57 lines
2.4 KiB
Diff
57 lines
2.4 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Shivaprasad G Bhat <sbhat@linux.ibm.com>
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Date: Tue, 2 May 2023 20:55:30 +0530
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Subject: [PATCH] softfloat: Fix the incorrect computation in float32_exp2
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The float32_exp2 function is computing wrong exponent of 2.
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For example, with the following set of values {0.1, 2.0, 2.0, -1.0},
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the expected output would be {1.071773, 4.000000, 4.000000, 0.500000}.
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Instead, the function is computing {1.119102, 3.382044, 3.382044, -0.191022}
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Looking at the code, the float32_exp2() attempts to do this
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2 3 4 5 n
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x x x x x x x
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e = 1 + --- + --- + --- + --- + --- + ... + --- + ...
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1! 2! 3! 4! 5! n!
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But because of the typo it ends up doing
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x x x x x x x
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e = 1 + --- + --- + --- + --- + --- + ... + --- + ...
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1! 2! 3! 4! 5! n!
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This is because instead of the xnp which holds the numerator, parts_muladd
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is using the xp which is just 'x'. Commit '572c4d862ff2' refactored this
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function, and mistakenly used xp instead of xnp.
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Cc: qemu-stable@nongnu.org
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Fixes: 572c4d862ff2 "softfloat: Convert float32_exp2 to FloatParts"
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Partially-Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1623
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Reported-By: Luca Barbato (https://gitlab.com/lu-zero)
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Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
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Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
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Message-Id: <168304110865.537992.13059030916325018670.stgit@localhost.localdomain>
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Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
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Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
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(cherry-picked from commit 1098cc3fcf952763fc9fd72c1c8fda30a18cc8ea)
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Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
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---
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fpu/softfloat.c | 2 +-
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1 file changed, 1 insertion(+), 1 deletion(-)
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diff --git a/fpu/softfloat.c b/fpu/softfloat.c
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index c7454c3eb1..108f9cb224 100644
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--- a/fpu/softfloat.c
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+++ b/fpu/softfloat.c
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@@ -5135,7 +5135,7 @@ float32 float32_exp2(float32 a, float_status *status)
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float64_unpack_canonical(&rp, float64_one, status);
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for (i = 0 ; i < 15 ; i++) {
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float64_unpack_canonical(&tp, float32_exp2_coefficients[i], status);
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- rp = *parts_muladd(&tp, &xp, &rp, 0, status);
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+ rp = *parts_muladd(&tp, &xnp, &rp, 0, status);
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xnp = *parts_mul(&xnp, &xp, status);
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}
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