LLVM 19.0.0git
CallBrPrepare.cpp
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1//===-- CallBrPrepare - Prepare callbr for code generation ----------------===//
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 pass lowers callbrs in LLVM IR in order to to assist SelectionDAG's
10// codegen.
11//
12// In particular, this pass assists in inserting register copies for the output
13// values of a callbr along the edges leading to the indirect target blocks.
14// Though the output SSA value is defined by the callbr instruction itself in
15// the IR representation, the value cannot be copied to the appropriate virtual
16// registers prior to jumping to an indirect label, since the jump occurs
17// within the user-provided assembly blob.
18//
19// Instead, those copies must occur separately at the beginning of each
20// indirect target. That requires that we create a separate SSA definition in
21// each of them (via llvm.callbr.landingpad), and may require splitting
22// critical edges so we have a location to place the intrinsic. Finally, we
23// remap users of the original callbr output SSA value to instead point to the
24// appropriate llvm.callbr.landingpad value.
25//
26// Ideally, this could be done inside SelectionDAG, or in the
27// MachineInstruction representation, without the use of an IR-level intrinsic.
28// But, within the current framework, it’s simpler to implement as an IR pass.
29// (If support for callbr in GlobalISel is implemented, it’s worth considering
30// whether this is still required.)
31//
32//===----------------------------------------------------------------------===//
33
35#include "llvm/ADT/ArrayRef.h"
38#include "llvm/ADT/iterator.h"
39#include "llvm/Analysis/CFG.h"
40#include "llvm/CodeGen/Passes.h"
41#include "llvm/IR/BasicBlock.h"
42#include "llvm/IR/Dominators.h"
43#include "llvm/IR/Function.h"
44#include "llvm/IR/IRBuilder.h"
47#include "llvm/IR/Intrinsics.h"
49#include "llvm/Pass.h"
52
53using namespace llvm;
54
55#define DEBUG_TYPE "callbr-prepare"
56
59 DominatorTree &DT);
60static void UpdateSSA(DominatorTree &DT, CallBrInst *CBR, CallInst *Intrinsic,
61 SSAUpdater &SSAUpdate);
63
64namespace {
65
66class CallBrPrepare : public FunctionPass {
67public:
68 CallBrPrepare() : FunctionPass(ID) {}
69 void getAnalysisUsage(AnalysisUsage &AU) const override;
70 bool runOnFunction(Function &Fn) override;
71 static char ID;
72};
73
74} // end anonymous namespace
75
78 bool Changed = false;
80
81 if (CBRs.empty())
83
84 auto &DT = FAM.getResult<DominatorTreeAnalysis>(Fn);
85
86 Changed |= SplitCriticalEdges(CBRs, DT);
87 Changed |= InsertIntrinsicCalls(CBRs, DT);
88
89 if (!Changed)
93 return PA;
94}
95
96char CallBrPrepare::ID = 0;
97INITIALIZE_PASS_BEGIN(CallBrPrepare, "callbrprepare", "Prepare callbr", false,
98 false)
100INITIALIZE_PASS_END(CallBrPrepare, "callbrprepare", "Prepare callbr", false,
101 false)
102
103FunctionPass *llvm::createCallBrPass() { return new CallBrPrepare(); }
104
105void CallBrPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
107}
108
111 for (BasicBlock &BB : Fn)
112 if (auto *CBR = dyn_cast<CallBrInst>(BB.getTerminator()))
113 if (!CBR->getType()->isVoidTy() && !CBR->use_empty())
114 CBRs.push_back(CBR);
115 return CBRs;
116}
117
119 bool Changed = false;
121 Options.setMergeIdenticalEdges();
122
123 // The indirect destination might be duplicated between another parameter...
124 // %0 = callbr ... [label %x, label %x]
125 // ...hence MergeIdenticalEdges and AllowIndentical edges, but we don't need
126 // to split the default destination if it's duplicated between an indirect
127 // destination...
128 // %1 = callbr ... to label %x [label %x]
129 // ...hence starting at 1 and checking against successor 0 (aka the default
130 // destination).
