Re: [PATCH v3] gdb: RISC-V: Refine lr/sc sequence support

[Andrew Burgess <aburgess@redhat.com>]

Yang Liu <liuyang22@iscas.ac.cn> writes:

> Per RISC-V spec, the lr/sc sequence can consist of up to 16 instructions, and we
> cannot insert breakpoints in the middle of this sequence. Before this, we only
> detected a specific pattern (the most common one). This patch improves this part
> and now supports more complex pattern detection.

Thanks for the update.  This looks great.

Approved-By: Andrew Burgess <aburgess@redhat.com>

Thanks,
Andrew

>
> Signed-off-by: Yang Liu <liuyang22@iscas.ac.cn>
> ---
>  gdb/riscv-tdep.c | 290 ++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 251 insertions(+), 39 deletions(-)
>
> diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
> index 3a2891c2c92..ffad7fe2a60 100644
> --- a/gdb/riscv-tdep.c
> +++ b/gdb/riscv-tdep.c
> @@ -1578,8 +1578,34 @@ class riscv_insn
>        BLTU,
>        BGEU,
>        /* These are needed for stepping over atomic sequences.  */
> -      LR,
> -      SC,
> +      SLTI,
> +      SLTIU,
> +      XORI,
> +      ORI,
> +      ANDI,
> +      SLLI,
> +      SLLIW,
> +      SRLI,
> +      SRLIW,
> +      SRAI,
> +      SRAIW,
> +      SUB,
> +      SUBW,
> +      SLL,
> +      SLLW,
> +      SLT,
> +      SLTU,
> +      XOR,
> +      SRL,
> +      SRLW,
> +      SRA,
> +      SRAW,
> +      OR,
> +      AND,
> +      LR_W,
> +      LR_D,
> +      SC_W,
> +      SC_D,
>        /* This instruction is used to do a syscall.  */
>        ECALL,
>
> @@ -1768,6 +1794,13 @@ class riscv_insn
>      m_imm.s = EXTRACT_CBTYPE_IMM (ival);
>    }
>
> +  void decode_ca_type_insn (enum opcode opcode, ULONGEST ival)
> +  {
> +    m_opcode = opcode;
> +    m_rs1 = decode_register_index_short (ival, OP_SH_CRS1S);
> +    m_rs2 = decode_register_index_short (ival, OP_SH_CRS2S);
> +  }
> +
>    /* Fetch instruction from target memory at ADDR, return the content of
>       the instruction, and update LEN with the instruction length.  */
>    static ULONGEST fetch_instruction (struct gdbarch *gdbarch,
> @@ -1882,14 +1915,62 @@ riscv_insn::decode (struct gdbarch *gdbarch, CORE_ADDR pc)
>  	decode_b_type_insn (BLTU, ival);
>        else if (is_bgeu_insn (ival))
>  	decode_b_type_insn (BGEU, ival);
> +      else if (is_slti_insn(ival))
> +	decode_i_type_insn (SLTI, ival);
> +      else if (is_sltiu_insn(ival))
> +	decode_i_type_insn (SLTIU, ival);
> +      else if (is_xori_insn(ival))
> +	decode_i_type_insn (XORI, ival);
> +      else if (is_ori_insn(ival))
> +	decode_i_type_insn (ORI, ival);
> +      else if (is_andi_insn(ival))
> +	decode_i_type_insn (ANDI, ival);
> +      else if (is_slli_insn(ival))
> +	decode_i_type_insn (SLLI, ival);
> +      else if (is_slliw_insn(ival))
> +	decode_i_type_insn (SLLIW, ival);
> +      else if (is_srli_insn(ival))
> +	decode_i_type_insn (SRLI, ival);
> +      else if (is_srliw_insn(ival))
> +	decode_i_type_insn (SRLIW, ival);
> +      else if (is_srai_insn(ival))
> +	decode_i_type_insn (SRAI, ival);
> +      else if (is_sraiw_insn(ival))
> +	decode_i_type_insn (SRAIW, ival);
> +      else if (is_sub_insn(ival))
> +	decode_r_type_insn (SUB, ival);
> +      else if (is_subw_insn(ival))
> +	decode_r_type_insn (SUBW, ival);
> +      else if (is_sll_insn(ival))
