1 //===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
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 forwards branches to unconditional branches to make them branch
11 // directly to the target block. This pass often results in dead MBB's, which
14 // Note that this pass must be run after register allocation, it cannot handle
17 //===----------------------------------------------------------------------===//
19 #include "llvm/CodeGen/Passes.h"
20 #include "llvm/CodeGen/MachineFunctionPass.h"
21 #include "llvm/Target/TargetInstrInfo.h"
22 #include "llvm/Target/TargetMachine.h"
23 #include "llvm/ADT/STLExtras.h"
27 struct BranchFolder : public MachineFunctionPass {
28 virtual bool runOnMachineFunction(MachineFunction &MF);
29 virtual const char *getPassName() const { return "Control Flow Optimizer"; }
30 const TargetInstrInfo *TII;
33 void OptimizeBlock(MachineFunction::iterator MBB);
37 FunctionPass *llvm::createBranchFoldingPass() { return new BranchFolder(); }
39 bool BranchFolder::runOnMachineFunction(MachineFunction &MF) {
40 TII = MF.getTarget().getInstrInfo();
41 if (!TII) return false;
49 bool EverMadeChange = false;
53 for (MachineFunction::iterator MBB = ++MF.begin(), E = MF.end(); MBB != E;
57 // If branches were folded away somehow, do a quick scan and delete any dead
60 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
61 MachineBasicBlock *MBB = I++;
63 if (MBB->pred_empty()) {
64 // drop all successors.
65 while (!MBB->succ_empty())
66 MBB->removeSuccessor(MBB->succ_end()-1);
67 MF.getBasicBlockList().erase(MBB);
72 EverMadeChange |= MadeChange;
75 return EverMadeChange;
78 /// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to
79 /// 'Old', change the code and CFG so that it branches to 'New' instead.
80 static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB,
81 MachineBasicBlock *Old,
82 MachineBasicBlock *New,
83 const TargetInstrInfo *TII) {
84 assert(Old != New && "Cannot replace self with self!");
86 MachineBasicBlock::iterator I = BB->end();
87 while (I != BB->begin()) {
89 if (!TII->isTerminatorInstr(I->getOpcode())) break;
91 // Scan the operands of this machine instruction, replacing any uses of Old
93 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
94 if (I->getOperand(i).isMachineBasicBlock() &&
95 I->getOperand(i).getMachineBasicBlock() == Old)
96 I->getOperand(i).setMachineBasicBlock(New);
99 // Update the successor information.
100 std::vector<MachineBasicBlock*> Succs(BB->succ_begin(), BB->succ_end());
101 for (int i = Succs.size()-1; i >= 0; --i)
102 if (Succs[i] == Old) {
103 BB->removeSuccessor(Old);
104 BB->addSuccessor(New);
108 /// OptimizeBlock - Analyze and optimize control flow related to the specified
109 /// block. This is never called on the entry block.
110 void BranchFolder::OptimizeBlock(MachineFunction::iterator MBB) {
111 // If this block is empty, make everyone use its fall-through, not the block
114 if (MBB->pred_empty()) return; // dead block? Leave for cleanup later.
116 MachineFunction::iterator FallThrough = next(MBB);
118 if (FallThrough != MBB->getParent()->end()) {
119 while (!MBB->pred_empty()) {
120 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
121 ReplaceUsesOfBlockWith(Pred, MBB, FallThrough, TII);
125 // TODO: CHANGE STUFF TO NOT BRANCH HERE!
129 // Check to see if we can simplify the terminator of the block before this
132 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
133 std::vector<MachineOperand> PriorCond;
134 if (!TII->AnalyzeBranch(*prior(MBB), PriorTBB, PriorFBB, PriorCond)) {
135 // If the previous branch is conditional and both conditions go to the same
136 // destination, remove the branch, replacing it with an unconditional one.
137 if (PriorTBB && PriorTBB == PriorFBB) {
138 TII->RemoveBranch(*prior(MBB));
140 if (PriorTBB != &*MBB)
141 TII->InsertBranch(*prior(MBB), PriorTBB, 0, PriorCond);
143 return OptimizeBlock(MBB);
146 // If the previous branch *only* branches to *this* block (conditional or
147 // not) remove the branch.
148 if (PriorTBB == &*MBB && PriorFBB == 0) {
149 TII->RemoveBranch(*prior(MBB));
151 return OptimizeBlock(MBB);
159 if (MBB->pred_size() == 1) {
160 // If this block has a single predecessor, and if that block has a single
161 // successor, merge this block into that block.
