Re: [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), c...@braap.org wrote:

From: Kito Cheng <kito.ch...@gmail.com>

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 <kito.ch...@gmail.com>
Signed-off-by: Emilio G. Cota <c...@braap.org>

Tested-by: Palmer Dabbelt <pal...@sifive.com>

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;
         }
     }