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

[Philippe Mathieu-Daudé]

+Tony

On 10/28/20 1:57 PM, Thomas Huth wrote:
> 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...?

I think the previous discussion is this thread:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg632245.html