[lldb][RISCV] fix LR/SC atomic sequence handling in lldb-server (#127505)

lldb-server had limited support for single-stepping through the lr/sc
atomic sequence. This patch enhances that support for all possible
atomic sequences.

Author: dlav-sc

lldb/include/lldb/Core/EmulateInstruction.h

@@ -22,6 +22,8 @@
 #include "lldb/lldb-private-types.h"
 #include "lldb/lldb-types.h"
 
+#include "llvm/Support/Error.h"
+
 #include <cstddef>
 #include <cstdint>
 
@@ -32,6 +34,38 @@ class RegisterValue;
 class Stream;
 class Target;
 class UnwindPlan;
+class EmulateInstruction;
+
+using BreakpointLocations = std::vector<lldb::addr_t>;
+
+class SingleStepBreakpointLocationsPredictor {
+public:
+  SingleStepBreakpointLocationsPredictor(
+      std::unique_ptr<EmulateInstruction> emulator_up)
+      : m_emulator_up{std::move(emulator_up)} {}
+
+  virtual BreakpointLocations GetBreakpointLocations(Status &status);
+
+  virtual llvm::Expected<unsigned>
+  GetBreakpointSize([[maybe_unused]] lldb::addr_t bp_addr) {
+    return 4;
+  }
+
+  virtual ~SingleStepBreakpointLocationsPredictor() = default;
+
+protected:
+  // This function retrieves the address of the next instruction as it appears
+  // in the binary file. Essentially, it reads the value of the PC register,
+  // determines the size of the current instruction (where the PC is pointing),
+  // and returns the sum of these two values.
+  lldb::addr_t GetNextInstructionAddress(Status &error);
+
+  lldb::addr_t GetBreakpointLocationAddress(lldb::addr_t entry_pc,
+                                            Status &error);
+
+  std::unique_ptr<EmulateInstruction> m_emulator_up;
+  bool m_emulation_result = false;
+};
 
 /// \class EmulateInstruction EmulateInstruction.h
 /// "lldb/Core/EmulateInstruction.h"
@@ -497,7 +531,19 @@ class EmulateInstruction : public PluginInterface {
   static uint32_t GetInternalRegisterNumber(RegisterContext *reg_ctx,
                                             const RegisterInfo &reg_info);
 
+  static std::unique_ptr<SingleStepBreakpointLocationsPredictor>
+  CreateBreakpointLocationPredictor(
+      std::unique_ptr<EmulateInstruction> emulator_up);
+
+  // Helper functions
+  std::optional<lldb::addr_t> ReadPC();
+  bool WritePC(lldb::addr_t addr);
+
 protected:
+  using BreakpointLocationsPredictorCreator =
+      std::function<std::unique_ptr<SingleStepBreakpointLocationsPredictor>(
+          std::unique_ptr<EmulateInstruction>)>;
+
   ArchSpec m_arch;
   void *m_baton = nullptr;
   ReadMemoryCallback m_read_mem_callback = &ReadMemoryDefault;
@@ -508,6 +554,21 @@ class EmulateInstruction : public PluginInterface {
   Opcode m_opcode;
 
 private:
+  virtual BreakpointLocationsPredictorCreator
+  GetSingleStepBreakpointLocationsPredictorCreator() {
+    if (!m_arch.IsMIPS() && !m_arch.GetTriple().isPPC64() &&
+        !m_arch.GetTriple().isLoongArch()) {
+      // Unsupported architecture
+      return [](std::unique_ptr<EmulateInstruction> emulator_up) {
+        return nullptr;
+      };
+    }
+    return [](std::unique_ptr<EmulateInstruction> emulator_up) {
+      return std::make_unique<SingleStepBreakpointLocationsPredictor>(
+          std::move(emulator_up));
+    };
+  }
+
   // For EmulateInstruction only
   EmulateInstruction(const EmulateInstruction &) = delete;
   const EmulateInstruction &operator=(const EmulateInstruction &) = delete;

lldb/source/Core/EmulateInstruction.cpp

@@ -588,7 +588,99 @@ EmulateInstruction::GetInternalRegisterNumber(RegisterContext *reg_ctx,
   return LLDB_INVALID_REGNUM;
 }
 
