Re: [Qemu-devel] [PATCH v2 3/5] target-m68k: Inline shifts

[Laurent Vivier]

Le 28/11/2016 à 15:54, Richard Henderson a écrit :
> On 11/27/2016 11:35 AM, Laurent Vivier wrote:
>>>> +        tcg_gen_extr_i64_i32(QREG_CC_N, QREG_CC_C, t64);
>> This does not extract correctly the C flag when the opsize is word or byte.
>> I think we should use a shift instead:
>>
>> -        tcg_gen_extr_i64_i32(QREG_CC_N, QREG_CC_C, t64);
>> -
>> -        /* Note that C=0 if shift count is 0, and we get that for free.  */
>> +        if (opsize == OS_LONG) {
>> +            tcg_gen_extr_i64_i32(QREG_CC_N, QREG_CC_C, t64);
>> +            /* Note that C=0 if shift count is 0, and we get that for
>> free.  */
>> +        } else {
>> +            TCGv zero = tcg_const_i32(0);
>> +            tcg_gen_extrl_i64_i32(QREG_CC_N, t64);
>> +            tcg_gen_shri_i64(t64, t64, bits);
>> +            tcg_gen_extrl_i64_i32(QREG_CC_C, t64);
>> +            tcg_gen_movcond_i32(TCG_COND_EQ, QREG_CC_C,
>> +                                s32, zero, zero, QREG_CC_C);
>> +            tcg_temp_free(zero);
>> +        }
>>
>> Do you have a better idea?
> 
>   if (opsize == OS_LONG) {
>     tcg_gen_extr_i64_i32(QREG_CC_N, QREG_CC_C, t64);
>   } else {
>     tcg_gen_extrl_i64_i32(QREG_CC_N, t64);
>     tcg_gen_shri_i32(QREG_CC_C, QREG_CC_N, bits);
>   }
> 
> Since we zero-extend the input from bits, it's still true that a zero shift
> gets C=0 for free.
> 

It doesn't work because we work with word/byte where the bit sign has
been extended to the long word. So in the case of 0 shift, with retrieve
C=<original bit sign> and not 0.

Laurent