[PATCH] softfloat: Fix the incorrect computation in float32_exp2()

[Shivaprasad G Bhat]

The float32_exp2() is computing wrong exponent of 2.
For example, with the following set of values {0.1, 2.0, 2.0, -1.0},
the expected output would be {1.071773, 4.000000, 4.000000, 0.500000}.
Instead, the function is computing {1.119102, 3.382044, 3.382044, -0.191022}

Looking at the code, the float32_exp2() attempts to do this

                  2     3     4     5           n
  x        x     x     x     x     x           x
 e  = 1 + --- + --- + --- + --- + --- + ... + --- + ...
           1!    2!    3!    4!    5!          n!

But because of the 'typo'/bug it ends up doing

 x        x     x     x     x     x           x
e  = 1 + --- + --- + --- + --- + --- + ... + --- + ...
          1!    2!    3!    4!    5!          n!

This is because instead of the xnp which holds the numerator,
parts_muladd is using the xp which is just 'x'. The commit '572c4d862ff2'
refactored this function, and it seems mistakenly using xp instead of xnp.

The patches fixes this possible typo.

Fixes: 572c4d862ff2 "softfloat: Convert float32_exp2 to FloatParts"
Partially-Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1623
Reported-By: Luca Barbato (https://gitlab.com/lu-zero)
Signed-off-by: Shivaprasad G Bhat <sb...@linux.ibm.com>
Signed-off-by: Vaibhav Jain <vaib...@linux.ibm.com>
---
 fpu/softfloat.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index c7454c3eb1a..108f9cb224a 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -5135,7 +5135,7 @@ float32 float32_exp2(float32 a, float_status *status)
     float64_unpack_canonical(&rp, float64_one, status);
     for (i = 0 ; i < 15 ; i++) {
         float64_unpack_canonical(&tp, float32_exp2_coefficients[i], status);
-        rp = *parts_muladd(&tp, &xp, &rp, 0, status);
+        rp = *parts_muladd(&tp, &xnp, &rp, 0, status);
         xnp = *parts_mul(&xnp, &xp, status);
     }