[llvm] [RISCV] Insert simple landing pad for taken address labels. (PR #91855)

[Craig Topper via llvm-commits]

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@@ -0,0 +1,102 @@
+//===------ RISCVIndirectBranchTracking.cpp - Enables lpad mechanism ------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// The pass adds LPAD (AUIPC with rs1 = X0) machine instructions at the
+// beginning of each basic block or function that is referenced by an indrect
+// jump/call instruction.
+//
+//===----------------------------------------------------------------------===//
+
+#include "RISCV.h"
+#include "RISCVInstrInfo.h"
+#include "RISCVSubtarget.h"
+#include "RISCVTargetMachine.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+
+using namespace llvm;
+
+static cl::opt<uint32_t> PreferredLandingPadLabel(
+    "riscv-landing-pad-label", cl::ReallyHidden,
+    cl::desc("Use preferred fixed label for all labels"));
+
+namespace {
+class RISCVIndirectBranchTrackingPass : public MachineFunctionPass {
+public:
+  RISCVIndirectBranchTrackingPass() : MachineFunctionPass(ID) {}
+
+  StringRef getPassName() const override {
+    return "RISC-V Indirect Branch Tracking";
+  }
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+
+private:
+  static char ID;
+  const Align LpadAlign = Align(4);
+};
+
+} // end anonymous namespace
+
+char RISCVIndirectBranchTrackingPass::ID = 0;
+
+FunctionPass *llvm::createRISCVIndirectBranchTrackingPass() {
+  return new RISCVIndirectBranchTrackingPass();
+}
+
+static void emitLpad(MachineBasicBlock &MBB, const RISCVInstrInfo *TII,
+                     uint32_t Label) {
+  auto I = MBB.begin();
+  BuildMI(MBB, I, MBB.findDebugLoc(I), TII->get(RISCV::AUIPC), RISCV::X0)
+      .addImm(Label);
+}
+
+bool RISCVIndirectBranchTrackingPass::runOnMachineFunction(
+    MachineFunction &MF) {
+  const auto &Subtarget = MF.getSubtarget<RISCVSubtarget>();
+  const RISCVInstrInfo *TII = Subtarget.getInstrInfo();
+  if (!Subtarget.hasStdExtZicfilp())
+    return false;
+
+  uint32_t FixedLabel = 0;
+  if (PreferredLandingPadLabel.getNumOccurrences() > 0) {
+    if (!isUInt<20>(PreferredLandingPadLabel))
+      report_fatal_error("riscv-landing-pad-label=<val>, <val> needs to fit in "
+                         "unsigned 20-bits");
+    FixedLabel = PreferredLandingPadLabel;
+  }
+
+  bool Changed = false;
+  for (MachineBasicBlock &MBB : MF) {
+    if (&MBB == &MF.front()) {
+      Function &F = MF.getFunction();
+      // When trap is taken, landing pad is not needed.
+      if (F.hasFnAttribute("interrupt"))
----------------
topperc wrote:

Nevermind, I think its not possible to take the address of the entry block. There is an IR verifier check for it.

https://github.com/llvm/llvm-project/pull/91855