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author | Joseph Myers <joseph@codesourcery.com> | 2021-09-21 21:54:37 +0000 |
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committer | Joseph Myers <joseph@codesourcery.com> | 2021-09-21 21:54:37 +0000 |
commit | 1356f38df5be0776823eb2c40cc4e607b86b9680 (patch) | |
tree | 52590f89365894844fac4bb98898e4725445c844 /sysdeps/ieee754/ldbl-128/s_fdivl.c | |
parent | 0b5ca7c3e551e5502f3be3b06453324fe8604e82 (diff) | |
download | glibc-1356f38df5be0776823eb2c40cc4e607b86b9680.tar.gz glibc-1356f38df5be0776823eb2c40cc4e607b86b9680.tar.xz glibc-1356f38df5be0776823eb2c40cc4e607b86b9680.zip |
Fix f64xdivf128, f64xmulf128 spurious underflows (bug 28358)
As described in bug 28358, the round-to-odd computations used in the libm functions that round their results to a narrower format can yield spurious underflow exceptions in the following circumstances: the narrowing only narrows the precision of the type and not the exponent range (i.e., it's narrowing _Float128 to _Float64x on x86_64, x86 or ia64), the architecture does after-rounding tininess detection (which applies to all those architectures), the result is inexact, tiny before rounding but not tiny after rounding (with the chosen rounding mode) for _Float64x (which is possible for narrowing mul, div and fma, not for narrowing add, sub or sqrt), so the underflow exception resulting from the toward-zero computation in _Float128 is spurious for _Float64x. Fixed by making ROUND_TO_ODD call feclearexcept (FE_UNDERFLOW) in the problem cases (as indicated by an extra argument to the macro); there is never any need to preserve underflow exceptions from this part of the computation, because the conversion of the round-to-odd value to the narrower type will underflow in exactly the cases in which the function should raise that exception, but it may be more efficient to avoid the extra manipulation of the floating-point environment when not needed. Tested for x86_64 and x86, and with build-many-glibcs.py.
Diffstat (limited to 'sysdeps/ieee754/ldbl-128/s_fdivl.c')
-rw-r--r-- | sysdeps/ieee754/ldbl-128/s_fdivl.c | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/sysdeps/ieee754/ldbl-128/s_fdivl.c b/sysdeps/ieee754/ldbl-128/s_fdivl.c index dbad331909..7ceb4db4f8 100644 --- a/sysdeps/ieee754/ldbl-128/s_fdivl.c +++ b/sysdeps/ieee754/ldbl-128/s_fdivl.c @@ -28,6 +28,6 @@ float __fdivl (_Float128 x, _Float128 y) { NARROW_DIV_ROUND_TO_ODD (x, y, float, union ieee854_long_double, l, - mantissa3); + mantissa3, false); } libm_alias_float_ldouble (div) |