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From: "jsm28 at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug math/18370] New: csqrt missing underflows
Date: Sat, 02 May 2015 20:43:00 -0000
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X-Bugzilla-Product: glibc
X-Bugzilla-Component: math
X-Bugzilla-Version: 2.21
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https://sourceware.org/bugzilla/show_bug.cgi?id=18370

            Bug ID: 18370
           Summary: csqrt missing underflows
           Product: glibc
           Version: 2.21
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org
  Target Milestone: ---

The csqrt implementations in glibc can miss underflow exceptions when the
result becomes tiny in the course of multiplying by 0.5 (or other scaling down)
and that scaling happens to be exact (but the mathematical result is not
exact).  E.g., for float on x86_64:

Failure: Real part of: csqrt_downward (-0x1.08e754p+16 - 0x7.ede03p-120 i):
Exception "Underflow" not set
Failure: Real part of: csqrt_downward (0xb.6554ep+0 + 0x1.247482p-124 i):
Exception "Underflow" not set
Failure: Real part of: csqrt_towardzero (0xb.6554ep+0 + 0x1.247482p-124 i):
Exception "Underflow" not set

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