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