1 //===-- CondPropagate.cpp - Propagate Conditional Expressions -------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass propagates information about conditional expressions through the
11 // program, allowing it to eliminate conditional branches in some cases.
13 //===----------------------------------------------------------------------===//
15 #define DEBUG_TYPE "condprop"
16 #include "llvm/Transforms/Scalar.h"
17 #include "llvm/Transforms/Utils/Local.h"
18 #include "llvm/Constants.h"
19 #include "llvm/Function.h"
20 #include "llvm/Instructions.h"
21 #include "llvm/Pass.h"
22 #include "llvm/Type.h"
23 #include "llvm/ADT/STLExtras.h"
24 #include "llvm/ADT/Statistic.h"
25 #include "llvm/Support/Compiler.h"
26 #include "llvm/Support/Streams.h"
29 STATISTIC(NumBrThread, "Number of CFG edges threaded through branches");
30 STATISTIC(NumSwThread, "Number of CFG edges threaded through switches");
33 struct VISIBILITY_HIDDEN CondProp : public FunctionPass {
34 static char ID; // Pass identification, replacement for typeid
35 CondProp() : FunctionPass((intptr_t)&ID) {}
37 virtual bool runOnFunction(Function &F);
39 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
40 AU.addRequiredID(BreakCriticalEdgesID);
41 //AU.addRequired<DominanceFrontier>();
46 void SimplifyBlock(BasicBlock *BB);
47 void SimplifyPredecessors(BranchInst *BI);
48 void SimplifyPredecessors(SwitchInst *SI);
49 void RevectorBlockTo(BasicBlock *FromBB, BasicBlock *ToBB);
52 char CondProp::ID = 0;
53 RegisterPass<CondProp> X("condprop", "Conditional Propagation");
56 FunctionPass *llvm::createCondPropagationPass() {
57 return new CondProp();
60 bool CondProp::runOnFunction(Function &F) {
61 bool EverMadeChange = false;
63 // While we are simplifying blocks, keep iterating.
66 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
68 EverMadeChange = EverMadeChange || MadeChange;
70 return EverMadeChange;
73 void CondProp::SimplifyBlock(BasicBlock *BB) {
74 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
75 // If this is a conditional branch based on a phi node that is defined in
76 // this block, see if we can simplify predecessors of this block.
77 if (BI->isConditional() && isa<PHINode>(BI->getCondition()) &&
78 cast<PHINode>(BI->getCondition())->getParent() == BB)
79 SimplifyPredecessors(BI);
81 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
82 if (isa<PHINode>(SI->getCondition()) &&
83 cast<PHINode>(SI->getCondition())->getParent() == BB)
84 SimplifyPredecessors(SI);
87 // If possible, simplify the terminator of this block.
88 if (ConstantFoldTerminator(BB))
91 // If this block ends with an unconditional branch and the only successor has
92 // only this block as a predecessor, merge the two blocks together.
93 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
94 if (BI->isUnconditional() && BI->getSuccessor(0)->getSinglePredecessor() &&
95 BB != BI->getSuccessor(0)) {
96 BasicBlock *Succ = BI->getSuccessor(0);
98 // If Succ has any PHI nodes, they are all single-entry PHI's.
99 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin())) {
100 assert(PN->getNumIncomingValues() == 1 &&
101 "PHI doesn't match parent block");
102 PN->replaceAllUsesWith(PN->getIncomingValue(0));
103 PN->eraseFromParent();
107 BI->eraseFromParent();
109 // Move over all of the instructions.
110 BB->getInstList().splice(BB->end(), Succ->getInstList());
112 // Any phi nodes that had entries for Succ now have entries from BB.
113 Succ->replaceAllUsesWith(BB);
115 // Succ is now dead, but we cannot delete it without potentially
116 // invalidating iterators elsewhere. Just insert an unreachable
117 // instruction in it.
118 new UnreachableInst(Succ);
123 // SimplifyPredecessors(branches) - We know that BI is a conditional branch
124 // based on a PHI node defined in this block. If the phi node contains constant
125 // operands, then the blocks corresponding to those operands can be modified to
126 // jump directly to the destination instead of going through this block.
