Re: [Qemu-devel] [PATCH] tcg: Initialize return value after exit_atomic

[Peter Maydell]

On 25 April 2017 at 11:43, Richard Henderson <address@hidden> wrote:
> Users of tcg_gen_atomic_cmpxchg and do_atomic_op rightfully utilize
> the output.  Even though this code is dead, it gets translated, and
> without the initialization we encounter a tcg_error.
>
> Reported-by: Nikunj A Dadhania <address@hidden>
> Signed-off-by: Richard Henderson <address@hidden>
> ---
>  tcg/tcg-op.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/tcg/tcg-op.c b/tcg/tcg-op.c
> index 95a39b7..6b1f415 100644
> --- a/tcg/tcg-op.c
> +++ b/tcg/tcg-op.c
> @@ -2861,6 +2861,9 @@ void tcg_gen_atomic_cmpxchg_i64(TCGv_i64 retv, TCGv 
> addr, TCGv_i64 cmpv,
>  #endif
>  #else
>          gen_helper_exit_atomic(tcg_ctx.tcg_env);
> +        /* Produce a result, so that we have a well-formed opcode stream
> +           with respect to uses of the result in the (dead) code following.  
> */
> +        tcg_gen_movi_i64(retv, 0);
>  #endif /* CONFIG_ATOMIC64 */
>      } else {
>          TCGv_i32 c32 = tcg_temp_new_i32();
> @@ -2966,6 +2969,9 @@ static void do_atomic_op_i64(TCGv_i64 ret, TCGv addr, 
> TCGv_i64 val,
>  #endif
>  #else
>          gen_helper_exit_atomic(tcg_ctx.tcg_env);
> +        /* Produce a result, so that we have a well-formed opcode stream
> +           with respect to uses of the result in the (dead) code following.  
> */
> +        tcg_gen_movi_i64(ret, 0);
>  #endif /* CONFIG_ATOMIC64 */
>      } else {
>          TCGv_i32 v32 = tcg_temp_new_i32();
> --

Tested-by: Peter Maydell <address@hidden>

Without this patch an AArch64 QEMU crashes on startup if I build it
with clang and with optimization enabled. We should probably get this
into master sooner rather than later...

thanks
-- PMM