LLVM  6.0.0svn
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/Statistic.h"
19 #include "llvm/IR/Dominators.h"
20 #include "llvm/IR/Function.h"
21 #include "llvm/Pass.h"
22 #include "llvm/Transforms/Scalar.h"
24 using namespace llvm;
25 
26 #define DEBUG_TYPE "div-rem-pairs"
27 STATISTIC(NumPairs, "Number of div/rem pairs");
28 STATISTIC(NumHoisted, "Number of instructions hoisted");
29 STATISTIC(NumDecomposed, "Number of instructions decomposed");
30 
31 /// Find matching pairs of integer div/rem ops (they have the same numerator,
32 /// denominator, and signedness). If they exist in different basic blocks, bring
33 /// them together by hoisting or replace the common division operation that is
34 /// implicit in the remainder:
35 /// X % Y <--> X - ((X / Y) * Y).
36 ///
37 /// We can largely ignore the normal safety and cost constraints on speculation
38 /// of these ops when we find a matching pair. This is because we are already
39 /// guaranteed that any exceptions and most cost are already incurred by the
40 /// first member of the pair.
41 ///
42 /// Note: This transform could be an oddball enhancement to EarlyCSE, GVN, or
43 /// SimplifyCFG, but it's split off on its own because it's different enough
44 /// that it doesn't quite match the stated objectives of those passes.
45 static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
46  const DominatorTree &DT) {
47  bool Changed = false;
48 
49  // Insert all divide and remainder instructions into maps keyed by their
50  // operands and opcode (signed or unsigned).
52  for (auto &BB : F) {
53  for (auto &I : BB) {
54  if (I.getOpcode() == Instruction::SDiv)
55  DivMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = &I;
56  else if (I.getOpcode() == Instruction::UDiv)
57  DivMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
58  else if (I.getOpcode() == Instruction::SRem)
59  RemMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = &I;
60  else if (I.getOpcode() == Instruction::URem)
61  RemMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
62  }
63  }
64 
65  // We can iterate over either map because we are only looking for matched
66  // pairs. Choose remainders for efficiency because they are usually even more
67  // rare than division.
68  for (auto &RemPair : RemMap) {
69  // Find the matching division instruction from the division map.
70  Instruction *DivInst = DivMap[RemPair.getFirst()];
71  if (!DivInst)
72  continue;
73 
74  // We have a matching pair of div/rem instructions. If one dominates the
75  // other, hoist and/or replace one.
76  NumPairs++;
77  Instruction *RemInst = RemPair.getSecond();
78  bool IsSigned = DivInst->getOpcode() == Instruction::SDiv;
79  bool HasDivRemOp = TTI.hasDivRemOp(DivInst->getType(), IsSigned);
80 
81  // If the target supports div+rem and the instructions are in the same block
82  // already, there's nothing to do. The backend should handle this. If the
83  // target does not support div+rem, then we will decompose the rem.
84  if (HasDivRemOp && RemInst->getParent() == DivInst->getParent())
85  continue;
86 
87  bool DivDominates = DT.dominates(DivInst, RemInst);
88  if (!DivDominates && !DT.dominates(RemInst, DivInst))
89  continue;
90 
91  if (HasDivRemOp) {
92  // The target has a single div/rem operation. Hoist the lower instruction
93  // to make the matched pair visible to the backend.
94  if (DivDominates)
95  RemInst->moveAfter(DivInst);
96  else
97  DivInst->moveAfter(RemInst);
98  NumHoisted++;
99  } else {
100  // The target does not have a single div/rem operation. Decompose the
101  // remainder calculation as:
102  // X % Y --> X - ((X / Y) * Y).