127 void CondProp::SimplifyPredecessors(BranchInst *BI) {
128 // TODO: We currently only handle the most trival case, where the PHI node has
129 // one use (the branch), and is the only instruction besides the branch in the
131 PHINode *PN = cast<PHINode>(BI->getCondition());
132 if (!PN->hasOneUse()) return;
134 BasicBlock *BB = BI->getParent();
135 if (&*BB->begin() != PN || &*next(BB->begin()) != BI)
138 // Ok, we have this really simple case, walk the PHI operands, looking for
139 // constants. Walk from the end to remove operands from the end when
140 // possible, and to avoid invalidating "i".
141 for (unsigned i = PN->getNumIncomingValues(); i != 0; --i)
142 if (ConstantInt *CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i-1))) {
143 // If we have a constant, forward the edge from its current to its
144 // ultimate destination.
145 bool PHIGone = PN->getNumIncomingValues() == 2;
146 RevectorBlockTo(PN->getIncomingBlock(i-1),
147 BI->getSuccessor(CB->isZero()));
150 // If there were two predecessors before this simplification, the PHI node
151 // will be deleted. Don't iterate through it the last time.
156 // SimplifyPredecessors(switch) - We know that SI is switch based on a PHI node
157 // defined in this block. If the phi node contains constant operands, then the
158 // blocks corresponding to those operands can be modified to jump directly to
159 // the destination instead of going through this block.
160 void CondProp::SimplifyPredecessors(SwitchInst *SI) {
161 // TODO: We currently only handle the most trival case, where the PHI node has
162 // one use (the branch), and is the only instruction besides the branch in the
164 PHINode *PN = cast<PHINode>(SI->getCondition());
165 if (!PN->hasOneUse()) return;
167 BasicBlock *BB = SI->getParent();
168 if (&*BB->begin() != PN || &*next(BB->begin()) != SI)
171 bool RemovedPreds = false;
173 // Ok, we have this really simple case, walk the PHI operands, looking for
174 // constants. Walk from the end to remove operands from the end when
175 // possible, and to avoid invalidating "i".
176 for (unsigned i = PN->getNumIncomingValues(); i != 0; --i)
177 if (ConstantInt *CI = dyn_cast<ConstantInt>(PN->getIncomingValue(i-1))) {
178 // If we have a constant, forward the edge from its current to its
179 // ultimate destination.
180 bool PHIGone = PN->getNumIncomingValues() == 2;
181 unsigned DestCase = SI->findCaseValue(CI);
182 RevectorBlockTo(PN->getIncomingBlock(i-1),
183 SI->getSuccessor(DestCase));
187 // If there were two predecessors before this simplification, the PHI node
188 // will be deleted. Don't iterate through it the last time.
194 // RevectorBlockTo - Revector the unconditional branch at the end of FromBB to
195 // the ToBB block, which is one of the successors of its current successor.
196 void CondProp::RevectorBlockTo(BasicBlock *FromBB, BasicBlock *ToBB) {
197 BranchInst *FromBr = cast<BranchInst>(FromBB->getTerminator());
198 assert(FromBr->isUnconditional() && "FromBB should end with uncond br!");
200 // Get the old block we are threading through.
201 BasicBlock *OldSucc = FromBr->getSuccessor(0);
203 // OldSucc had multiple successors. If ToBB has multiple predecessors, then
204 // the edge between them would be critical, which we already took care of.
205 // If ToBB has single operand PHI node then take care of it here.
206 while (PHINode *PN = dyn_cast<PHINode>(ToBB->begin())) {
207 assert(PN->getNumIncomingValues() == 1 && "Critical Edge Found!");
208 PN->replaceAllUsesWith(PN->getIncomingValue(0));
209 PN->eraseFromParent();
212 // Update PHI nodes in OldSucc to know that FromBB no longer branches to it.
213 OldSucc->removePredecessor(FromBB);
215 // Change FromBr to branch to the new destination.
216 FromBr->setSuccessor(0, ToBB);