Re: [Qemu-riscv] [Qemu-devel] [PATCH v2 for-4.0] hardfloat: fix float32/64 fused multiply-add

[Palmer Dabbelt]

On Fri, 22 Mar 2019 13:43:20 PDT (-0700), address@hidden wrote:
> From: Kito Cheng <address@hidden>
>
> Before falling back to softfloat FMA, we do not restore the original
> values of inputs A and C. Fix it.
>
> This bug was caught by running gcc's testsuite on RISC-V qemu.
>
> Note that this change gives a small perf increase for fp-bench:
>
>   Host: Intel(R) Core(TM) i7-4790K CPU @ 4.00GHz
>   Command: perf stat -r 3 taskset -c 0 ./fp-bench -o mulAdd -p $prec
>
> - $prec = single:
>   - before:
>     101.71 MFlops
>     102.18 MFlops
>     100.96 MFlops
>   - after:
>     103.63 MFlops
>     103.05 MFlops
>     102.96 MFlops
>
> - $prec = double:
>   - before:
>     173.10 MFlops
>     173.93 MFlops
>     172.11 MFlops
>   - after:
>     178.49 MFlops
>     178.88 MFlops
>     178.66 MFlops
>
> Signed-off-by: Kito Cheng <address@hidden>
> Signed-off-by: Emilio G. Cota <address@hidden>

Tested-by: Palmer Dabbelt <address@hidden>

Thanks for fixing this!

> ---
>  fpu/softfloat.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index 4610738ab1..2ba36ec370 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -1596,6 +1596,9 @@ float32_muladd(float32 xa, float32 xb, float32 xc, int 
> flags, float_status *s)
>          }
>          ur.h = up.h + uc.h;
>      } else {
> +        union_float32 ua_orig = ua;
> +        union_float32 uc_orig = uc;
> +
>          if (flags & float_muladd_negate_product) {
>              ua.h = -ua.h;
>          }
> @@ -1608,6 +1611,8 @@ float32_muladd(float32 xa, float32 xb, float32 xc, int 
> flags, float_status *s)
>          if (unlikely(f32_is_inf(ur))) {
>              s->float_exception_flags |= float_flag_overflow;
>          } else if (unlikely(fabsf(ur.h) <= FLT_MIN)) {
> +            ua = ua_orig;
> +            uc = uc_orig;
>              goto soft;
>          }
>      }
> @@ -1662,6 +1667,9 @@ float64_muladd(float64 xa, float64 xb, float64 xc, int 
> flags, float_status *s)
>          }
>          ur.h = up.h + uc.h;
>      } else {
> +        union_float64 ua_orig = ua;
> +        union_float64 uc_orig = uc;
> +
>          if (flags & float_muladd_negate_product) {
>              ua.h = -ua.h;
>          }
> @@ -1674,6 +1682,8 @@ float64_muladd(float64 xa, float64 xb, float64 xc, int 
> flags, float_status *s)
>          if (unlikely(f64_is_inf(ur))) {
>              s->float_exception_flags |= float_flag_overflow;
>          } else if (unlikely(fabs(ur.h) <= FLT_MIN)) {
> +            ua = ua_orig;
> +            uc = uc_orig;
>              goto soft;
>          }
>      }