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

[Richard Henderson <rth@twiddle.net>]

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 <nikunj@linux.vnet.ibm.com>
> ---
>  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.

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)


r~