> +	decode_r_type_insn (SLL, ival);
> +      else if (is_sllw_insn(ival))
> +	decode_r_type_insn (SLLW, ival);
> +      else if (is_slt_insn(ival))
> +	decode_r_type_insn (SLT, ival);
> +      else if (is_sltu_insn(ival))
> +	decode_r_type_insn (SLTU, ival);
> +      else if (is_xor_insn(ival))
> +	decode_r_type_insn (XOR, ival);
> +      else if (is_srl_insn(ival))
> +	decode_r_type_insn (SRL, ival);
> +      else if (is_srlw_insn(ival))
> +	decode_r_type_insn (SRLW, ival);
> +      else if (is_sra_insn(ival))
> +	decode_r_type_insn (SRA, ival);
> +      else if (is_sraw_insn(ival))
> +	decode_r_type_insn (SRAW, ival);
> +      else if (is_or_insn(ival))
> +	decode_r_type_insn (OR, ival);
> +      else if (is_and_insn(ival))
> +	decode_r_type_insn (AND, ival);
>        else if (is_lr_w_insn (ival))
> -	decode_r_type_insn (LR, ival);
> +	decode_r_type_insn (LR_W, ival);
>        else if (is_lr_d_insn (ival))
> -	decode_r_type_insn (LR, ival);
> +	decode_r_type_insn (LR_D, ival);
>        else if (is_sc_w_insn (ival))
> -	decode_r_type_insn (SC, ival);
> +	decode_r_type_insn (SC_W, ival);
>        else if (is_sc_d_insn (ival))
> -	decode_r_type_insn (SC, ival);
> +	decode_r_type_insn (SC_D, ival);
>        else if (is_ecall_insn (ival))
>  	decode_i_type_insn (ECALL, ival);
>        else if (is_ld_insn (ival))
> @@ -1944,6 +2025,24 @@ riscv_insn::decode (struct gdbarch *gdbarch, CORE_ADDR pc)
>  	  m_rd = decode_register_index (ival, OP_SH_CRS1S);
>  	  m_imm.s = EXTRACT_CITYPE_LUI_IMM (ival);
>  	}
> +      else if (is_c_srli_insn (ival))
> +	decode_cb_type_insn (SRLI, ival);
> +      else if (is_c_srai_insn (ival))
> +	decode_cb_type_insn (SRAI, ival);
> +      else if (is_c_andi_insn (ival))
> +	decode_cb_type_insn (ANDI, ival);
> +      else if (is_c_sub_insn (ival))
> +	decode_ca_type_insn (SUB, ival);
> +      else if (is_c_xor_insn (ival))
> +	decode_ca_type_insn (XOR, ival);
> +      else if (is_c_or_insn (ival))
> +	decode_ca_type_insn (OR, ival);
> +      else if (is_c_and_insn (ival))
> +	decode_ca_type_insn (AND, ival);
> +      else if (is_c_subw_insn (ival))
> +	decode_ca_type_insn (SUBW, ival);
> +      else if (is_c_addw_insn (ival))
> +	decode_ca_type_insn (ADDW, ival);
>        else if (is_c_li_insn (ival))
>  	decode_ci_type_insn (LI, ival);
>        /* C_SD and C_FSW have the same opcode.  C_SD is RV64 and RV128 only,
> @@ -4404,51 +4503,164 @@ riscv_next_pc (struct regcache *regcache, CORE_ADDR pc)
>    return next_pc;
>  }
>
> +/* Return true if INSN is not a control transfer instruction and is allowed to
> +   appear in the middle of the lr/sc sequence.  */
> +
> +static bool
> +riscv_insn_is_non_cti_and_allowed_in_atomic_sequence
> +  (const struct riscv_insn &insn)
> +{
> +  switch (insn.opcode ())
> +    {
> +    case riscv_insn::LUI:
> +    case riscv_insn::AUIPC:
> +    case riscv_insn::ADDI:
> +    case riscv_insn::ADDIW:
> +    case riscv_insn::SLTI:
> +    case riscv_insn::SLTIU:
> +    case riscv_insn::XORI:
> +    case riscv_insn::ORI:
> +    case riscv_insn::ANDI:
> +    case riscv_insn::SLLI:
> +    case riscv_insn::SLLIW:
> +    case riscv_insn::SRLI:
> +    case riscv_insn::SRLIW:
> +    case riscv_insn::SRAI:
> +    case riscv_insn::ADD:
> +    case riscv_insn::ADDW:
