LLVM  8.0.0svn
DivRemPairs.cpp
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1 //===- DivRemPairs.cpp - Hoist/decompose division and remainder -*- C++ -*-===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass hoists and/or decomposes integer division and remainder
11 // instructions to enable CFG improvements and better codegen.
12 //
13 //===----------------------------------------------------------------------===//
14 
16 #include "llvm/ADT/DenseMap.h"
17 #include "llvm/ADT/MapVector.h"
18 #include "llvm/ADT/Statistic.h"
21 #include "llvm/IR/Dominators.h"
22 #include "llvm/IR/Function.h"
23 #include "llvm/Pass.h"
25 #include "llvm/Transforms/Scalar.h"
27 using namespace llvm;
28 
29 #define DEBUG_TYPE "div-rem-pairs"
30 STATISTIC(NumPairs, "Number of div/rem pairs");
31 STATISTIC(NumHoisted, "Number of instructions hoisted");
32 STATISTIC(NumDecomposed, "Number of instructions decomposed");
33 DEBUG_COUNTER(DRPCounter, "div-rem-pairs-transform",
34  "Controls transformations in div-rem-pairs pass");
35 
36 /// Find matching pairs of integer div/rem ops (they have the same numerator,
37 /// denominator, and signedness). If they exist in different basic blocks, bring
38 /// them together by hoisting or replace the common division operation that is
39 /// implicit in the remainder:
40 /// X % Y <--> X - ((X / Y) * Y).
41 ///
42 /// We can largely ignore the normal safety and cost constraints on speculation
43 /// of these ops when we find a matching pair. This is because we are already
44 /// guaranteed that any exceptions and most cost are already incurred by the
45 /// first member of the pair.
46 ///
47 /// Note: This transform could be an oddball enhancement to EarlyCSE, GVN, or
48 /// SimplifyCFG, but it's split off on its own because it's different enough
49 /// that it doesn't quite match the stated objectives of those passes.
50 static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
51  const DominatorTree &DT) {
52  bool Changed = false;
53 
54  // Insert all divide and remainder instructions into maps keyed by their
55  // operands and opcode (signed or unsigned).
57  // Use a MapVector for RemMap so that instructions are moved/inserted in a
58  // deterministic order.
60  for (auto &BB : F) {
61  for (auto &I : BB) {
62  if (I.getOpcode() == Instruction::SDiv)
63  DivMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = &I;
64  else if (I.getOpcode() == Instruction::UDiv)
65  DivMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
66  else if (I.getOpcode() == Instruction::SRem)
67  RemMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = &I;
68  else if (I.getOpcode() == Instruction::URem)
69  RemMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
70  }
71  }
72 
73  // We can iterate over either map because we are only looking for matched
74  // pairs. Choose remainders for efficiency because they are usually even more
75  // rare than division.
76  for (auto &RemPair : RemMap) {
77  // Find the matching division instruction from the division map.
78  Instruction *DivInst = DivMap[RemPair.first];
79  if (!DivInst)
80  continue;
81 
82  // We have a matching pair of div/rem instructions. If one dominates the
83  // other, hoist and/or replace one.
84  NumPairs++;
85  Instruction *RemInst = RemPair.second;
86  bool IsSigned = DivInst->getOpcode() == Instruction::SDiv;
87  bool HasDivRemOp = TTI.hasDivRemOp(DivInst->getType(), IsSigned);
88 
89  // If the target supports div+rem and the instructions are in the same block
90  // already, there's nothing to do. The backend should handle this. If the
91  // target does not support div+rem, then we will decompose the rem.
92  if (HasDivRemOp && RemInst->getParent() == DivInst->getParent())
93  continue;
94 
95  bool DivDominates = DT.dominates(DivInst, RemInst);
96  if (!DivDominates && !DT.dominates(RemInst, DivInst))
97  continue;
98 
99  if (!DebugCounter::shouldExecute(DRPCounter))
100  continue;
101 
102  if (HasDivRemOp) {
103  // The target has a single div/rem operation. Hoist the lower instruction
104  // to make the matched pair visible to the backend.
