Add a simple select instruction lowering pass

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12540 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Transforms/Scalar.h

@@ -254,6 +254,12 @@ FunctionPass *createLowerAllocationsPass();
 //
 FunctionPass *createLowerSwitchPass();
 
+//===----------------------------------------------------------------------===//
+// 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 *createLowerSelectPass(bool OnlyFP = false);
 
 //===----------------------------------------------------------------------===//
 // This pass converts 'invoke' instructions calls, and 'unwind' instructions

lib/Transforms/Utils/LowerSelect.cpp

@@ -0,0 +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;
+}