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Re: [Qemu-devel] [RFC v2 05/13] target-ppc: add modulo word operations


From: Nikunj A Dadhania
Subject: Re: [Qemu-devel] [RFC v2 05/13] target-ppc: add modulo word operations
Date: Mon, 25 Jul 2016 11:07:33 +0530
User-agent: Notmuch/0.21 (https://notmuchmail.org) Emacs/25.0.94.1 (x86_64-redhat-linux-gnu)

Richard Henderson <address@hidden> writes:

> On 07/23/2016 02:14 PM, Nikunj A Dadhania wrote:
>> Adding following instructions:
>>
>> moduw: Modulo Unsigned Word
>> modsw: Modulo Signed Word
>>
>> Signed-off-by: Nikunj A Dadhania <address@hidden>
>> ---
>>  target-ppc/helper.h     |  2 ++
>>  target-ppc/int_helper.c | 15 +++++++++++++++
>>  target-ppc/translate.c  | 19 +++++++++++++++++++
>>  3 files changed, 36 insertions(+)
>>
>> diff --git a/target-ppc/helper.h b/target-ppc/helper.h
>> index 1f5cfd0..76072fd 100644
>> --- a/target-ppc/helper.h
>> +++ b/target-ppc/helper.h
>> @@ -41,6 +41,8 @@ DEF_HELPER_FLAGS_1(cntlzw, TCG_CALL_NO_RWG_SE, tl, tl)
>>  DEF_HELPER_FLAGS_1(popcntb, TCG_CALL_NO_RWG_SE, tl, tl)
>>  DEF_HELPER_FLAGS_1(popcntw, TCG_CALL_NO_RWG_SE, tl, tl)
>>  DEF_HELPER_FLAGS_2(cmpb, TCG_CALL_NO_RWG_SE, tl, tl, tl)
>> +DEF_HELPER_FLAGS_2(modsw, TCG_CALL_NO_RWG_SE, i32, i32, i32)
>> +DEF_HELPER_FLAGS_2(moduw, TCG_CALL_NO_RWG_SE, i32, i32, i32)
>>  DEF_HELPER_3(sraw, tl, env, tl, tl)
>>  #if defined(TARGET_PPC64)
>>  DEF_HELPER_FLAGS_1(cntlzd, TCG_CALL_NO_RWG_SE, tl, tl)
>> diff --git a/target-ppc/int_helper.c b/target-ppc/int_helper.c
>> index 7445376..631e0b4 100644
>> --- a/target-ppc/int_helper.c
>> +++ b/target-ppc/int_helper.c
>> @@ -139,6 +139,21 @@ uint64_t helper_divde(CPUPPCState *env, uint64_t rau, 
>> uint64_t rbu, uint32_t oe)
>>
>>  #endif
>>
>> +uint32_t helper_modsw(uint32_t rau, uint32_t rbu)
>> +{
>> +    int32_t ra = (int32_t) rau;
>> +    int32_t rb = (int32_t) rbu;
>> +
>> +    if ((rb == 0) || (ra == INT32_MIN && rb == -1)) {
>> +        return 0;
>> +    }
>> +    return ra % rb;
>> +}
>> +
>> +uint32_t helper_moduw(uint32_t ra, uint32_t rb)
>> +{
>> +    return rb ? ra % rb : 0;
>> +}
>
> I think, like you, I got distracted by the current div implementation in ppc. 
> I've just re-read the spec and seen the "undefined" language.  Which of 
> course 
> gives us much more freedom.

Right, I too thought of the same but didn't do it in this series.
Current div implementation is pretty complicated.

> With this freedom, we can do the division inline, without branches.  Please 
> see 
> target-mips/translate.c, gen_r6_muldiv.
>
> Basically, we check for the offending cases and modify the divisor prior to 
> the 
> division.  For unsigned:
>
>       a / (b == 0 ? 1 : b)
>
> For signed:
>
>       a / ((a == INT_MAX & b == -1) | (b == 0) ? : b)

Sure, will add it in this series.

Regards
Nikunj




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