Re: [Qemu-devel] [PATCH 2/2] target-i386: Don't left shift negative constant

[Richard Henderson <rth@twiddle.net>]

On 09/30/2015 06:34 AM, Eduardo Habkost wrote:
> Left shift of negative values is undefined behavior. Detected by clang:
>    qemu/target-i386/translate.c:2423:26: runtime error:
>      left shift of negative value -8
>
> This changes the code to reverse the sign after the left shift.
>
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> ---
>   target-i386/translate.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/target-i386/translate.c b/target-i386/translate.c
> index 8b35de1..cc59b7e 100644
> --- a/target-i386/translate.c
> +++ b/target-i386/translate.c
> @@ -2420,7 +2420,7 @@ static void gen_pusha(DisasContext *s)
>   {
>       int i;
>       gen_op_movl_A0_reg(R_ESP);
> -    gen_op_addl_A0_im(-8 << s->dflag);
> +    gen_op_addl_A0_im(-(8 << s->dflag));

Better as -8U << s->d_flag?



>       if (!s->ss32)
>           tcg_gen_ext16u_tl(cpu_A0, cpu_A0);
>       tcg_gen_mov_tl(cpu_T[1], cpu_A0);
>