1 //===-- Local.cpp - Functions to perform local transformations ------------===//
3 // This family of functions perform various local transformations to the
6 //===----------------------------------------------------------------------===//
8 #include "llvm/Transforms/Utils/Local.h"
9 #include "llvm/iTerminators.h"
10 #include "llvm/iOperators.h"
11 #include "llvm/ConstantHandling.h"
13 //===----------------------------------------------------------------------===//
14 // Local constant propagation...
17 // ConstantFoldInstruction - If an instruction references constants, try to fold
20 bool doConstantPropagation(BasicBlock::iterator &II) {
21 if (Constant *C = ConstantFoldInstruction(II)) {
22 // Replaces all of the uses of a variable with uses of the constant.
23 II->replaceAllUsesWith(C);
25 // Remove the instruction from the basic block...
26 II = II->getParent()->getInstList().erase(II);
33 // ConstantFoldTerminator - If a terminator instruction is predicated on a
34 // constant value, convert it into an unconditional branch to the constant
37 bool ConstantFoldTerminator(BasicBlock *BB) {
38 TerminatorInst *T = BB->getTerminator();
40 // Branch - See if we are conditional jumping on constant
41 if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
42 if (BI->isUnconditional()) return false; // Can't optimize uncond branch
43 BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
44 BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));
46 if (ConstantBool *Cond = dyn_cast<ConstantBool>(BI->getCondition())) {
47 // Are we branching on constant?
48 // YES. Change to unconditional branch...
49 BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2;
50 BasicBlock *OldDest = Cond->getValue() ? Dest2 : Dest1;
52 //cerr << "Function: " << T->getParent()->getParent()
53 // << "\nRemoving branch from " << T->getParent()
54 // << "\n\nTo: " << OldDest << endl;
56 // Let the basic block know that we are letting go of it. Based on this,
57 // it will adjust it's PHI nodes.
58 assert(BI->getParent() && "Terminator not inserted in block!");
59 OldDest->removePredecessor(BI->getParent());
61 // Set the unconditional destination, and change the insn to be an
62 // unconditional branch.
63 BI->setUnconditionalDest(Destination);
65 } else if (Dest2 == Dest1) { // Conditional branch to same location?
66 // This branch matches something like this:
67 // br bool %cond, label %Dest, label %Dest
68 // and changes it into: br label %Dest
70 // Let the basic block know that we are letting go of one copy of it.
71 assert(BI->getParent() && "Terminator not inserted in block!");
72 Dest1->removePredecessor(BI->getParent());
74 // Change a conditional branch to unconditional.
75 BI->setUnconditionalDest(Dest1);
78 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
79 // If we are switching on a constant, we can convert the switch into a
80 // single branch instruction!
81 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
82 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest
84 // Figure out which case it goes to...
85 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
86 // Found case matching a constant operand?
87 if (SI->getSuccessorValue(i) == CI) {
88 TheOnlyDest = SI->getSuccessor(i);
92 // Otherwise, check to see if the switch only branches to one destination.
93 // We do this by reseting "TheOnlyDest" to null when we find two non-equal
95 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0;
98 if (CI && !TheOnlyDest) {
99 // Branching on a constant, but not any of the cases, go to the default
101 TheOnlyDest = SI->getDefaultDest();
104 // If we found a single destination that we can fold the switch into, do so
107 // Insert the new branch..
108 new BranchInst(TheOnlyDest, SI);
109 BasicBlock *BB = SI->getParent();
111 // Remove entries from PHI nodes which we no longer branch to...
112 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
113 // Found case matching a constant operand?
114 BasicBlock *Succ = SI->getSuccessor(i);
115 if (Succ == TheOnlyDest)
116 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest
118 Succ->removePredecessor(BB);
121 // Delete the old switch...
122 BB->getInstList().erase(SI);
124 } else if (SI->getNumSuccessors() == 2) {
125 // Otherwise, we can fold this switch into a conditional branch
126 // instruction if it has only one non-default destination.
127 Value *Cond = new SetCondInst(Instruction::SetEQ, SI->getCondition(),
128 SI->getSuccessorValue(1), "cond", SI);
129 // Insert the new branch...
130 new BranchInst(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);
132 // Delete the old switch...
133 SI->getParent()->getInstList().erase(SI);
142 //===----------------------------------------------------------------------===//
143 // Local dead code elimination...
146 bool isInstructionTriviallyDead(Instruction *I) {
147 return I->use_empty() && !I->mayWriteToMemory() && !isa<TerminatorInst>(I);
150 // dceInstruction - Inspect the instruction at *BBI and figure out if it's
151 // [trivially] dead. If so, remove the instruction and update the iterator
152 // to point to the instruction that immediately succeeded the original
155 bool dceInstruction(BasicBlock::iterator &BBI) {
156 // Look for un"used" definitions...
157 if (isInstructionTriviallyDead(BBI)) {
158 BBI = BBI->getParent()->getInstList().erase(BBI); // Bye bye