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From: "adhemerval.zanella at linaro dot org"
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Subject: [Bug math/27460] hypot(__DBL_DENORM_MIN__, -0.) incorrectly raises
 FE_INEXACT and FE_UNDERFLOW
Date: Tue, 23 Feb 2021 20:13:25 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D27460

Adhemerval Zanella <adhemerval.zanella at linaro dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |adhemerval.zanella at lina=
ro dot o
                   |                            |rg

--- Comment #1 from Adhemerval Zanella <adhemerval.zanella at linaro dot or=
g> ---
Afaik since we define __STDC_IEC_559__ POSIX allows the implementation to
return an error range [1].  This is in fact what the fix for BZ#18803 does,
where it explicit raise underflow exception when the result is tiny and
inexact.

I think we are in fact missing a underflow check for the case where second
argument is '0':

diff --git a/sysdeps/ieee754/dbl-64/e_hypot.c
b/sysdeps/ieee754/dbl-64/e_hypot.c
index 9ec4c1ced0..bcd172130e 100644
--- a/sysdeps/ieee754/dbl-64/e_hypot.c
+++ b/sysdeps/ieee754/dbl-64/e_hypot.c
@@ -100,7 +100,10 @@ __ieee754_hypot (double x, double y)
          uint32_t low;
          GET_LOW_WORD (low, b);
          if ((hb | low) =3D=3D 0)
-           return a;
+           {
+             math_check_force_underflow (x);
+             return a;
+           }
          t1 =3D 0;
          SET_HIGH_WORD (t1, 0x7fd00000);       /* t1=3D2^1022 */
          b *=3D t1;


[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/hypot.html

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