[PATCH v3 2/5] powerpc/lib/sstep: Add popcnt instruction emulation

[Balbir Singh]

On Tue, Jul 25, 2017 at 8:24 PM, David Laight <David.Laight at aculab.com> wrote:
> From: Linuxppc-dev [mailto:linuxppc-dev-bounces+david.laight=aculab.com at lists.ozlabs.org] On Behalf Of
>> Matt Brown
>> Sent: 25 July 2017 04:33
>> To: linuxppc-dev at lists.ozlabs.org
>> Subject: [PATCH v3 2/5] powerpc/lib/sstep: Add popcnt instruction emulation
>>
>> This adds emulations for the popcntb, popcntw, and popcntd instructions.
>> Tested for correctness against the popcnt{b,w,d} instructions on ppc64le.
>>
>> Signed-off-by: Matt Brown <matthew.brown.dev at gmail.com>
>> ---
>> v3:
>>       - optimised using the Giles-Miller method of side-ways addition
>> v2:
>>       - fixed opcodes
>>       - fixed typecasting
>>       - fixed bitshifting error for both 32 and 64bit arch
>> ---
>>  arch/powerpc/lib/sstep.c | 40 +++++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 39 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
>> index 87d277f..c1f9cdb 100644
>> --- a/arch/powerpc/lib/sstep.c
>> +++ b/arch/powerpc/lib/sstep.c
>> @@ -612,6 +612,32 @@ static nokprobe_inline void do_cmpb(struct pt_regs *regs, unsigned long v1,
>>       regs->gpr[rd] = out_val;
>>  }
>>
>> +/*
>> + * The size parameter is used to adjust the equivalent popcnt instruction.
>> + * popcntb = 8, popcntw = 32, popcntd = 64
>> + */
>> +static nokprobe_inline void do_popcnt(struct pt_regs *regs, unsigned long v1,
>> +                             int size, int ra)
>> +{
>> +     unsigned long long out = v1;
>> +
>> +     out = (0x5555555555555555 & out) + (0x5555555555555555 & (out >> 1));
>> +     out = (0x3333333333333333 & out) + (0x3333333333333333 & (out >> 2));
>> +     out = (0x0f0f0f0f0f0f0f0f & out) + (0x0f0f0f0f0f0f0f0f & (out >> 4));
>> +     if (size == 8) {        /* popcntb */
>> +             regs->gpr[ra] = out;
>
> I'm pretty sure you need to mask the result with 7.
>
Absolutely! Good catch!

Balbir Singh.