105  if (DivDominates)
106  RemInst->moveAfter(DivInst);
107  else
108  DivInst->moveAfter(RemInst);
109  NumHoisted++;
110  } else {
111  // The target does not have a single div/rem operation. Decompose the
112  // remainder calculation as:
113  // X % Y --> X - ((X / Y) * Y).
114  Value *X = RemInst->getOperand(0);
115  Value *Y = RemInst->getOperand(1);
116  Instruction *Mul = BinaryOperator::CreateMul(DivInst, Y);
117  Instruction *Sub = BinaryOperator::CreateSub(X, Mul);
118 
119  // If the remainder dominates, then hoist the division up to that block:
120  //
121  // bb1:
122  // %rem = srem %x, %y
123  // bb2:
124  // %div = sdiv %x, %y
125  // -->
126  // bb1:
127  // %div = sdiv %x, %y
128  // %mul = mul %div, %y
129  // %rem = sub %x, %mul
130  //
131  // If the division dominates, it's already in the right place. The mul+sub
132  // will be in a different block because we don't assume that they are
133  // cheap to speculatively execute:
134  //
135  // bb1:
136  // %div = sdiv %x, %y
137  // bb2:
138  // %rem = srem %x, %y
139  // -->
140  // bb1:
141  // %div = sdiv %x, %y
142  // bb2:
143  // %mul = mul %div, %y
144  // %rem = sub %x, %mul
145  //
146  // If the div and rem are in the same block, we do the same transform,
147  // but any code movement would be within the same block.
148 
149  if (!DivDominates)
150  DivInst->moveBefore(RemInst);
151  Mul->insertAfter(RemInst);
152  Sub->insertAfter(Mul);
153 
154  // Now kill the explicit remainder. We have replaced it with:
155  // (sub X, (mul (div X, Y), Y)
156  RemInst->replaceAllUsesWith(Sub);
157  RemInst->eraseFromParent();
158  NumDecomposed++;
159  }
160  Changed = true;
161  }
162 
163  return Changed;
164 }
165 
166 // Pass manager boilerplate below here.
167 
168 namespace {
169 struct DivRemPairsLegacyPass : public FunctionPass {
170  static char ID;
171  DivRemPairsLegacyPass() : FunctionPass(ID) {
173  }
174 
175  void getAnalysisUsage(AnalysisUsage &AU) const override {
178  AU.setPreservesCFG();
182  }
183 
184  bool runOnFunction(Function &F) override {
185  if (skipFunction(F))
186  return false;
187  auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
188  auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
189  return optimizeDivRem(F, TTI, DT);
190  }
191 };
192 }
193 
195 INITIALIZE_PASS_BEGIN(DivRemPairsLegacyPass, "div-rem-pairs",
196  "Hoist/decompose integer division and remainder", false,
197  false)
199 INITIALIZE_PASS_END(DivRemPairsLegacyPass, "div-rem-pairs",
200  "Hoist/decompose integer division and remainder", false,
201  false)
203  return new DivRemPairsLegacyPass();
204 }
205 
210  if (!optimizeDivRem(F, TTI, DT))
211  return PreservedAnalyses::all();
212  // TODO: This pass just hoists/replaces math ops - all analyses are preserved?
214  PA.preserveSet<CFGAnalyses>();
215  PA.preserve<GlobalsAA>();
216  return PA;
217 }
Legacy wrapper pass to provide the GlobalsAAResult object.
SymbolTableList< Instruction >::iterator eraseFromParent()
This method unlinks &#39;this&#39; from the containing basic block and deletes it.
Definition: Instruction.cpp:68
static GCMetadataPrinterRegistry::Add< ErlangGCPrinter > X("erlang", "erlang-compatible garbage collector")
AnalysisUsage & addPreserved()
Add the specified Pass class to the set of analyses preserved by this pass.
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI, const DominatorTree &DT)
Find matching pairs of integer div/rem ops (they have the same numerator, denominator, and signedness).
Definition: DivRemPairs.cpp:50
PassT::Result & getResult(IRUnitT &IR, ExtraArgTs... ExtraArgs)
Get the result of an analysis pass for a given IR unit.
Definition: PassManager.h:770
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
This is the interface for a simple mod/ref and alias analysis over globals.
