1 //===-- CondPropagate.cpp - Propagate Conditional Expressions -------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // 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/Constants.h"
18 #include "llvm/Function.h"
19 #include "llvm/Instructions.h"
20 #include "llvm/IntrinsicInst.h"
21 #include "llvm/Pass.h"
22 #include "llvm/Type.h"
23 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
24 #include "llvm/Transforms/Utils/Local.h"
25 #include "llvm/ADT/STLExtras.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/ADT/SmallVector.h"
28 #include "llvm/Support/Compiler.h"
29 #include "llvm/Support/Streams.h"
32 STATISTIC(NumBrThread, "Number of CFG edges threaded through branches");
33 STATISTIC(NumSwThread, "Number of CFG edges threaded through switches");
36 struct VISIBILITY_HIDDEN CondProp : public FunctionPass {
37 static char ID; // Pass identification, replacement for typeid
38 CondProp() : FunctionPass(&ID) {}
40 virtual bool runOnFunction(Function &F);
42 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
43 AU.addRequiredID(BreakCriticalEdgesID);
44 //AU.addRequired<DominanceFrontier>();
49 SmallVector<BasicBlock *, 4> DeadBlocks;
50 void SimplifyBlock(BasicBlock *BB);
51 void SimplifyPredecessors(BranchInst *BI);
52 void SimplifyPredecessors(SwitchInst *SI);
53 void RevectorBlockTo(BasicBlock *FromBB, BasicBlock *ToBB);
57 char CondProp::ID = 0;
58 static RegisterPass<CondProp> X("condprop", "Conditional Propagation");
60 FunctionPass *llvm::createCondPropagationPass() {
61 return new CondProp();
64 bool CondProp::runOnFunction(Function &F) {
65 bool EverMadeChange = false;
68 // While we are simplifying blocks, keep iterating.
71 for (Function::iterator BB = F.begin(), E = F.end(); BB != E;)
73 EverMadeChange = EverMadeChange || MadeChange;
77 while (!DeadBlocks.empty()) {
78 BasicBlock *BB = DeadBlocks.back(); DeadBlocks.pop_back();
82 return EverMadeChange;
85 void CondProp::SimplifyBlock(BasicBlock *BB) {
86 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
87 // If this is a conditional branch based on a phi node that is defined in
88 // this block, see if we can simplify predecessors of this block.
89 if (BI->isConditional() && isa<PHINode>(BI->getCondition()) &&
90 cast<PHINode>(BI->getCondition())->getParent() == BB)
91 SimplifyPredecessors(BI);
93 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
94 if (isa<PHINode>(SI->getCondition()) &&
95 cast<PHINode>(SI->getCondition())->getParent() == BB)
96 SimplifyPredecessors(SI);
99 // If possible, simplify the terminator of this block.
100 if (ConstantFoldTerminator(BB))
103 // If this block ends with an unconditional branch and the only successor has
104 // only this block as a predecessor, merge the two blocks together.
105 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
106 if (BI->isUnconditional() && BI->getSuccessor(0)->getSinglePredecessor() &&
107 BB != BI->getSuccessor(0)) {
108 BasicBlock *Succ = BI->getSuccessor(0);
110 // If Succ has any PHI nodes, they are all single-entry PHI's. Eliminate
112 FoldSingleEntryPHINodes(Succ);
115 BI->eraseFromParent();
117 // Move over all of the instructions.
118 BB->getInstList().splice(BB->end(), Succ->getInstList());
120 // Any phi nodes that had entries for Succ now have entries from BB.
121 Succ->replaceAllUsesWith(BB);
123 // Succ is now dead, but we cannot delete it without potentially
124 // invalidating iterators elsewhere. Just insert an unreachable
125 // instruction in it and delete this block later on.
126 new UnreachableInst(Succ);
127 DeadBlocks.push_back(Succ);
132 // SimplifyPredecessors(branches) - We know that BI is a conditional branch
133 // based on a PHI node defined in this block. If the phi node contains constant
134 // operands, then the blocks corresponding to those operands can be modified to
135 // jump directly to the destination instead of going through this block.
136 void CondProp::SimplifyPredecessors(BranchInst *BI) {
137 // TODO: We currently only handle the most trival case, where the PHI node has
138 // one use (the branch), and is the only instruction besides the branch and dbg
139 // intrinsics in the block.
