Re: [PATCH 1/9] target/i386: silence the compiler warnings in gen_shiftd_rm_T1

[Thomas Huth]

On 28/10/2020 05.18, Chen Qun wrote:
> The current "#ifdef TARGET_X86_64" statement affects
> the compiler's determination of fall through.
> 
> When using -Wimplicit-fallthrough in our CFLAGS, the compiler showed warning:
> target/i386/translate.c: In function ‘gen_shiftd_rm_T1’:
> target/i386/translate.c:1773:12: warning: this statement may fall through 
> [-Wimplicit-fallthrough=]
>          if (is_right) {
>             ^
> target/i386/translate.c:1782:5: note: here
>      case MO_32:
>      ^~~~
> 
> Reported-by: Euler Robot <euler.robot@huawei.com>
> Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
> ---
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Richard Henderson <richard.henderson@linaro.org>
> Cc: Eduardo Habkost <ehabkost@redhat.com>
> ---
>  target/i386/translate.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/target/i386/translate.c b/target/i386/translate.c
> index caea6f5fb1..4c353427d7 100644
> --- a/target/i386/translate.c
> +++ b/target/i386/translate.c
> @@ -1777,9 +1777,9 @@ static void gen_shiftd_rm_T1(DisasContext *s, MemOp ot, 
> int op1,
>          } else {
>              tcg_gen_deposit_tl(s->T1, s->T0, s->T1, 16, 16);
>          }
> -        /* FALLTHRU */
> -#ifdef TARGET_X86_64
> +        /* fall through */
>      case MO_32:
> +#ifdef TARGET_X86_64
>          /* Concatenate the two 32-bit values and use a 64-bit shift.  */
>          tcg_gen_subi_tl(s->tmp0, count, 1);
>          if (is_right) {

The whole code here looks a little bit fishy to me ... in case TARGET_X86_64
is defined, the MO_16 code falls through to MO_32 ... but in case it is not
defined, it falls through to the default case that comes after the #ifdef
block? Is this really the right thing here? If so, I think there should be
some additional comments explaining this behavior.

Richard, maybe you could help to judge what is right here...?

 Thomas