1 //===- LowerSelect.cpp - Transform select insts to branches ---------------===//
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 lowers select instructions into conditional branches for targets
11 // that do not have conditional moves or that have not implemented the select
14 // Note that this pass could be improved. In particular it turns every select
15 // instruction into a new conditional branch, even though some common cases have
16 // select instructions on the same predicate next to each other. It would be
17 // better to use the same branch for the whole group of selects.
19 //===----------------------------------------------------------------------===//
21 #include "llvm/Transforms/Scalar.h"
22 #include "llvm/Function.h"
23 #include "llvm/Instructions.h"
24 #include "llvm/Pass.h"
25 #include "llvm/Type.h"
26 #include "Support/Statistic.h"
30 Statistic<> NumLowered("lowerselect","Number of select instructions lowered");
32 /// LowerSelect - Turn select instructions into conditional branches.
34 class LowerSelect : public FunctionPass {
35 bool OnlyFP; // Only lower FP select instructions?
37 LowerSelect(bool onlyfp = false) : OnlyFP(onlyfp) {}
39 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
40 // Doesn't really preserve anything. It can certainly destroy the CFG.
43 bool runOnFunction(Function &F);
46 RegisterOpt<LowerSelect>
47 X("lowerselect", "Lower select instructions to branches");
50 //===----------------------------------------------------------------------===//
51 // This pass converts SelectInst instructions into conditional branch and PHI
52 // instructions. If the OnlyFP flag is set to true, then only floating point
53 // select instructions are lowered.
55 FunctionPass *llvm::createLowerSelectPass(bool OnlyFP) {
56 return new LowerSelect(OnlyFP);
60 bool LowerSelect::runOnFunction(Function &F) {
62 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
63 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
64 if (SelectInst *SI = dyn_cast<SelectInst>(I))
65 if (!OnlyFP || SI->getType()->isFloatingPoint()) {
66 // Split this basic block in half right before the select instruction.
68 BB->splitBasicBlock(I, BB->getName()+".selectcont");
70 // Make the true block, and make it branch to the continue block.
71 BasicBlock *NewTrue = new BasicBlock(BB->getName()+".selecttrue",
72 BB->getParent(), NewCont);
73 new BranchInst(NewCont, NewTrue);
75 // Make the unconditional branch in the incoming block be a
76 // conditional branch on the select predicate.
77 BB->getInstList().erase(BB->getTerminator());
78 new BranchInst(NewTrue, NewCont, SI->getCondition(), BB);
80 // Create a new PHI node in the cont block with the entries we need.
81 std::string Name = SI->getName(); SI->setName("");
82 PHINode *PN = new PHINode(SI->getType(), Name, NewCont->begin());
83 PN->addIncoming(SI->getTrueValue(), NewTrue);
84 PN->addIncoming(SI->getFalseValue(), BB);
86 // Use the PHI instead of the select.
87 SI->replaceAllUsesWith(PN);
88 NewCont->getInstList().erase(SI);
91 break; // This block is done with.