Re: [Qemu-devel] [PATCH v2 4/7] softfloat: fallback to __int128 maths for s390x and others

[Richard Henderson <richard.henderson@linaro.org>]

On 1/17/19 7:23 AM, Alex Bennée wrote:
> Apparently some versions of clang can't handle inline assembly with
> __int128 parameters, especially on s390. Instead of hand-coding the
> s390 divide provide a generic fallback for anything that provides
> __int128 capable maths.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Cc: Thomas Huth <thuth@redhat.com>
> ---
>  include/fpu/softfloat-macros.h | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/include/fpu/softfloat-macros.h b/include/fpu/softfloat-macros.h
> index b1d772e6d4..1a43609eef 100644
> --- a/include/fpu/softfloat-macros.h
> +++ b/include/fpu/softfloat-macros.h
> @@ -641,12 +641,6 @@ static inline uint64_t udiv_qrnnd(uint64_t *r, uint64_t n1,
>      uint64_t q;
>      asm("divq %4" : "=a"(q), "=d"(*r) : "0"(n0), "1"(n1), "rm"(d));
>      return q;
> -#elif defined(__s390x__)
> -    /* Need to use a TImode type to get an even register pair for DLGR.  */
> -    unsigned __int128 n = (unsigned __int128)n1 << 64 | n0;
> -    asm("dlgr %0, %1" : "+r"(n) : "r"(d));
> -    *r = n >> 64;
> -    return n;
>  #elif defined(_ARCH_PPC64) && defined(_ARCH_PWR7)
>      /* From Power ISA 2.06, programming note for divdeu.  */
>      uint64_t q1, q2, Q, r1, r2, R;
> @@ -663,6 +657,10 @@ static inline uint64_t udiv_qrnnd(uint64_t *r, uint64_t n1,
>      }
>      *r = R;
>      return Q;
> +#elif defined(CONFIG_INT128)
> +    unsigned __int128 n = (unsigned __int128)n1 << 64 | n0;
> +    *r = n % d;
> +    return n / d;
>  #else

I thought that we'd shown that, at least at present, no compiler is taking
advantage of hardware insns for this, and is promoting this to a full 128-bit
divide.  And further that the version using 64-bit arithmetic was competitive
with the hardware insn.

I'd rather not include this hunk for now.


r~