Analysis pass providing the TargetTransformInfo.
void initializeDivRemPairsLegacyPassPass(PassRegistry &)
This class implements a map that also provides access to all stored values in a deterministic order...
Definition: MapVector.h:38
STATISTIC(NumFunctions, "Total number of functions")
Analysis pass which computes a DominatorTree.
Definition: Dominators.h:231
F(f)
INITIALIZE_PASS_BEGIN(DivRemPairsLegacyPass, "div-rem-pairs", "Hoist/decompose integer division and remainder", false, false) INITIALIZE_PASS_END(DivRemPairsLegacyPass
static GCMetadataPrinterRegistry::Add< OcamlGCMetadataPrinter > Y("ocaml", "ocaml 3.10-compatible collector")
AnalysisUsage & addRequired()
#define INITIALIZE_PASS_DEPENDENCY(depName)
Definition: PassSupport.h:51
This file provides an implementation of debug counters.
div rem Hoist decompose integer division and remainder
virtual void getAnalysisUsage(AnalysisUsage &) const
getAnalysisUsage - This function should be overriden by passes that need analysis information to do t...
Definition: Pass.cpp:92
div rem pairs
Type * getType() const
All values are typed, get the type of this value.
Definition: Value.h:245
bool hasDivRemOp(Type *DataType, bool IsSigned) const
Return true if the target has a unified operation to calculate division and remainder.
unsigned getOpcode() const
Returns a member of one of the enums like Instruction::Add.
Definition: Instruction.h:126
void replaceAllUsesWith(Value *V)
Change all uses of this to point to a new Value.
Definition: Value.cpp:439
Concrete subclass of DominatorTreeBase that is used to compute a normal dominator tree...
Definition: Dominators.h:145
Value * getOperand(unsigned i) const
Definition: User.h:170
static bool runOnFunction(Function &F, bool PostInlining)
Wrapper pass for TargetTransformInfo.
A set of analyses that are preserved following a run of a transformation pass.
Definition: PassManager.h:154
FunctionPass * createDivRemPairsPass()
Represent the analysis usage information of a pass.
Analysis pass providing a never-invalidated alias analysis result.
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:285
static bool shouldExecute(unsigned CounterName)
Definition: DebugCounter.h:72
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: PassManager.h:160
INITIALIZE_PASS_END(RegBankSelect, DEBUG_TYPE, "Assign register bank of generic virtual registers", false, false) RegBankSelect
This pass provides access to the codegen interfaces that are needed for IR-level transformations.
bool dominates(const Instruction *Def, const Use &U) const
Return true if Def dominates a use in User.
Definition: Dominators.cpp:249
void setPreservesCFG()
This function should be called by the pass, iff they do not:
Definition: Pass.cpp:286
Represents analyses that only rely on functions&#39; control flow.
Definition: PassManager.h:115
DEBUG_COUNTER(DRPCounter, "div-rem-pairs-transform", "Controls transformations in div-rem-pairs pass")
PreservedAnalyses run(Function &F, FunctionAnalysisManager &)
void preserveSet()
Mark an analysis set as preserved.
Definition: PassManager.h:190
void insertAfter(Instruction *InsertPos)
Insert an unlinked instruction into a basic block immediately after the specified instruction...
Definition: Instruction.cpp:80
#define I(x, y, z)
Definition: MD5.cpp:58
void preserve()
Mark an analysis as preserved.
Definition: PassManager.h:175
void moveAfter(Instruction *MovePos)
Unlink this instruction from its current basic block and insert it into the basic block that MovePos ...
Definition: Instruction.cpp:91
LLVM Value Representation.
Definition: Value.h:73
void moveBefore(Instruction *MovePos)
Unlink this instruction from its current basic block and insert it into the basic block that MovePos ...
Definition: Instruction.cpp:87
A container for analyses that lazily runs them and caches their results.
Legacy analysis pass which computes a DominatorTree.
Definition: Dominators.h:260
This pass exposes codegen information to IR-level passes.
static BinaryOperator * CreateMul(Value *S1, Value *S2, const Twine &Name, Instruction *InsertBefore, Value *FlagsOp)
const BasicBlock * getParent() const
Definition: Instruction.h:67