[llvm-commits] CVS: llvm/lib/Transforms/Scalar/LowerSelect.cpp

[Chris Lattner]

Changes in directory llvm/lib/Transforms/Scalar:

LowerSelect.cpp added (r1.1)

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Log message:

Add a simple select instruction lowering pass



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Diffs of the changes:  (+95 -0)

Index: llvm/lib/Transforms/Scalar/LowerSelect.cpp
diff -c /dev/null llvm/lib/Transforms/Scalar/LowerSelect.cpp:1.1
*** /dev/null	Tue Mar 30 12:41:20 2004
--- llvm/lib/Transforms/Scalar/LowerSelect.cpp	Tue Mar 30 12:41:10 2004
***************
*** 0 ****
--- 1,95 ----
+ //===- LowerSelect.cpp - Transform select insts to branches ---------------===//
+ // 
+ //                     The LLVM Compiler Infrastructure
+ //
+ // This file was developed by the LLVM research group and is distributed under
+ // the University of Illinois Open Source License. See LICENSE.TXT for details.
+ // 
+ //===----------------------------------------------------------------------===//
+ //
+ // This pass lowers select instructions into conditional branches for targets
+ // that do not have conditional moves or that have not implemented the select
+ // instruction yet.
+ //
+ // Note that this pass could be improved.  In particular it turns every select
+ // instruction into a new conditional branch, even though some common cases have
+ // select instructions on the same predicate next to each other.  It would be
+ // better to use the same branch for the whole group of selects.
+ //
+ //===----------------------------------------------------------------------===//
+ 
+ #include "llvm/Transforms/Scalar.h"
+ #include "llvm/Function.h"
+ #include "llvm/Instructions.h"
+ #include "llvm/Pass.h"
+ #include "llvm/Type.h"
+ #include "Support/Statistic.h"
+ using namespace llvm;
+ 
+ namespace {
+   Statistic<> NumLowered("lowerselect","Number of select instructions lowered");
+ 
+   /// LowerSelect - Turn select instructions into conditional branches.
+   ///
+   class LowerSelect : public FunctionPass {
+     bool OnlyFP;   // Only lower FP select instructions?
+   public:
+     LowerSelect(bool onlyfp = false) : OnlyFP(onlyfp) {}
+ 
+     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+       // Doesn't really preserve anything.  It can certainly destroy the CFG.
+     }
+ 
+     bool runOnFunction(Function &F);
+   };
+ 
+   RegisterOpt<LowerSelect>
+   X("lowerselect", "Lower select instructions to branches");
+ }
+ 
+ //===----------------------------------------------------------------------===//
+ // This pass converts SelectInst instructions into conditional branch and PHI
+ // instructions.  If the OnlyFP flag is set to true, then only floating point
+ // select instructions are lowered.
+ //
+ FunctionPass *llvm::createLowerSelectPass(bool OnlyFP) {
+   return new LowerSelect(OnlyFP);
+ }
+ 
+ 
+ bool LowerSelect::runOnFunction(Function &F) {
+   bool Changed = false;
+   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
+     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
+       if (SelectInst *SI = dyn_cast<SelectInst>(I))
+         if (!OnlyFP || SI->getType()->isFloatingPoint()) {
+           // Split this basic block in half right before the select instruction.
+           BasicBlock *NewCont =
+             BB->splitBasicBlock(I, BB->getName()+".selectcont");
+ 
+           // Make the true block, and make it branch to the continue block.
+           BasicBlock *NewTrue = new BasicBlock(BB->getName()+".selecttrue",
+                                                BB->getParent(), NewCont);
+           new BranchInst(NewCont, NewTrue);
+ 
+           // Make the unconditional branch in the incoming block be a
+           // conditional branch on the select predicate.
+           BB->getInstList().erase(BB->getTerminator());
+           new BranchInst(NewTrue, NewCont, SI->getCondition(), BB);
+ 
+           // Create a new PHI node in the cont block with the entries we need.
+           std::string Name = SI->getName(); SI->setName("");
+           PHINode *PN = new PHINode(SI->getType(), Name, NewCont->begin());
+           PN->addIncoming(SI->getTrueValue(), NewTrue);
+           PN->addIncoming(SI->getFalseValue(), BB);
+ 
+           // Use the PHI instead of the select.
+           SI->replaceAllUsesWith(PN);
+           NewCont->getInstList().erase(SI);
+         
+           Changed = true;
+           break; // This block is done with.
+         }
+     }
+   return Changed;
+ }