> +    case riscv_insn::SRAIW:
> +    case riscv_insn::SUB:
> +    case riscv_insn::SUBW:
> +    case riscv_insn::SLL:
> +    case riscv_insn::SLLW:
> +    case riscv_insn::SLT:
> +    case riscv_insn::SLTU:
> +    case riscv_insn::XOR:
> +    case riscv_insn::SRL:
> +    case riscv_insn::SRLW:
> +    case riscv_insn::SRA:
> +    case riscv_insn::SRAW:
> +    case riscv_insn::OR:
> +    case riscv_insn::AND:
> +      return true;
> +    }
> +
> +    return false;
> +}
> +
> +/* Return true if INSN is a direct branch instruction.  */
> +
> +static bool
> +riscv_insn_is_direct_branch (const struct riscv_insn &insn)
> +{
> +  switch (insn.opcode ())
> +    {
> +    case riscv_insn::BEQ:
> +    case riscv_insn::BNE:
> +    case riscv_insn::BLT:
> +    case riscv_insn::BGE:
> +    case riscv_insn::BLTU:
> +    case riscv_insn::BGEU:
> +    case riscv_insn::JAL:
> +      return true;
> +    }
> +
> +    return false;
> +}
> +
>  /* We can't put a breakpoint in the middle of a lr/sc atomic sequence, so look
>     for the end of the sequence and put the breakpoint there.  */
>
> -static bool
> -riscv_next_pc_atomic_sequence (struct regcache *regcache, CORE_ADDR pc,
> -			       CORE_ADDR *next_pc)
> +static std::vector<CORE_ADDR>
> +riscv_deal_with_atomic_sequence (struct regcache *regcache, CORE_ADDR pc)
>  {
>    struct gdbarch *gdbarch = regcache->arch ();
>    struct riscv_insn insn;
> -  CORE_ADDR cur_step_pc = pc;
> -  CORE_ADDR last_addr = 0;
> +  CORE_ADDR cur_step_pc = pc, next_pc;
> +  std::vector<CORE_ADDR> next_pcs;
> +  bool found_valid_atomic_sequence = false;
> +  enum riscv_insn::opcode lr_opcode;
>
>    /* First instruction has to be a load reserved.  */
>    insn.decode (gdbarch, cur_step_pc);
> -  if (insn.opcode () != riscv_insn::LR)
> -    return false;
> -  cur_step_pc = cur_step_pc + insn.length ();
> +  lr_opcode = insn.opcode ();
> +  if (lr_opcode != riscv_insn::LR_D && lr_opcode != riscv_insn::LR_W)
> +    return {};
> +
> +  /* The loop comprises only an LR/SC sequence and code to retry the sequence in
> +     the case of failure, and must comprise at most 16 instructions placed
> +     sequentially in memory.  While our code tries to follow these restrictions,
> +     it has the following limitations:
> +
> +       (a) We expect the loop to start with an LR and end with a BNE.
> +	   Apparently this does not cover all cases for a valid sequence.
> +       (b) The atomic limitations only apply to the code that is actually
> +	   executed, so here again it's overly restrictive.
> +       (c) The lr/sc are required to be for the same target address, but this
> +	   information is only known at runtime.  Same as (b), in order to check
> +	   this we will end up needing to simulate the sequence, which is more
> +	   complicated than what we're doing right now.
> +
> +     Also note that we only expect a maximum of (16-2) instructions in the for
> +     loop as we have assumed the presence of LR and BNE at the beginning and end
> +     respectively.  */
> +  for (int insn_count = 0; insn_count < 16 - 2; ++insn_count)
> +    {
> +      cur_step_pc += insn.length ();
> +      insn.decode (gdbarch, cur_step_pc);
>
> -  /* Next instruction should be branch to exit.  */
> -  insn.decode (gdbarch, cur_step_pc);
