From 9b300a100c6db039d32e444d587c292369585eee Mon Sep 17 00:00:00 2001 From: Shivaprasad G Bhat Date: Tue, 2 May 2023 20:55:30 +0530 Subject: [PATCH] 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 Signed-off-by: Vaibhav Jain Message-Id: <168304110865.537992.13059030916325018670.stgit@localhost.localdomain> Reviewed-by: Richard Henderson Signed-off-by: Richard Henderson (cherry picked from commit 1098cc3fcf952763fc9fd72c1c8fda30a18cc8ea) Signed-off-by: Michael Tokarev --- fpu/softfloat.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fpu/softfloat.c b/fpu/softfloat.c index c7454c3eb1..108f9cb224 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); }