArg->getType()->getPointerElementType();

      // Skip this argument if converting it will push us over the register
      // count to return limit.

      // TODO: This is an approximation. When legalized this could be more. We
      // can ask TLI for exactly how many.
      unsigned ArgNumRegs = DL->getTypeStoreSize(ArgTy) / 4;
      if (ArgNumRegs + ReturnNumRegs > MaxNumRetRegs)
        continue;

      // An argument is convertible only if all exit blocks are able to replace
      // it.
      for (ReturnInst *RI : Returns) {
        BasicBlock *BB = RI->getParent();

        MemDepResult Q = MDA->getPointerDependencyFrom(MemoryLocation(OutArg),
                                                       true, BB->end(), BB, RI);
        StoreInst *SI = nullptr;
        if (Q.isDef())
          SI = dyn_cast<StoreInst>(Q.getInst());

        if (SI) {
          LLVM_DEBUG(dbgs() << "Found out argument store: " << *SI << '\n');
          ReplaceableStores.emplace_back(RI, SI);
        } else {
          ThisReplaceable = false;
          break;
        }
      }

      if (!ThisReplaceable)
        continue; // Try the next argument candidate.

      for (std::pair<ReturnInst *, StoreInst *> Store : ReplaceableStores) {
        Value *ReplVal = Store.second->getValueOperand();

        auto &ValVec = Replacements[Store.first];
        if (llvm::find_if(ValVec,
              [OutArg](const std::pair<Argument *, Value *> &Entry) {
                 return Entry.first == OutArg;}) != ValVec.end()) {
          LLVM_DEBUG(dbgs()
                     << "Saw multiple out arg stores" << *OutArg << '\n');
          // It is possible to see stores to the same argument multiple times,
          // but we expect these would have been optimized out already.
          ThisReplaceable = false;
          break;
        }

        ValVec.emplace_back(OutArg, ReplVal);
        Store.second->eraseFromParent();
      }

      if (ThisReplaceable) {
        ReturnTypes.push_back(ArgTy);
        OutArgIndexes.insert(OutArg->getArgNo());
        ++NumOutArgumentsReplaced;
        Changing = true;
      }
    }
  } while (Changing);

  if (Replacements.empty())
    return false;

  LLVMContext &Ctx = F.getParent()->getContext();
  StructType *NewRetTy = StructType::create(Ctx, ReturnTypes, F.getName());

  FunctionType *NewFuncTy = FunctionType::get(NewRetTy,
                                              F.getFunctionType()->params(),
                                              F.isVarArg());

  LLVM_DEBUG(dbgs() << "Computed new return type: " << *NewRetTy << '\n');

  Function *NewFunc = Function::Create(NewFuncTy, Function::PrivateLinkage,
                                       F.getName() + ".body");
  F.getParent()->getFunctionList().insert(F.getIterator(), NewFunc);
  NewFunc->copyAttributesFrom(&F);
  NewFunc->setComdat(F.getComdat());

  // We want to preserve the function and param attributes, but need to strip
  // off any return attributes, e.g. zeroext doesn't make sense with a struct.
  NewFunc->stealArgumentListFrom(F);

  AttrBuilder RetAttrs;
  RetAttrs.addAttribute(Attribute::SExt);
  RetAttrs.addAttribute(Attribute::ZExt);
  RetAttrs.addAttribute(Attribute::NoAlias);
  NewFunc->removeAttributes(AttributeList::ReturnIndex, RetAttrs);
  // TODO: How to preserve metadata?

  // Move the body of the function into the new rewritten function, and replace
  // this function with a stub.
  NewFunc->getBasicBlockList().splice(NewFunc->begin(), F.getBasicBlockList());

  for (std::pair<ReturnInst *, ReplacementVec> &Replacement : Replacements) {
    ReturnInst *RI = Replacement.first;
    IRBuilder<> B(RI);
    B.SetCurrentDebugLocation(RI->getDebugLoc());

    int RetIdx = 0;
    Value *NewRetVal = UndefValue::get(NewRetTy);

    Value *RetVal = RI->getReturnValue();
    if (RetVal)
      NewRetVal = B.CreateInsertValue(NewRetVal, RetVal, RetIdx++);

    for (std::pair<Argument *, Value *> ReturnPoint : Replacement.second) {
      Argument *Arg = ReturnPoint.first;
      Value *Val = ReturnPoint.second;
      Type *EltTy = Arg->getType()->getPointerElementType();
      if (Val->getType() != EltTy) {
        Type *EffectiveEltTy = EltTy;
        if (StructType *CT = dyn_cast<StructType>(EltTy)) {
          assert(CT->getNumElements() == 1);
          EffectiveEltTy = CT->getElementType(0);
        }

        if (DL->getTypeSizeInBits(EffectiveEltTy) !=
            DL->getTypeSizeInBits(Val->getType())) {
          assert(isVec3ToVec4Shuffle(EffectiveEltTy, Val->getType()));
          Val = B.CreateShuffleVector(Val, UndefValue::get(Val->getType()),
                                      { 0, 1, 2 });
        }

        Val = B.CreateBitCast(Val, EffectiveEltTy);

        // Re-create single element composite.
        if (EltTy != EffectiveEltTy)
          Val = B.CreateInsertValue(UndefValue::get(EltTy), Val, 0);
      }

      NewRetVal = B.CreateInsertValue(NewRetVal, Val, RetIdx++);
    }

    if (RetVal)
      RI->setOperand(0, NewRetVal);
    else {
      B.CreateRet(NewRetVal);
      RI->eraseFromParent();
    }
  }

  SmallVector<Value *, 16> StubCallArgs;
  for (Argument &Arg : F.args()) {
    if (OutArgIndexes.count(Arg.getArgNo())) {
      // It's easier to preserve the type of the argument list. We rely on
      // DeadArgumentElimination to take care of these.
      StubCallArgs.push_back(UndefValue::get(Arg.getType()));
    } else {
      StubCallArgs.push_back(&Arg);
    }
  }

  BasicBlock *StubBB = BasicBlock::Create(Ctx, "", &F);
  IRBuilder<> B(StubBB);
  CallInst *StubCall = B.CreateCall(NewFunc, StubCallArgs);

  int RetIdx = RetTy->isVoidTy() ? 0 : 1;
  for (Argument &Arg : F.args()) {
    if (!OutArgIndexes.count(Arg.getArgNo()))
      continue;

    PointerType *ArgType = cast<PointerType>(Arg.getType());

    auto *EltTy = ArgType->getElementType();
    unsigned Align = Arg.getParamAlignment();
    if (Align == 0)
      Align = DL->getABITypeAlignment(EltTy);

    Value *Val = B.CreateExtractValue(StubCall, RetIdx++);
    Type *PtrTy = Val->getType()->getPointerTo(ArgType->getAddressSpace());

    // We can peek through bitcasts, so the type may not match.
    Value *PtrVal = B.CreateBitCast(&Arg, PtrTy);

    B.CreateAlignedStore(Val, PtrVal, Align);
  }

  if (!RetTy->isVoidTy()) {
    B.CreateRet(B.CreateExtractValue(StubCall, 0));
  } else {
    B.CreateRetVoid();
  }

  // The function is now a stub we want to inline.
  F.addFnAttr(Attribute::AlwaysInline);

  ++NumOutArgumentFunctionsReplaced;
  return true;
}

FunctionPass *llvm::createAMDGPURewriteOutArgumentsPass() {
  return new AMDGPURewriteOutArguments();
}