+std::unique_ptr<SingleStepBreakpointLocationsPredictor>
+EmulateInstruction::CreateBreakpointLocationPredictor(
+    std::unique_ptr<EmulateInstruction> emulator_up) {
+  auto creator =
+      emulator_up->GetSingleStepBreakpointLocationsPredictorCreator();
+  return creator(std::move(emulator_up));
+}
+
+std::optional<lldb::addr_t> EmulateInstruction::ReadPC() {
+  bool success = false;
+  auto addr = ReadRegisterUnsigned(eRegisterKindGeneric, LLDB_REGNUM_GENERIC_PC,
+                                   LLDB_INVALID_ADDRESS, &success);
+  return success ? std::optional<addr_t>(addr) : std::nullopt;
+}
+
+bool EmulateInstruction::WritePC(lldb::addr_t addr) {
+  EmulateInstruction::Context ctx;
+  ctx.type = eContextAdvancePC;
+  ctx.SetNoArgs();
+  return WriteRegisterUnsigned(ctx, eRegisterKindGeneric,
+                               LLDB_REGNUM_GENERIC_PC, addr);
+}
+
 bool EmulateInstruction::CreateFunctionEntryUnwind(UnwindPlan &unwind_plan) {
   unwind_plan.Clear();
   return false;
 }
+
+BreakpointLocations
+SingleStepBreakpointLocationsPredictor::GetBreakpointLocations(Status &status) {
+  if (!m_emulator_up->ReadInstruction()) {
+    // try to get at least the size of next instruction to set breakpoint.
+    lldb::addr_t next_pc = GetNextInstructionAddress(status);
+    return BreakpointLocations{next_pc};
+  }
+
+  auto entry_pc = m_emulator_up->ReadPC();
+  if (!entry_pc) {
+    status = Status("Can't read PC");
+    return {};
+  }
+
+  m_emulation_result = m_emulator_up->EvaluateInstruction(
+      eEmulateInstructionOptionAutoAdvancePC);
+
+  lldb::addr_t next_pc = GetBreakpointLocationAddress(*entry_pc, status);
+  return BreakpointLocations{next_pc};
+}
+
+lldb::addr_t SingleStepBreakpointLocationsPredictor::GetNextInstructionAddress(
+    Status &error) {
+  auto instr_size = m_emulator_up->GetLastInstrSize();
+  if (!instr_size) {
+    error = Status("Read instruction failed!");
+    return LLDB_INVALID_ADDRESS;
+  }
+
+  auto pc = m_emulator_up->ReadPC();
+  if (!pc) {
+    error = Status("Can't read PC");
+    return LLDB_INVALID_ADDRESS;
+  }
+
+  lldb::addr_t next_pc = *pc + *instr_size;
+  return next_pc;
+}
+
+lldb::addr_t
+SingleStepBreakpointLocationsPredictor::GetBreakpointLocationAddress(
+    lldb::addr_t entry_pc, Status &error) {
+  auto addr = m_emulator_up->ReadPC();
+  if (!addr) {
+    error = Status("Can't read PC");
+    return LLDB_INVALID_ADDRESS;
+  }
+  lldb::addr_t pc = *addr;
+
+  if (m_emulation_result) {
+    assert(entry_pc != pc && "Emulation was successfull but PC wasn't updated");
+    return pc;
+  }
+
+  if (entry_pc == pc) {
+    // Emulate instruction failed and it hasn't changed PC. Advance PC with
+    // the size of the current opcode because the emulation of all
+    // PC modifying instruction should be successful. The failure most
+    // likely caused by an unsupported instruction which does not modify PC.
+    return pc + m_emulator_up->GetOpcode().GetByteSize();
+  }
+
+  // The instruction emulation failed after it modified the PC. It is an
+  // unknown error where we can't continue because the next instruction is
+  // modifying the PC but we don't  know how.
+  error = Status("Instruction emulation failed unexpectedly.");
+  return LLDB_INVALID_ADDRESS;
+}

lldb/source/Plugins/Instruction/ARM/EmulateInstructionARM.cpp

@@ -14471,3 +14471,16 @@ bool EmulateInstructionARM::CreateFunctionEntryUnwind(UnwindPlan &unwind_plan) {
   unwind_plan.SetReturnAddressRegister(dwarf_lr);
   return true;
 }
+
+llvm::Expected<unsigned>
+ARMSingleStepBreakpointLocationsPredictor::GetBreakpointSize(
+    lldb::addr_t bp_addr) {
+  auto flags = m_emulator_up->ReadRegisterUnsigned(
+      eRegisterKindGeneric, LLDB_REGNUM_GENERIC_FLAGS, LLDB_INVALID_ADDRESS,
+      nullptr);
+  if (flags == LLDB_INVALID_ADDRESS)
+    return llvm::createStringError(llvm::inconvertibleErrorCode(),
+                                   "Reading flags failed!");
+
+  return (flags & 0x20) ? /* Thumb mode */ 2 : /* Arm mode */ 4;
+}

lldb/source/Plugins/Instruction/ARM/EmulateInstructionARM.h

@@ -16,6 +16,16 @@
 
 namespace lldb_private {
 
+class ARMSingleStepBreakpointLocationsPredictor
+    : public SingleStepBreakpointLocationsPredictor {
+public:
+  ARMSingleStepBreakpointLocationsPredictor(
+      std::unique_ptr<EmulateInstruction> emulator_up)
+      : SingleStepBreakpointLocationsPredictor{std::move(emulator_up)} {}
+
+  llvm::Expected<unsigned> GetBreakpointSize(lldb::addr_t bp_addr) override;
+};
+
 // ITSession - Keep track of the IT Block progression.
 class ITSession {
 public:
@@ -770,6 +780,14 @@ class EmulateInstructionARM : public EmulateInstruction {
   // B6.2.13 SUBS PC, LR and related instructions
   bool EmulateSUBSPcLrEtc(const uint32_t opcode, const ARMEncoding encoding);
 
+  BreakpointLocationsPredictorCreator
+  GetSingleStepBreakpointLocationsPredictorCreator() override {
+    return [](std::unique_ptr<EmulateInstruction> emulator_up) {
+      return std::make_unique<ARMSingleStepBreakpointLocationsPredictor>(
+          std::move(emulator_up));
+    };
+  }
+
   uint32_t m_arm_isa;
   Mode m_opcode_mode;
   uint32_t m_opcode_cpsr;