140 PHINode *PN = cast<PHINode>(BI->getCondition());
142 if (PN->getNumIncomingValues() == 1) {
143 // Eliminate single-entry PHI nodes.
144 FoldSingleEntryPHINodes(PN->getParent());
149 if (!PN->hasOneUse()) return;
151 BasicBlock *BB = BI->getParent();
152 if (&*BB->begin() != PN)
154 BasicBlock::iterator BBI = BB->begin();
155 BasicBlock::iterator BBE = BB->end();
156 while (BBI != BBE && isa<DbgInfoIntrinsic>(++BBI)) ;
160 // Ok, we have this really simple case, walk the PHI operands, looking for
161 // constants. Walk from the end to remove operands from the end when
162 // possible, and to avoid invalidating "i".
163 for (unsigned i = PN->getNumIncomingValues(); i != 0; --i)
164 if (ConstantInt *CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i-1))) {
165 // If we have a constant, forward the edge from its current to its
166 // ultimate destination.
167 RevectorBlockTo(PN->getIncomingBlock(i-1),
168 BI->getSuccessor(CB->isZero()));
171 // If there were two predecessors before this simplification, or if the
172 // PHI node contained all the same value except for the one we just
173 // substituted, the PHI node may be deleted. Don't iterate through it the
175 if (BI->getCondition() != PN) return;
179 // SimplifyPredecessors(switch) - We know that SI is switch based on a PHI node
180 // defined in this block. If the phi node contains constant operands, then the
181 // blocks corresponding to those operands can be modified to jump directly to
182 // the destination instead of going through this block.
183 void CondProp::SimplifyPredecessors(SwitchInst *SI) {
184 // TODO: We currently only handle the most trival case, where the PHI node has
185 // one use (the branch), and is the only instruction besides the branch and
186 // dbg intrinsics in the block.
187 PHINode *PN = cast<PHINode>(SI->getCondition());
188 if (!PN->hasOneUse()) return;
190 BasicBlock *BB = SI->getParent();
191 if (&*BB->begin() != PN)
193 BasicBlock::iterator BBI = BB->begin();
194 BasicBlock::iterator BBE = BB->end();
195 while (BBI != BBE && isa<DbgInfoIntrinsic>(++BBI)) ;
199 bool RemovedPreds = false;
201 // Ok, we have this really simple case, walk the PHI operands, looking for
202 // constants. Walk from the end to remove operands from the end when
203 // possible, and to avoid invalidating "i".
204 for (unsigned i = PN->getNumIncomingValues(); i != 0; --i)
205 if (ConstantInt *CI = dyn_cast<ConstantInt>(PN->getIncomingValue(i-1))) {
206 // If we have a constant, forward the edge from its current to its
207 // ultimate destination.
208 unsigned DestCase = SI->findCaseValue(CI);
209 RevectorBlockTo(PN->getIncomingBlock(i-1),
210 SI->getSuccessor(DestCase));
214 // If there were two predecessors before this simplification, or if the
215 // PHI node contained all the same value except for the one we just
216 // substituted, the PHI node may be deleted. Don't iterate through it the
218 if (SI->getCondition() != PN) return;
223 // RevectorBlockTo - Revector the unconditional branch at the end of FromBB to
224 // the ToBB block, which is one of the successors of its current successor.
225 void CondProp::RevectorBlockTo(BasicBlock *FromBB, BasicBlock *ToBB) {
226 BranchInst *FromBr = cast<BranchInst>(FromBB->getTerminator());
227 assert(FromBr->isUnconditional() && "FromBB should end with uncond br!");
229 // Get the old block we are threading through.
230 BasicBlock *OldSucc = FromBr->getSuccessor(0);
232 // OldSucc had multiple successors. If ToBB has multiple predecessors, then
233 // the edge between them would be critical, which we already took care of.
234 // If ToBB has single operand PHI node then take care of it here.
235 FoldSingleEntryPHINodes(ToBB);
237 // Update PHI nodes in OldSucc to know that FromBB no longer branches to it.
238 OldSucc->removePredecessor(FromBB);
240 // Change FromBr to branch to the new destination.
241 FromBr->setSuccessor(0, ToBB);