softfloat: Fix the incorrect computation in float32_exp2
The float32_exp2 function 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 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'. Commit '572c4d862ff2' refactored this
function, and mistakenly used xp instead of xnp.
Cc: qemu-stable@nongnu.org
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 <sbhat@linux.ibm.com>
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Message-Id: <168304110865.537992.13059030916325018670.stgit@localhost.localdomain>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index c7454c3..108f9cb 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -5135,7 +5135,7 @@
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);
}
