Add a new MachineInstr-level DCE pass. It is very simple, and is intended to

be used with fast-isel.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56268 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/LinkAllCodegenComponents.h

@@ -29,6 +29,8 @@ namespace {
       if (std::getenv("bar") != (char*) -1)
         return;
 
+      (void) llvm::createDeadMachineInstructionElimPass();
+
       (void) llvm::createSimpleRegisterAllocator();
       (void) llvm::createLocalRegisterAllocator();
       (void) llvm::createBigBlockRegisterAllocator();

include/llvm/CodeGen/Passes.h

@@ -75,6 +75,11 @@ namespace llvm {
   /// machine basic blocks.
   extern const PassInfo *const UnreachableMachineBlockElimID;
 
+  /// DeadMachineInstructionElim pass - This pass removes dead machine
+  /// instructions.
+  ///
+  FunctionPass *createDeadMachineInstructionElimPass();
+
   /// Creates a register allocator as the user specified on the command line.
   ///
   FunctionPass *createRegisterAllocator();

lib/CodeGen/DeadMachineInstructionElim.cpp

@@ -0,0 +1,99 @@
+//===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This is an extremely simple MachineInstr-level dead-code-elimination pass.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/Pass.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetMachine.h"
+using namespace llvm;
+
+namespace {
+  class VISIBILITY_HIDDEN DeadMachineInstructionElim : 
+        public MachineFunctionPass {
+    virtual bool runOnMachineFunction(MachineFunction &MF);
+    
+  public:
+    static char ID; // Pass identification, replacement for typeid
+    DeadMachineInstructionElim() : MachineFunctionPass(&ID) {}
+  };
+}
+char DeadMachineInstructionElim::ID = 0;
+
+static RegisterPass<DeadMachineInstructionElim>
+Y("dead-mi-elimination",
+  "Remove dead machine instructions");
+
+FunctionPass *llvm::createDeadMachineInstructionElimPass() {
+  return new DeadMachineInstructionElim();
+}
+
+bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
+  bool AnyChanges = false;
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+  const TargetInstrInfo &TII = *MF.getTarget().getInstrInfo();
+  bool SawStore = true;
+
+  // Loop over all instructions in all blocks, from bottom to top, so that it's
+  // more likely that chains of dependent but ultimately dead instructions will
+  // be cleaned up.
+  for (MachineFunction::reverse_iterator I = MF.rbegin(), E = MF.rend();
+       I != E; ++I) {
+    MachineBasicBlock *MBB = &*I;
+    for (MachineBasicBlock::reverse_iterator MII = MBB->rbegin(),
+         MIE = MBB->rend(); MII != MIE; ) {
+      MachineInstr *MI = &*MII;
+
+      // Don't delete instructions with side effects.
+      if (MI->isSafeToMove(&TII, SawStore)) {
+        // Examine each operand.
+        bool AllDefsDead = true;
+        for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+          const MachineOperand &MO = MI->getOperand(i);
+          if (MO.isRegister() && MO.isDef()) {
+            unsigned Reg = MO.getReg();
+            if (TargetRegisterInfo::isPhysicalRegister(Reg) ||
+                !MRI.use_empty(Reg)) {
+              // This def has a use. Don't delete the instruction!
+              AllDefsDead = false;
+              break;
+            }
+          }
+        }
+
+        // If there are no defs with uses, the instruction is dead.
+        if (AllDefsDead) {
+          // Clear out the operands to take the registers out of their
+          // use chains.
+          while (unsigned Num = MI->getNumOperands())
+            MI->RemoveOperand(Num-1);
+
+          // Delete the actual instruction.
+          AnyChanges = true;
+          MI->eraseFromParent();
+          MIE = MBB->rend();
+          // MII is now pointing to the next instruction to process,
+          // so don't increment it.
+          continue;
+        }
+      }
+      // We didn't delete the current instruction, so increment MII to
+      // the next one.
+      ++MII;
+    }
+  }
+
+  return AnyChanges;
+}