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
diff --git a/lib/CodeGen/DeadMachineInstructionElim.cpp b/lib/CodeGen/DeadMachineInstructionElim.cpp
new file mode 100644
index 0000000..a3bba61
--- /dev/null
+++ b/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;
+}