131 for (CallBrInst *CBR : CBRs)
132 for (unsigned i = 1, e = CBR->getNumSuccessors(); i != e; ++i)
133 if (CBR->getSuccessor(i) == CBR->getSuccessor(0) ||
134 isCriticalEdge(CBR, i, /*AllowIdenticalEdges*/ true))
135 if (SplitKnownCriticalEdge(CBR, i, Options))
136 Changed = true;
137 return Changed;
138}
139
141 bool Changed = false;
143 IRBuilder<> Builder(CBRs[0]->getContext());
144 for (CallBrInst *CBR : CBRs) {
145 if (!CBR->getNumIndirectDests())
146 continue;
147
148 SSAUpdater SSAUpdate;
149 SSAUpdate.Initialize(CBR->getType(), CBR->getName());
150 SSAUpdate.AddAvailableValue(CBR->getParent(), CBR);
151 SSAUpdate.AddAvailableValue(CBR->getDefaultDest(), CBR);
152
153 for (BasicBlock *IndDest : CBR->getIndirectDests()) {
154 if (!Visited.insert(IndDest).second)
155 continue;
156 Builder.SetInsertPoint(&*IndDest->begin());
157 CallInst *Intrinsic = Builder.CreateIntrinsic(
158 CBR->getType(), Intrinsic::callbr_landingpad, {CBR});
159 SSAUpdate.AddAvailableValue(IndDest, Intrinsic);
160 UpdateSSA(DT, CBR, Intrinsic, SSAUpdate);
161 Changed = true;
162 }
163 }
164 return Changed;
165}
166
167static bool IsInSameBasicBlock(const Use &U, const BasicBlock *BB) {
168 const auto *I = dyn_cast<Instruction>(U.getUser());
169 return I && I->getParent() == BB;
170}
171
172#ifndef NDEBUG
173static void PrintDebugDomInfo(const DominatorTree &DT, const Use &U,
174 const BasicBlock *BB, bool IsDefaultDest) {
175 if (!isa<Instruction>(U.getUser()))
176 return;
177 LLVM_DEBUG(dbgs() << "Use: " << *U.getUser() << ", in block "
178 << cast<Instruction>(U.getUser())->getParent()->getName()
179 << ", is " << (DT.dominates(BB, U) ? "" : "NOT ")
180 << "dominated by " << BB->getName() << " ("
181 << (IsDefaultDest ? "in" : "") << "direct)\n");
182}
183#endif
184
185void UpdateSSA(DominatorTree &DT, CallBrInst *CBR, CallInst *Intrinsic,
186 SSAUpdater &SSAUpdate) {
187
188 SmallPtrSet<Use *, 4> Visited;
189 BasicBlock *DefaultDest = CBR->getDefaultDest();
190 BasicBlock *LandingPad = Intrinsic->getParent();
191
193 for (Use *U : Uses) {
194 if (!Visited.insert(U).second)
195 continue;
196
197#ifndef NDEBUG
198 PrintDebugDomInfo(DT, *U, LandingPad, /*IsDefaultDest*/ false);
199 PrintDebugDomInfo(DT, *U, DefaultDest, /*IsDefaultDest*/ true);
200#endif
201
202 // Don't rewrite the use in the newly inserted intrinsic.
203 if (const auto *II = dyn_cast<IntrinsicInst>(U->getUser()))
204 if (II->getIntrinsicID() == Intrinsic::callbr_landingpad)
205 continue;
206
207 // If the Use is in the same BasicBlock as the Intrinsic call, replace
208 // the Use with the value of the Intrinsic call.
209 if (IsInSameBasicBlock(*U, LandingPad)) {
210 U->set(Intrinsic);
211 continue;
212 }
213
214 // If the Use is dominated by the default dest, do not touch it.
215 if (DT.dominates(DefaultDest, *U))
216 continue;
217
218 SSAUpdate.RewriteUse(*U);
219 }
220}
221
222bool CallBrPrepare::runOnFunction(Function &Fn) {
223 bool Changed = false;
225
226 if (CBRs.empty())
227 return Changed;
228
229 // It's highly likely that most programs do not contain CallBrInsts. Follow a
230 // similar pattern from SafeStackLegacyPass::runOnFunction to reuse previous
231 // domtree analysis if available, otherwise compute it lazily. This avoids
232 // forcing Dominator Tree Construction at -O0 for programs that likely do not
233 // contain CallBrInsts. It does pessimize programs with callbr at higher
234 // optimization levels, as the DominatorTree created here is not reused by
235 // subsequent passes.
236 DominatorTree *DT;
237 std::optional<DominatorTree> LazilyComputedDomTree;
238 if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
239 DT = &DTWP->getDomTree();
240 else {
241 LazilyComputedDomTree.emplace(Fn);
242 DT = &*LazilyComputedDomTree;
243 }
244
245 if (SplitCriticalEdges(CBRs, *DT))
246 Changed = true;
247
248 if (InsertIntrinsicCalls(CBRs, *DT))
249 Changed = true;
250
251 return Changed;
252}
static bool InsertIntrinsicCalls(ArrayRef< CallBrInst * > CBRs, DominatorTree &DT)
static bool SplitCriticalEdges(ArrayRef< CallBrInst * > CBRs, DominatorTree &DT)
static bool IsInSameBasicBlock(const Use &U, const BasicBlock *BB)
static void UpdateSSA(DominatorTree &DT, CallBrInst *CBR, CallInst *Intrinsic, SSAUpdater &SSAUpdate)
static void PrintDebugDomInfo(const DominatorTree &DT, const Use &U, const BasicBlock *BB, bool IsDefaultDest)
Prepare callbr
callbrprepare
static SmallVector< CallBrInst *, 2 > FindCallBrs(Function &Fn)
#define LLVM_DEBUG(X)
Definition: Debug.h:101
Rewrite Partial Register Uses
static LVOptions Options
Definition: LVOptions.cpp:25
#define I(x, y, z)
Definition: MD5.cpp:58
uint64_t IntrinsicInst * II
FunctionAnalysisManager FAM
#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 SmallPtrSet class.
