[CodeGen] NFC: Move isDead to MachineInstr (#123531)

Provide isDead interface for access to ad-hoc isDead queries.
LivePhysRegs is optional: if not provided, pessimistically check
deadness of a single MI without doing the LivePhysReg walk; if provided
it is assumed to be at the position of MI.

Author: jrbyrnes

llvm/include/llvm/CodeGen/MachineInstr.h

@@ -46,6 +46,7 @@ class BatchAAResults;
 template <typename T> class ArrayRef;
 class DIExpression;
 class DILocalVariable;
+class LiveRegUnits;
 class MachineBasicBlock;
 class MachineFunction;
 class MachineRegisterInfo;
@@ -1744,6 +1745,18 @@ class MachineInstr
   /// defined registers were dead.
   bool wouldBeTriviallyDead() const;
 
+  /// Check whether an MI is dead. If \p LivePhysRegs is provided, it is assumed
+  /// to be at the position of MI and will be used to check the Liveness of
+  /// physical register defs. If \p LivePhysRegs is not provided, this will
+  /// pessimistically assume any PhysReg def is live.
+  /// For trivially dead instructions (i.e. those without hard to model effects
+  /// / wouldBeTriviallyDead), this checks deadness by analyzing defs of the
+  /// MachineInstr. If the instruction wouldBeTriviallyDead, and  all the defs
+  /// either have dead flags or have no uses, then the instruction is said to be
+  /// dead.
+  bool isDead(const MachineRegisterInfo &MRI,
+              LiveRegUnits *LivePhysRegs = nullptr) const;
+
   /// Returns true if this instruction's memory access aliases the memory
   /// access of Other.
   //

llvm/lib/CodeGen/DeadMachineInstructionElim.cpp

@@ -38,7 +38,6 @@ class DeadMachineInstructionElimImpl {
   bool runImpl(MachineFunction &MF);
 
 private:
-  bool isDead(const MachineInstr *MI) const;
   bool eliminateDeadMI(MachineFunction &MF);
 };
 
@@ -79,47 +78,6 @@ char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID;
 INITIALIZE_PASS(DeadMachineInstructionElim, DEBUG_TYPE,
                 "Remove dead machine instructions", false, false)
 
-bool DeadMachineInstructionElimImpl::isDead(const MachineInstr *MI) const {
-  // Instructions without side-effects are dead iff they only define dead regs.
-  // This function is hot and this loop returns early in the common case,
-  // so only perform additional checks before this if absolutely necessary.
-  for (const MachineOperand &MO : MI->all_defs()) {
-    Register Reg = MO.getReg();
-    if (Reg.isPhysical()) {
-      // Don't delete live physreg defs, or any reserved register defs.
-      if (!LivePhysRegs.available(Reg) || MRI->isReserved(Reg))
-        return false;
-    } else {
-      if (MO.isDead()) {
-#ifndef NDEBUG
-        // Basic check on the register. All of them should be 'undef'.
-        for (auto &U : MRI->use_nodbg_operands(Reg))
-          assert(U.isUndef() && "'Undef' use on a 'dead' register is found!");
-#endif
-        continue;
-      }
-      for (const MachineInstr &Use : MRI->use_nodbg_instructions(Reg)) {
-        if (&Use != MI)
-          // This def has a non-debug use. Don't delete the instruction!
-          return false;
-      }
-    }
-  }
-
-  // Technically speaking inline asm without side effects and no defs can still
-  // be deleted. But there is so much bad inline asm code out there, we should
-  // let them be.
-  if (MI->isInlineAsm())
-    return false;
-
-  // FIXME: See issue #105950 for why LIFETIME markers are considered dead here.
-  if (MI->isLifetimeMarker())
-    return true;
-
-  // If there are no defs with uses, the instruction might be dead.
-  return MI->wouldBeTriviallyDead();
-}
-
 bool DeadMachineInstructionElimImpl::runImpl(MachineFunction &MF) {
   MRI = &MF.getRegInfo();
 
@@ -146,7 +104,7 @@ bool DeadMachineInstructionElimImpl::eliminateDeadMI(MachineFunction &MF) {
     // liveness as we go.
     for (MachineInstr &MI : make_early_inc_range(reverse(*MBB))) {
       // If the instruction is dead, delete it!
-      if (isDead(&MI)) {
+      if (MI.isDead(*MRI, &LivePhysRegs)) {
         LLVM_DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << MI);
         // It is possible that some DBG_VALUE instructions refer to this
         // instruction. They will be deleted in the live debug variable
@@ -156,11 +114,9 @@ bool DeadMachineInstructionElimImpl::eliminateDeadMI(MachineFunction &MF) {
         ++NumDeletes;
         continue;
       }
-
       LivePhysRegs.stepBackward(MI);
     }
   }
-
   LivePhysRegs.clear();
   return AnyChanges;
 }

llvm/lib/CodeGen/MachineInstr.cpp

@@ -18,6 +18,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/MemoryLocation.h"
+#include "llvm/CodeGen/LiveRegUnits.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
@@ -1350,6 +1351,43 @@ bool MachineInstr::wouldBeTriviallyDead() const {
   return isPHI() || isSafeToMove(SawStore);
 }
 
+bool MachineInstr::isDead(const MachineRegisterInfo &MRI,
+                          LiveRegUnits *LivePhysRegs) const {
+  // Technically speaking inline asm without side effects and no defs can still
+  // be deleted. But there is so much bad inline asm code out there, we should
+  // let them be.
+  if (isInlineAsm())
+    return false;
+
+  // If we suspect this instruction may have some side-effects, then we say
+  // this instruction cannot be dead.
+  // FIXME: See issue #105950 for why LIFETIME markers are considered dead here.
+  if (!isLifetimeMarker() && !wouldBeTriviallyDead())
+    return false;
+
+  // Instructions without side-effects are dead iff they only define dead regs.
+  // This function is hot and this loop returns early in the common case,
+  // so only perform additional checks before this if absolutely necessary.
+  for (const MachineOperand &MO : all_defs()) {
+    Register Reg = MO.getReg();
+    if (Reg.isPhysical()) {
+      // Don't delete live physreg defs, or any reserved register defs.
+      if (!LivePhysRegs || !LivePhysRegs->available(Reg) || MRI.isReserved(Reg))
+        return false;
+    } else {
+      if (MO.isDead())
+        continue;
+      for (const MachineInstr &Use : MRI.use_nodbg_instructions(Reg)) {
+        if (&Use != this)
+          // This def has a non-debug use. Don't delete the instruction!
+          return false;
+      }
+    }
+  }
+
+  return true;
+}
+
 static bool MemOperandsHaveAlias(const MachineFrameInfo &MFI,
                                  BatchAAResults *AA, bool UseTBAA,
                                  const MachineMemOperand *MMOa,