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

[Usha Gupta 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 =
+            MI.getOperand(Opc == AArch64::TBZW || Opc == AArch64::TBZX ? 2 : 1)
+                .getMBB();
+        MachineBasicBlock *MBB = MI.getParent();
+        SmallVector<MachineBasicBlock *> Succs(MBB->successors());
+        for (auto *S : Succs)
+          if (S != Target)
+            MBB->removeSuccessor(S);
+        SmallVector<MachineInstr *> DeadInstrs;
+        for (auto It = MI.getIterator(); It != MBB->end(); ++It)
+          DeadInstrs.push_back(&*It);
+        const MachineFunction *MF = MBB->getParent();
+        const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
+        BuildMI(MBB, MI.getDebugLoc(), TII->get(AArch64::B)).addMBB(Target);
+        for (auto It : DeadInstrs)
+          It->eraseFromParent();
+        return true;
+      }
+      break;
+    case AArch64::CBNZW:
+    case AArch64::CBNZX:
+    case AArch64::TBNZW:
+    case AArch64::TBNZX:
+      // CBNZ XZR -> nop
----------------
usha1830 wrote:

```suggestion
      // CBNZ/TBNZ with WZR/XZR -> never taken, remove branch and successor
```

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