This file defines the SmallVector class.
A container for analyses that lazily runs them and caches their results.
Definition: PassManager.h:321
PassT::Result & getResult(IRUnitT &IR, ExtraArgTs... ExtraArgs)
Get the result of an analysis pass for a given IR unit.
Definition: PassManager.h:473
Represent the analysis usage information of a pass.
AnalysisUsage & addPreserved()
Add the specified Pass class to the set of analyses preserved by this pass.
ArrayRef - Represent a constant reference to an array (0 or more elements consecutively in memory),...
Definition: ArrayRef.h:41
LLVM Basic Block Representation.
Definition: BasicBlock.h:60
CallBr instruction, tracking function calls that may not return control but instead transfer it to a ...
BasicBlock * getDefaultDest() const
PreservedAnalyses run(Function &Fn, FunctionAnalysisManager &FAM)
This class represents a function call, abstracting a target machine's calling convention.
Analysis pass which computes a DominatorTree.
Definition: Dominators.h:279
Legacy analysis pass which computes a DominatorTree.
Definition: Dominators.h:317
Concrete subclass of DominatorTreeBase that is used to compute a normal dominator tree.
Definition: Dominators.h:162
bool dominates(const BasicBlock *BB, const Use &U) const
Return true if the (end of the) basic block BB dominates the use U.
Definition: Dominators.cpp:122
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.
CallInst * CreateIntrinsic(Intrinsic::ID ID, ArrayRef< Type * > Types, ArrayRef< Value * > Args, Instruction *FMFSource=nullptr, const Twine &Name="")
Create a call to intrinsic ID with Args, mangled using Types.
Definition: IRBuilder.cpp:932
void SetInsertPoint(BasicBlock *TheBB)
This specifies that created instructions should be appended to the end of the specified block.
Definition: IRBuilder.h:180
This provides a uniform API for creating instructions and inserting them into a basic block: either a...
Definition: IRBuilder.h:2666
virtual void getAnalysisUsage(AnalysisUsage &) const
getAnalysisUsage - This function should be overriden by passes that need analysis information to do t...
Definition: Pass.cpp:98
A set of analyses that are preserved following a run of a transformation pass.
Definition: Analysis.h:109
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: Analysis.h:115
void preserve()
Mark an analysis as preserved.
Definition: Analysis.h:129
Helper class for SSA formation on a set of values defined in multiple blocks.
Definition: SSAUpdater.h:40
void RewriteUse(Use &U)
Rewrite a use of the symbolic value.
Definition: SSAUpdater.cpp:188
void Initialize(Type *Ty, StringRef Name)
Reset this object to get ready for a new set of SSA updates with type 'Ty'.
Definition: SSAUpdater.cpp:53
void AddAvailableValue(BasicBlock *BB, Value *V)
Indicate that a rewritten value is available in the specified block with the specified value.
Definition: SSAUpdater.cpp:70
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:342
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements.
Definition: SmallPtrSet.h:427
bool empty() const
Definition: SmallVector.h:94
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
A Use represents the edge between a Value definition and its users.
Definition: Use.h:43
iterator_range< use_iterator > uses()
Definition: Value.h:376
StringRef getName() const
Return a constant reference to the value's name.
Definition: Value.cpp:309
unsigned ID
LLVM IR allows to use arbitrary numbers as calling convention identifiers.
Definition: CallingConv.h:24
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
FunctionPass * createCallBrPass()
BasicBlock * SplitKnownCriticalEdge(Instruction *TI, unsigned SuccNum, const CriticalEdgeSplittingOptions &Options=CriticalEdgeSplittingOptions(), const Twine &BBName="")
If it is known that an edge is critical, SplitKnownCriticalEdge can be called directly,...
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:163
bool isCriticalEdge(const Instruction *TI, unsigned SuccNum, bool AllowIdenticalEdges=false)
Return true if the specified edge is a critical edge.
Definition: CFG.cpp:95
iterator_range< pointer_iterator< WrappedIteratorT > > make_pointer_range(RangeT &&Range)
Definition: iterator.h:363
Option class for critical edge splitting.