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authorRichard Henderson <richard.henderson@linaro.org>2018-04-16 16:53:28 -1000
committerPeter Maydell <peter.maydell@linaro.org>2018-04-17 14:52:38 +0100
commitce8d4082054519f2eaac39958edde502860a7fc6 (patch)
tree67a7121204daef2a5b8fac2ce6145c72ac3739f1
parent1b2503fcf7b5932c5a3779ca2ceb92bd403c4ee7 (diff)
fpu: Bound increment for scalbn
Without bounding the increment, we can overflow exp either here in scalbn_decomposed or when adding the bias in round_canonical. This can result in e.g. underflowing to 0 instead of overflowing to infinity. The old softfloat code did bound the increment. Signed-off-by: Richard Henderson <richard.henderson@linaro.org> Reviewed-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Alex Bennée <alex.bennee@linaro.org> Tested-by: Alex Bennée <alex.bennee@linaro.org> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
-rw-r--r--fpu/softfloat.c6
1 files changed, 6 insertions, 0 deletions
diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index d90d79d777..70e0c40a1c 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -1878,6 +1878,12 @@ static FloatParts scalbn_decomposed(FloatParts a, int n, float_status *s)
return return_nan(a, s);
}
if (a.cls == float_class_normal) {
+ /* The largest float type (even though not supported by FloatParts)
+ * is float128, which has a 15 bit exponent. Bounding N to 16 bits
+ * still allows rounding to infinity, without allowing overflow
+ * within the int32_t that backs FloatParts.exp.
+ */
+ n = MIN(MAX(n, -0x10000), 0x10000);
a.exp += n;
}
return a;