> -  if (insn.opcode () != riscv_insn::BNE)
> -    return false;
> -  last_addr = cur_step_pc + insn.imm_signed ();
> -  cur_step_pc = cur_step_pc + insn.length ();
> +      /* The dynamic code executed between lr/sc can only contain instructions
> +	 from the base I instruction set, excluding loads, stores, backward
> +	 jumps, taken backward branches, JALR, FENCE, FENCE.I, and SYSTEM
> +	 instructions.  If the C extension is supported, then compressed forms
> +	 of the aforementioned I instructions are also permitted.  */
>
> -  /* Next instruction should be store conditional.  */
> -  insn.decode (gdbarch, cur_step_pc);
> -  if (insn.opcode () != riscv_insn::SC)
> -    return false;
> -  cur_step_pc = cur_step_pc + insn.length ();
> +      if (riscv_insn_is_non_cti_and_allowed_in_atomic_sequence (insn))
> +	continue;
> +      /* Look for a conditional branch instruction, check if it's taken forward
> +	 or not.  */
> +      else if (riscv_insn_is_direct_branch (insn))
> +	{
> +	  if (insn.imm_signed () > 0)
> +	    {
> +	      next_pc = cur_step_pc + insn.imm_signed ();
> +	      next_pcs.push_back (next_pc);
> +	    }
> +	  else
> +	    break;
> +	}
> +      /* Look for a paired SC instruction which closes the atomic sequence.  */
> +      else if ((insn.opcode () == riscv_insn::SC_D
> +		&& lr_opcode == riscv_insn::LR_D)
> +	       || (insn.opcode () == riscv_insn::SC_W
> +		   && lr_opcode == riscv_insn::LR_W))
> +	found_valid_atomic_sequence = true;
> +      else
> +	break;
> +    }
> +
> +  if (!found_valid_atomic_sequence)
> +    return {};
>
>    /* Next instruction should be branch to start.  */
>    insn.decode (gdbarch, cur_step_pc);
>    if (insn.opcode () != riscv_insn::BNE)
> -    return false;
> +    return {};
>    if (pc != (cur_step_pc + insn.imm_signed ()))
> -    return false;
> -  cur_step_pc = cur_step_pc + insn.length ();
> +    return {};
> +  cur_step_pc += insn.length ();
>
> -  /* We should now be at the end of the sequence.  */
> -  if (cur_step_pc != last_addr)
> -    return false;
> +  /* Remove all PCs that jump within the sequence.  */
> +  auto matcher = [cur_step_pc] (const CORE_ADDR addr) -> bool
> +    {
> +      return addr < cur_step_pc;
> +    };
> +  auto it = std::remove_if (next_pcs.begin (), next_pcs.end (), matcher);
> +  next_pcs.erase (it, next_pcs.end ());
>
> -  *next_pc = cur_step_pc;
> -  return true;
> +  next_pc = cur_step_pc;
> +  next_pcs.push_back (next_pc);
> +  return next_pcs;
>  }
>
>  /* This is called just before we want to resume the inferior, if we want to
> @@ -4458,14 +4670,14 @@ riscv_next_pc_atomic_sequence (struct regcache *regcache, CORE_ADDR pc,
>  std::vector<CORE_ADDR>
>  riscv_software_single_step (struct regcache *regcache)
>  {
> -  CORE_ADDR pc, next_pc;
> -
> -  pc = regcache_read_pc (regcache);
> +  CORE_ADDR cur_pc = regcache_read_pc (regcache), next_pc;
> +  std::vector<CORE_ADDR> next_pcs
> +    = riscv_deal_with_atomic_sequence (regcache, cur_pc);
>
> -  if (riscv_next_pc_atomic_sequence (regcache, pc, &next_pc))
> -    return {next_pc};
> +  if (!next_pcs.empty ())
> +    return next_pcs;
>
> -  next_pc = riscv_next_pc (regcache, pc);
> +  next_pc = riscv_next_pc (regcache, cur_pc);
>
>    return {next_pc};
>  }
> --
> 2.42.0