[llvm] [DeadMachineInstructionElim] NFC: Provide DeadMachineInstructionInfo (PR #121818)

[Jeffrey Byrnes via llvm-commits]

https://github.com/jrbyrnes created https://github.com/llvm/llvm-project/pull/121818

Provide DeadMachineInstructionInfo interface so independent passes have access to ad-hoc isDead queries / eliminateDeadMI walks. LivePhysRegs is optional in isDead for the case where a user wants to pessimistically check deadness of a single MI without doing the LivePhysReg walk.

>From 049f741bff249c12057d6b931f16b3328b238aba Mon Sep 17 00:00:00 2001
From: Jeffrey Byrnes <Jeffrey.Byrnes at amd.com>
Date: Mon, 6 Jan 2025 10:16:37 -0800
Subject: [PATCH] [DeadMachineInstructionElim] NFC: Provide
 DeadMachineInstructionInfo

Change-Id: I01bc6daa07c9d446f072b526f0067a0b93e6b526
---
 .../llvm/CodeGen/DeadMachineInstructionElim.h | 20 ++++++++
 .../CodeGen/DeadMachineInstructionElim.cpp    | 48 +++++++++----------
 2 files changed, 42 insertions(+), 26 deletions(-)

diff --git a/llvm/include/llvm/CodeGen/DeadMachineInstructionElim.h b/llvm/include/llvm/CodeGen/DeadMachineInstructionElim.h
index 56cfa1e087181a..22243515e93d98 100644
--- a/llvm/include/llvm/CodeGen/DeadMachineInstructionElim.h
+++ b/llvm/include/llvm/CodeGen/DeadMachineInstructionElim.h
@@ -9,10 +9,30 @@
 #ifndef LLVM_CODEGEN_DEADMACHINEINSTRUCTIONELIM_H
 #define LLVM_CODEGEN_DEADMACHINEINSTRUCTIONELIM_H
 
+#include "llvm/CodeGen/LiveRegUnits.h"
 #include "llvm/CodeGen/MachinePassManager.h"
 
 namespace llvm {
 
+class DeadMachineInstructionInfo {
+private:
+  const MachineRegisterInfo *MRI = nullptr;
+
+public:
+  /// Check whether \p 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.
+  bool isDead(const MachineInstr *MI,
+              LiveRegUnits *LivePhysRegs = nullptr) const;
+
+  /// Do a function walk over \p MF and delete any dead MachineInstrs. Uses
+  /// LivePhysRegs to track liveness of PhysRegs.
+  bool eliminateDeadMI(MachineFunction &MF);
+
+  DeadMachineInstructionInfo(const MachineRegisterInfo *MRI) : MRI(MRI) {}
+};
+
 class DeadMachineInstructionElimPass
     : public PassInfoMixin<DeadMachineInstructionElimPass> {
 public:
diff --git a/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp b/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
index 2f17c04487de7c..974d0706d75ff5 100644
--- a/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
+++ b/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
@@ -13,7 +13,6 @@
 #include "llvm/CodeGen/DeadMachineInstructionElim.h"
 #include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/ADT/Statistic.h"
-#include "llvm/CodeGen/LiveRegUnits.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
@@ -32,14 +31,9 @@ namespace {
 class DeadMachineInstructionElimImpl {
   const MachineRegisterInfo *MRI = nullptr;
   const TargetInstrInfo *TII = nullptr;
-  LiveRegUnits LivePhysRegs;
 
 public:
   bool runImpl(MachineFunction &MF);
-
-private:
-  bool isDead(const MachineInstr *MI) const;
-  bool eliminateDeadMI(MachineFunction &MF);
 };
 
 class DeadMachineInstructionElim : public MachineFunctionPass {
@@ -79,7 +73,21 @@ char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID;
 INITIALIZE_PASS(DeadMachineInstructionElim, DEBUG_TYPE,
                 "Remove dead machine instructions", false, false)
 
-bool DeadMachineInstructionElimImpl::isDead(const MachineInstr *MI) const {
+bool DeadMachineInstructionElimImpl::runImpl(MachineFunction &MF) {
+  MRI = &MF.getRegInfo();
+
+  const TargetSubtargetInfo &ST = MF.getSubtarget();
+  TII = ST.getInstrInfo();
+  DeadMachineInstructionInfo DMII(MRI);
+
+  bool AnyChanges = DMII.eliminateDeadMI(MF);
+  while (AnyChanges && DMII.eliminateDeadMI(MF))
+    ;
+  return AnyChanges;
+}
+
+bool DeadMachineInstructionInfo::isDead(const MachineInstr *MI,
+                                        LiveRegUnits *LivePhysRegs) 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.
@@ -87,7 +95,8 @@ bool DeadMachineInstructionElimImpl::isDead(const MachineInstr *MI) const {
     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))
+      if (!LivePhysRegs || !LivePhysRegs->available(Reg) ||
+          MRI->isReserved(Reg))
         return false;
     } else {
       if (MO.isDead()) {
@@ -120,22 +129,12 @@ bool DeadMachineInstructionElimImpl::isDead(const MachineInstr *MI) const {
   return MI->wouldBeTriviallyDead();
 }
 
-bool DeadMachineInstructionElimImpl::runImpl(MachineFunction &MF) {
-  MRI = &MF.getRegInfo();
-
-  const TargetSubtargetInfo &ST = MF.getSubtarget();
-  TII = ST.getInstrInfo();
-  LivePhysRegs.init(*ST.getRegisterInfo());
-
-  bool AnyChanges = eliminateDeadMI(MF);
-  while (AnyChanges && eliminateDeadMI(MF))
-    ;
-  return AnyChanges;
-}
-
-bool DeadMachineInstructionElimImpl::eliminateDeadMI(MachineFunction &MF) {
+bool DeadMachineInstructionInfo::eliminateDeadMI(MachineFunction &MF) {
   bool AnyChanges = false;
+  const TargetSubtargetInfo &ST = MF.getSubtarget();
 
+  LiveRegUnits LivePhysRegs;
+  LivePhysRegs.init(*ST.getRegisterInfo());
   // 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.
@@ -146,7 +145,7 @@ 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 (isDead(&MI, &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 +155,8 @@ bool DeadMachineInstructionElimImpl::eliminateDeadMI(MachineFunction &MF) {
         ++NumDeletes;
         continue;
       }
-
       LivePhysRegs.stepBackward(MI);
     }
   }
-
-  LivePhysRegs.clear();
   return AnyChanges;
 }