103  Value *X = RemInst->getOperand(0);
104  Value *Y = RemInst->getOperand(1);
105  Instruction *Mul = BinaryOperator::CreateMul(DivInst, Y);
106  Instruction *Sub = BinaryOperator::CreateSub(X, Mul);
107 
108  // If the remainder dominates, then hoist the division up to that block:
109  //
110  // bb1:
111  // %rem = srem %x, %y
112  // bb2:
113  // %div = sdiv %x, %y
114  // -->
115  // bb1:
116  // %div = sdiv %x, %y
117  // %mul = mul %div, %y
118  // %rem = sub %x, %mul
119  //
120  // If the division dominates, it's already in the right place. The mul+sub
121  // will be in a different block because we don't assume that they are
122  // cheap to speculatively execute:
123  //
124  // bb1:
125  // %div = sdiv %x, %y
126  // bb2:
127  // %rem = srem %x, %y
128  // -->
129  // bb1:
130  // %div = sdiv %x, %y
131  // bb2:
132  // %mul = mul %div, %y
133  // %rem = sub %x, %mul
134  //
135  // If the div and rem are in the same block, we do the same transform,
136  // but any code movement would be within the same block.
137 
138  if (!DivDominates)
139  DivInst->moveBefore(RemInst);
140  Mul->insertAfter(RemInst);
141  Sub->insertAfter(Mul);
142 
143  // Now kill the explicit remainder. We have replaced it with:
144  // (sub X, (mul (div X, Y), Y)
145  RemInst->replaceAllUsesWith(Sub);
146  RemInst->eraseFromParent();
147  NumDecomposed++;
148  }
149  Changed = true;
150  }
151 
152  return Changed;
153 }
154 
155 // Pass manager boilerplate below here.
156 
157 namespace {
158 struct DivRemPairsLegacyPass : public FunctionPass {
159  static char ID;
160  DivRemPairsLegacyPass() : FunctionPass(ID) {
162  }
163 
164  void getAnalysisUsage(AnalysisUsage &AU) const override {
167  AU.setPreservesCFG();
171  }
172 
173  bool runOnFunction(Function &F) override {
174  if (skipFunction(F))
175  return false;
176  auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
177  auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
178  return optimizeDivRem(F, TTI, DT);
179  }
180 };
181 }
182 
184 INITIALIZE_PASS_BEGIN(DivRemPairsLegacyPass, "div-rem-pairs",
185  "Hoist/decompose integer division and remainder", false,
186  false)
188 INITIALIZE_PASS_END(DivRemPairsLegacyPass, "div-rem-pairs",
189  "Hoist/decompose integer division and remainder", false,
190  false)
192  return new DivRemPairsLegacyPass();
193 }
194 
199  if (!optimizeDivRem(F, TTI, DT))
200  return PreservedAnalyses::all();
201  // TODO: This pass just hoists/replaces math ops - all analyses are preserved?
203  PA.preserveSet<CFGAnalyses>();
204  PA.preserve<GlobalsAA>();
205  return PA;
206 }
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:69
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:45
PassT::Result & getResult(IRUnitT &IR, ExtraArgTs... ExtraArgs)
Get the result of an analysis pass for a given IR unit.
Definition: PassManager.h:687
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 &)
STATISTIC(NumFunctions, "Total number of functions")
Analysis pass which computes a DominatorTree.
Definition: Dominators.h:238
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
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:91
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:125
void replaceAllUsesWith(Value *V)
Change all uses of this to point to a new Value.
Definition: Value.cpp:430
Concrete subclass of DominatorTreeBase that is used to compute a normal dominator tree...
Definition: Dominators.h:140
Value * getOperand(unsigned i) const
Definition: User.h:154
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:153
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 PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: PassManager.h:159
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:239
void setPreservesCFG()
This function should be called by the pass, iff they do not:
Definition: Pass.cpp:285
Represents analyses that only rely on functions&#39; control flow.
Definition: PassManager.h:114
PreservedAnalyses run(Function &F, FunctionAnalysisManager &)
void preserveSet()
Mark an analysis set as preserved.
Definition: PassManager.h:189
void insertAfter(Instruction *InsertPos)
Insert an unlinked instruction into a basic block immediately after the specified instruction...
Definition: Instruction.cpp:81
#define I(x, y, z)
Definition: MD5.cpp:58
void preserve()
Mark an analysis as preserved.
Definition: PassManager.h:174
void moveAfter(Instruction *MovePos)
Unlink this instruction from its current basic block and insert it into the basic block that MovePos ...
Definition: Instruction.cpp:92
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:88
A container for analyses that lazily runs them and caches their results.
Legacy analysis pass which computes a DominatorTree.
Definition: Dominators.h:267
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:66