To: J3                                                     J3/17-252
From: Robert Corbett
Subject: IEEE_UNDERFLOW
Date: 2017 October 19
Reference: 17-007r2

1. Introduction

This paper modifies the standard's description of underflow handling
to bring it in line with ISO/IEC/IEEE 60559:2011.  The change is a
technical fix.


2. Description

ISO/IEC/IEEE 60559:2011 specifies that an underflow exception is
signaled whenever a tiny result is produced.  IEEE default exception
handling does not set the underflow flag if the result of the
operation that caused the underflow was the exact result.
ISO/IEC/IEEE 60559:2011 says that in such a case, signaling
underflow causes no observable effects.

Underflow handling as described in 17-007r2 does not conform to
ISO/IEC/IEEE 60559:2011.  In particular, if the halting mode for
IEEE_UNDERFLOW is true, halting should occur even if there is no
loss of accuracy.

Edits to improve conformance with ISO/IEC/IEEE 60559:2011 are
provided.


3. Edits to 17-007r2


[449:16] 17.3 The exceptions, p1
         Delete "and loss of accuracy is detected"

[449:18] 17.3 The exceptions, p1
         Delete "and loss of accuracy is detected"

[449:21] 17.3 The exceptions, p2
         Replace the sentence
         "Its initial value is quiet and it signals when the
          associated exception occurs."
         with
         "Its initial value is quiet.  It is set to signaling
          when the associated exception occurs, except that
          the flag for IEEE_UNDERFLOW is not set if the result
          of the operation that caused the exception was exact
          and default IEEE exception handling is in effect for
          IEEE_UNDERFLOW."