[llvm] [AArch64] Optimize CBZ wzr and friends. (PR #161508)

[David Tellenbach via llvm-commits]

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@@ -0,0 +1,113 @@
+//=- AArch64RedundantCondBranch.cpp - Remove redundant cbz wzr --------------=//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Late in the pipeline, especially with zero phi operands propagated after tail
+// duplications, we can end up with CBZ/CBNZ/TBZ/TBNZ with a zero register. This
+// simple pass looks at the terminators to a block, removing the redundant
+// instructions where necessary.
+//
+//===----------------------------------------------------------------------===//
+
+#include "AArch64.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/TargetInstrInfo.h"
+#include "llvm/Support/Debug.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "aarch64-redundantcondbranch"
+
+namespace {
+class AArch64RedundantCondBranch : public MachineFunctionPass {
+public:
+  static char ID;
+  AArch64RedundantCondBranch() : MachineFunctionPass(ID) {}
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+
+  MachineFunctionProperties getRequiredProperties() const override {
+    return MachineFunctionProperties().setNoVRegs();
+  }
+  StringRef getPassName() const override {
+    return "AArch64 Redundant Conditional Branch Elimination";
+  }
+};
+char AArch64RedundantCondBranch::ID = 0;
+} // namespace
+
+INITIALIZE_PASS(AArch64RedundantCondBranch, "aarch64-redundantcondbranch",
+                "AArch64 Redundant Conditional Branch Elimination pass", false,
+                false)
+
+static bool optimizeTerminators(MachineBasicBlock *MBB) {
+  for (MachineInstr &MI : make_early_inc_range(MBB->terminators())) {
+    unsigned Opc = MI.getOpcode();
+    switch (Opc) {
+    case AArch64::CBZW:
+    case AArch64::CBZX:
+    case AArch64::TBZW:
+    case AArch64::TBZX:
+      // CBZ XZR -> B
+      if (MI.getOperand(0).getReg() == AArch64::WZR ||
+          MI.getOperand(0).getReg() == AArch64::XZR) {
+        LLVM_DEBUG(dbgs() << "Removing redundant branch: " << MI);
+        MachineBasicBlock *Target =
----------------
dtellenbach wrote:

Can this be `AArch64InstrInfo::getBranchDestBlock`?

https://github.com/llvm/llvm-project/pull/161508