Re: [PATCH] target/riscv: Update $pc after linking to $ra in trans_cm_jalt()

[Frank Chang <frank.chang@sifive.com>]

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Reviewed-by: Frank Chang <frank.chang@sifive.com>

On Tue, Feb 6, 2024 at 9:19 PM Jason Chien <jason.chien@sifive.com> wrote:

> The original implementation sets $pc to the address read from the jump
> vector table first and links $ra with the address of the next instruction
> after the updated $pc. After jumping to the updated $pc and executing the
> next ret instruction, the program jumps to $ra, which is in the same
> function currently executing, which results in an infinite loop.
> This commit reverses the two action. Firstly, $ra is updated with the
> address of the next instruction after $pc, and sets $pc to the address
> read from the jump vector table.
>
> Signed-off-by: Jason Chien <jason.chien@sifive.com>
> ---
>  target/riscv/insn_trans/trans_rvzce.c.inc | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/target/riscv/insn_trans/trans_rvzce.c.inc
> b/target/riscv/insn_trans/trans_rvzce.c.inc
> index 8d8a64f493..a185e2315f 100644
> --- a/target/riscv/insn_trans/trans_rvzce.c.inc
> +++ b/target/riscv/insn_trans/trans_rvzce.c.inc
> @@ -293,13 +293,6 @@ static bool trans_cm_jalt(DisasContext *ctx,
> arg_cm_jalt *a)
>  {
>      REQUIRE_ZCMT(ctx);
>
> -    /*
> -     * Update pc to current for the non-unwinding exception
> -     * that might come from cpu_ld*_code() in the helper.
> -     */
> -    gen_update_pc(ctx, 0);
> -    gen_helper_cm_jalt(cpu_pc, cpu_env, tcg_constant_i32(a->index));
> -
>      /* c.jt vs c.jalt depends on the index. */
>      if (a->index >= 32) {
>          TCGv succ_pc = dest_gpr(ctx, xRA);
> @@ -307,6 +300,13 @@ static bool trans_cm_jalt(DisasContext *ctx,
> arg_cm_jalt *a)
>          gen_set_gpr(ctx, xRA, succ_pc);
>      }
>
> +    /*
> +     * Update pc to current for the non-unwinding exception
> +     * that might come from cpu_ld*_code() in the helper.
> +     */
> +    gen_update_pc(ctx, 0);
> +    gen_helper_cm_jalt(cpu_pc, cpu_env, tcg_constant_i32(a->index));
> +
>      tcg_gen_lookup_and_goto_ptr();
>      ctx->base.is_jmp = DISAS_NORETURN;
>      return true;
> --
> 2.43.0
>
>
>

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