forked from OSchip/llvm-project
103 lines
4.0 KiB
C++
103 lines
4.0 KiB
C++
//===- LowerSelect.cpp - Transform select insts to branches ---------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass lowers select instructions into conditional branches for targets
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// that do not have conditional moves or that have not implemented the select
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// instruction yet.
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//
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// Note that this pass could be improved. In particular it turns every select
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// instruction into a new conditional branch, even though some common cases have
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// select instructions on the same predicate next to each other. It would be
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// better to use the same branch for the whole group of selects.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Pass.h"
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#include "llvm/Type.h"
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#include "llvm/Support/Compiler.h"
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using namespace llvm;
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namespace {
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/// LowerSelect - Turn select instructions into conditional branches.
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///
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class VISIBILITY_HIDDEN LowerSelect : public FunctionPass {
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bool OnlyFP; // Only lower FP select instructions?
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public:
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LowerSelect(bool onlyfp = false) : OnlyFP(onlyfp) {}
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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// This certainly destroys the CFG.
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// This is a cluster of orthogonal Transforms:
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AU.addPreserved<UnifyFunctionExitNodes>();
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AU.addPreservedID(PromoteMemoryToRegisterID);
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AU.addPreservedID(LowerSwitchID);
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AU.addPreservedID(LowerInvokePassID);
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AU.addPreservedID(LowerAllocationsID);
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}
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bool runOnFunction(Function &F);
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};
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RegisterPass<LowerSelect>
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X("lowerselect", "Lower select instructions to branches");
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}
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// Publically exposed interface to pass...
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const PassInfo *llvm::LowerSelectID = X.getPassInfo();
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//===----------------------------------------------------------------------===//
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// This pass converts SelectInst instructions into conditional branch and PHI
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// instructions. If the OnlyFP flag is set to true, then only floating point
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// select instructions are lowered.
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//
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FunctionPass *llvm::createLowerSelectPass(bool OnlyFP) {
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return new LowerSelect(OnlyFP);
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}
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bool LowerSelect::runOnFunction(Function &F) {
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bool Changed = false;
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for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
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if (SelectInst *SI = dyn_cast<SelectInst>(I))
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if (!OnlyFP || SI->getType()->isFloatingPoint()) {
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// Split this basic block in half right before the select instruction.
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BasicBlock *NewCont =
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BB->splitBasicBlock(I, BB->getName()+".selectcont");
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// Make the true block, and make it branch to the continue block.
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BasicBlock *NewTrue = new BasicBlock(BB->getName()+".selecttrue",
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BB->getParent(), NewCont);
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new BranchInst(NewCont, NewTrue);
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// Make the unconditional branch in the incoming block be a
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// conditional branch on the select predicate.
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BB->getInstList().erase(BB->getTerminator());
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new BranchInst(NewTrue, NewCont, SI->getCondition(), BB);
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// Create a new PHI node in the cont block with the entries we need.
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PHINode *PN = new PHINode(SI->getType(), "", NewCont->begin());
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PN->takeName(SI);
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PN->addIncoming(SI->getTrueValue(), NewTrue);
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PN->addIncoming(SI->getFalseValue(), BB);
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// Use the PHI instead of the select.
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SI->replaceAllUsesWith(PN);
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NewCont->getInstList().erase(SI);
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Changed = true;
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break; // This block is done with.
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}
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}
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return Changed;
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}
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