1 files changed, 112 insertions, 0 deletions
diff --git a/llvm/lib/CodeGen/OptimizeCmps.cpp b/llvm/lib/CodeGen/OptimizeCmps.cpp
new file mode 100644
index 00000000000..cd396cc2eae
--- /dev/null
+++ b/llvm/lib/CodeGen/OptimizeCmps.cpp
@@ -0,0 +1,112 @@
+//===-- OptimizeCmps.cpp - Optimize comparison instrs ---------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass performs optimization of comparison instructions. For instance, in
+// this code:
+//
+//     sub r1, 1
+//     cmp r1, 0
+//     bz  L1
+//
+// If the "sub" instruction all ready sets (or could be modified to set) the
+// same flag that the "cmp" instruction sets and that "bz" uses, then we can
+// eliminate the "cmp" instruction.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "opt-compares"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetRegisterInfo.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/ADT/Statistic.h"
+using namespace llvm;
+
+STATISTIC(NumEliminated, "Number of compares eliminated");
+
+static cl::opt<bool>
+EnableOptCmps("enable-optimize-cmps", cl::init(false), cl::Hidden);
+
+namespace {
+  class OptimizeCmps : public MachineFunctionPass {
+    const TargetMachine   *TM;
+    const TargetInstrInfo *TII;
+    MachineRegisterInfo   *MRI;
+
+    bool OptimizeCmpInstr(MachineInstr *MI, MachineBasicBlock *MBB);
+
+  public:
+    static char ID; // Pass identification
+    OptimizeCmps() : MachineFunctionPass(&ID) {}
+
+    virtual bool runOnMachineFunction(MachineFunction &MF);
+
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.setPreservesCFG();
+      MachineFunctionPass::getAnalysisUsage(AU);
+    }
+  };
+}
+
+char OptimizeCmps::ID = 0;
+INITIALIZE_PASS(OptimizeCmps, "opt-cmps",
+                "Optimize comparison instrs", false, false);
+
+FunctionPass *llvm::createOptimizeCmpsPass() { return new OptimizeCmps(); }
+
+/// OptimizeCmpInstr - If the instruction is a compare and the previous
+/// instruction it's comparing against all ready sets (or could be modified to
+/// set) the same flag as the compare, then we can remove the comparison and use
+/// the flag from the previous instruction.
+bool OptimizeCmps::OptimizeCmpInstr(MachineInstr *MI, MachineBasicBlock *MBB) {
+  // If this instruction is a comparison against zero and isn't comparing a
+  // physical register, we can try to optimize it.
+  unsigned SrcReg;
+  int CmpValue;
+  if (!TII->isCompareInstr(MI, SrcReg, CmpValue) ||
+      TargetRegisterInfo::isPhysicalRegister(SrcReg) ||
+      CmpValue != 0)
+    return false;
+
+  MachineRegisterInfo::def_iterator DI = MRI->def_begin(SrcReg);
+  if (llvm::next(DI) != MRI->def_end())
+    // Only support one definition.
+    return false;
+
+  // Attempt to convert the defining instruction to set the "zero" flag.
+  if (TII->convertToSetZeroFlag(&*DI, MI)) {
+    ++NumEliminated;
+    return true;
+  }
+
+  return false;
+}
+
+bool OptimizeCmps::runOnMachineFunction(MachineFunction &MF) {
+  TM = &MF.getTarget();
+  TII = TM->getInstrInfo();
+  MRI = &MF.getRegInfo();
+
+  if (!EnableOptCmps) return false;
+
+  bool Changed = false;
+  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
+    MachineBasicBlock *MBB = &*I;
+    for (MachineBasicBlock::iterator
+           MII = MBB->begin(), ME = MBB->end(); MII != ME; ) {
+      MachineInstr *MI = &*MII++;
+      Changed |= OptimizeCmpInstr(MI, MBB);
+    }
+  }
+
+  return Changed;
+}