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From: Richard Henderson <rth@twiddle.net>
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Date: Fri, 17 Feb 2017 07:52:18 +1100
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Subject: Re: [Qemu-devel] [PATCH 5/6] target-ppc: support for 32-bit carry
 and overflow
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To: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>, David Gibson <david@gibson.dropbear.id.au>
Cc: bharata@linux.vnet.ibm.com, qemu-ppc@nongnu.org, qemu-devel@nongnu.org

On 02/16/2017 04:08 PM, Nikunj A Dadhania wrote:
> Richard Henderson <rth@twiddle.net> writes:
>
>> On 02/14/2017 02:05 PM, Nikunj A Dadhania wrote:
>>> Yes, you are right. I had a discussion with Paul Mackerras yesterday, he
>>> explained to me in detail about the bits. I am working on the revised
>>> implementation. Will detail it in the commit message.
>>
>> As you're working on this, consider changing the definition of cpu_ov such that
>> the MSB is OV and bit 31 is OV32.
>>
>> E.g.
>>
>>
>>   static inline void gen_op_arith_compute_ov(DisasContext *ctx, TCGv arg0,
>>                                              TCGv arg1, TCGv arg2, int sub)
>>   {
>>       TCGv t0 = tcg_temp_new();
>>
>>       tcg_gen_xor_tl(cpu_ov, arg0, arg2);
>>       tcg_gen_xor_tl(t0, arg1, arg2);
>>       if (sub) {
>>           tcg_gen_and_tl(cpu_ov, cpu_ov, t0);
>>       } else {
>>           tcg_gen_andc_tl(cpu_ov, cpu_ov, t0);
>>       }
>>       tcg_temp_free(t0);
>>       if (NARROW_MODE(ctx)) {
>>           tcg_gen_ext32s_tl(cpu_ov, cpu_ov);
>>       }
>> -    tcg_gen_shri_tl(cpu_ov, cpu_ov, TARGET_LONG_BITS - 1);
>>       tcg_gen_or_tl(cpu_so, cpu_so, cpu_ov);
>>   }
>>
>>
>> is all that is required for arithmetic to compute OV and OV32 into those two bits.
>
> How about the below?
>
> @@ -809,10 +809,11 @@ static inline void gen_op_arith_compute_ov(DisasContext *ctx, TCGv arg0,
>          tcg_gen_andc_tl(cpu_ov, cpu_ov, t0);
>      }
>      tcg_temp_free(t0);
> +    tcg_gen_extract_tl(cpu_ov32, cpu_ov, 31, 1);
> +    tcg_gen_extract_tl(cpu_ov, cpu_ov, 63, 1);
>      if (NARROW_MODE(ctx)) {
> -        tcg_gen_ext32s_tl(cpu_ov, cpu_ov);
> +        tcg_gen_mov_tl(cpu_ov, cpu_ov32);
>      }
> -    tcg_gen_shri_tl(cpu_ov, cpu_ov, TARGET_LONG_BITS - 1);
>      tcg_gen_or_tl(cpu_so, cpu_so, cpu_ov);
>  }

Why do you want to extract these bits?


r~