162 MachineBasicBlock *Pred = *MBB->pred_begin();
163 if (Pred->succ_size() == 1) {
164 // Delete all of the terminators from end of the pred block. NOTE, this
165 // assumes that terminators do not have side effects!
166 // FIXME: This doesn't work for FP_REG_KILL.
168 while (!Pred->empty() && TII.isTerminatorInstr(Pred->back().getOpcode()))
171 // Splice the instructions over.
172 Pred->splice(Pred->end(), MBB, MBB->begin(), MBB->end());
174 // If MBB does not end with a barrier, add a goto instruction to the end.
175 if (Pred->empty() || !TII.isBarrier(Pred->back().getOpcode()))
176 TII.insertGoto(*Pred, *next(MBB));
178 // Update the CFG now.
179 Pred->removeSuccessor(Pred->succ_begin());
180 while (!MBB->succ_empty()) {
181 Pred->addSuccessor(*(MBB->succ_end()-1));
182 MBB->removeSuccessor(MBB->succ_end()-1);
188 // If BB falls through into Old, insert an unconditional branch to New.
189 MachineFunction::iterator BBSucc = BB; ++BBSucc;
190 if (BBSucc != BB->getParent()->end() && &*BBSucc == Old)
191 TII.insertGoto(*BB, *New);
194 if (MBB->pred_size() == 1) {
195 // If this block has a single predecessor, and if that block has a single
196 // successor, merge this block into that block.
197 MachineBasicBlock *Pred = *MBB->pred_begin();
198 if (Pred->succ_size() == 1) {
199 // Delete all of the terminators from end of the pred block. NOTE, this
200 // assumes that terminators do not have side effects!
201 // FIXME: This doesn't work for FP_REG_KILL.
203 while (!Pred->empty() && TII.isTerminatorInstr(Pred->back().getOpcode()))
206 // Splice the instructions over.
207 Pred->splice(Pred->end(), MBB, MBB->begin(), MBB->end());
209 // If MBB does not end with a barrier, add a goto instruction to the end.
210 if (Pred->empty() || !TII.isBarrier(Pred->back().getOpcode()))
211 TII.insertGoto(*Pred, *next(MBB));
213 // Update the CFG now.
214 Pred->removeSuccessor(Pred->succ_begin());
215 while (!MBB->succ_empty()) {
216 Pred->addSuccessor(*(MBB->succ_end()-1));
217 MBB->removeSuccessor(MBB->succ_end()-1);
223 // If the first instruction in this block is an unconditional branch, and if
224 // there are predecessors, fold the branch into the predecessors.
225 if (!MBB->pred_empty() && isUncondBranch(MBB->begin(), TII)) {
226 MachineInstr *Br = MBB->begin();
227 assert(Br->getNumOperands() == 1 && Br->getOperand(0).isMachineBasicBlock()
228 && "Uncond branch should take one MBB argument!");
229 MachineBasicBlock *Dest = Br->getOperand(0).getMachineBasicBlock();
231 while (!MBB->pred_empty()) {
232 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
233 ReplaceUsesOfBlockWith(Pred, MBB, Dest, TII);
238 // If the last instruction is an unconditional branch and the fall through
239 // block is the destination, just delete the branch.
240 if (isUncondBranch(--MBB->end(), TII)) {
241 MachineBasicBlock::iterator MI = --MBB->end();
242 MachineInstr *UncondBr = MI;
243 MachineFunction::iterator FallThrough = next(MBB);
245 MachineFunction::iterator UncondDest =
246 MI->getOperand(0).getMachineBasicBlock();
247 if (UncondDest == FallThrough) {
248 // Just delete the branch. This does not effect the CFG.
249 MBB->erase(UncondBr);
253 // Okay, so we don't have a fall-through. Check to see if we have an
254 // conditional branch that would be a fall through if we reversed it. If
255 // so, invert the condition and delete the uncond branch.
256 if (MI != MBB->begin() && isCondBranch(--MI, TII)) {
257 // We assume that conditional branches always have the branch dest as the
258 // last operand. This could be generalized in the future if needed.
259 unsigned LastOpnd = MI->getNumOperands()-1;
260 if (MachineFunction::iterator(
261 MI->getOperand(LastOpnd).getMachineBasicBlock()) == FallThrough) {
262 // Change the cond branch to go to the uncond dest, nuke the uncond,
263 // then reverse the condition.
264 MI->getOperand(LastOpnd).setMachineBasicBlock(UncondDest);
265 MBB->erase(UncondBr);
266 TII.reverseBranchCondition(MI);