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/Transforms/Utils/UnifyFunctionExitNodes.h"
23 #include "llvm/Function.h"
24 #include "llvm/Instructions.h"
25 #include "llvm/Pass.h"
26 #include "llvm/Type.h"
30 /// LowerSelect - Turn select instructions into conditional branches.
32 class LowerSelect : public FunctionPass {
33 bool OnlyFP; // Only lower FP select instructions?
35 LowerSelect(bool onlyfp = false) : OnlyFP(onlyfp) {}
37 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
38 // This certainly destroys the CFG.
39 // This is a cluster of orthogonal Transforms:
40 AU.addPreserved<UnifyFunctionExitNodes>();
41 AU.addPreservedID(PromoteMemoryToRegisterID);
42 AU.addPreservedID(LowerSwitchID);
43 AU.addPreservedID(LowerInvokePassID);
44 AU.addPreservedID(LowerAllocationsID);
47 bool runOnFunction(Function &F);
50 RegisterPass<LowerSelect>
51 X("lowerselect", "Lower select instructions to branches");
54 // Publically exposed interface to pass...
55 const PassInfo *llvm::LowerSelectID = X.getPassInfo();
56 //===----------------------------------------------------------------------===//
57 // This pass converts SelectInst instructions into conditional branch and PHI
58 // instructions. If the OnlyFP flag is set to true, then only floating point
59 // select instructions are lowered.
61 FunctionPass *llvm::createLowerSelectPass(bool OnlyFP) {
62 return new LowerSelect(OnlyFP);
66 bool LowerSelect::runOnFunction(Function &F) {
68 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
69 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
70 if (SelectInst *SI = dyn_cast<SelectInst>(I))
71 if (!OnlyFP || SI->getType()->isFloatingPoint()) {
72 // Split this basic block in half right before the select instruction.
74 BB->splitBasicBlock(I, BB->getName()+".selectcont");
76 // Make the true block, and make it branch to the continue block.
77 BasicBlock *NewTrue = new BasicBlock(BB->getName()+".selecttrue",
78 BB->getParent(), NewCont);
79 new BranchInst(NewCont, NewTrue);
81 // Make the unconditional branch in the incoming block be a
82 // conditional branch on the select predicate.
83 BB->getInstList().erase(BB->getTerminator());
84 new BranchInst(NewTrue, NewCont, SI->getCondition(), BB);
86 // Create a new PHI node in the cont block with the entries we need.
87 std::string Name = SI->getName(); SI->setName("");
88 PHINode *PN = new PHINode(SI->getType(), Name, NewCont->begin());
89 PN->addIncoming(SI->getTrueValue(), NewTrue);
90 PN->addIncoming(SI->getFalseValue(), BB);
92 // Use the PHI instead of the select.
93 SI->replaceAllUsesWith(PN);
94 NewCont->getInstList().erase(SI);
97 break; // This block is done with.