Re: [Qemu-devel] [PATCH v2 09/14] hardfloat: support float32/64 multiplication

[Emilio G. Cota]

On Wed, Mar 28, 2018 at 14:26:30 +0100, Alex Bennée wrote:
> Emilio G. Cota <address@hidden> writes:
> OK I've had a bit more of a play and I think we can drop the macro abuse
> and have common wrappers for the host_fpu. We don't want to intermingle
> with the soft float slow path to stop the compiler adding overhead. We
> also need a wrapper for each float size and op count due to differences
> in the classify functions. However the boiler plate is pretty common and
> where there are differences the compiler is smart enough to fix it.
> 
> See branch:
> https://github.com/stsquad/qemu/tree/hostfloat/common-fpu-wrapper
> 
> I keep the numbers for add/sub and doubled the speed of float32_mul on
> my box, without any macros ;-)

I really like the idea of letting the compiler unfold everything.
In fact I just did that to re-implement fp-bench (now with support
for -t host/soft, yay).

> Full patch inline:
> 
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index d0f1f65c12..89217b5e67 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -879,56 +879,72 @@ soft_float64_sub(float64 a, float64 b, float_status 
> *status)
>      return float64_round_pack_canonical(pr, status);
>  }
(snip)
> +static float fpu_mul32(float a, float b, bool *nocheck) {
> +
> +    if (float32_is_zero(a) || float32_is_zero(b)) {
> +        bool signbit = float32_is_neg(a) ^ float32_is_neg(b);
> +        *nocheck = true;
> +        return float32_set_sign((0), signbit);
> +    } else {
> +        float ha = float32_to_float(a);
> +        float hb = float32_to_float(b);
> +        float hr = ha * hb;
> +        return hr;
>      }
> +}

This function is wrong :-(

Note that a and b are floats, not float32's. So if any of
them is 0.X then they get silently converted to 0, which goes via the
fast(er) path above. This explains the speedup.

Note that you could have caught this with:

  $ ./fp-test -t soft ibm/* -w whitelist.txt -e x

Compiling with -Wconversion would also point these out, but the output
is way too noisy to be useful.


That said, I'll take inspiration from your approach for v3--hopefully
without (many) macros this time round.

Thanks!

                Emilio