J3/11-222
To: J3
From: Malcolm Cohen
Subject: Relational equivalence
Date: 2011 June 30

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NUMBER: F03/0065
TITLE: Relational equivalence
KEYWORDS: Relational equivalence
DEFECT TYPE: Interpretation
STATUS: J3 consideration in progress

QUESTION:

Given
  REAL X
  X = ... some value ...
may
  IF( X+3.0 .EQ. 3.0 )...
be transformed into
  IF( X .EQ. 0.0 )...
by the processor?

In Fortran 2003, 7.1.8.5 Evaluation of relational intrinsic operations
says
  "Two relational intrinsic operations are relationally equivalent if
   their logical values are equal for all possible values of their
   primaries."

On a machine where addition of 3.0 to a small value is not exact, the
logical values for X+3.0==3.0 are not the same as X==0.0 for all
possible values of X, therefore it would seem that this transformation
would not be possible.

However, Note 7.22 in Fortran 2003 shows this transformation as being
acceptable.

ANSWER:

No, the transformation is not permitted unless it gives the same
answer for all possible values (of X and X+3.0).

The erroneous example has been removed in Fortran 2008.

EDIT:

None.

SUBMITTED BY: Fred Tydeman

HISTORY: 05-192    m173  F03/0065 submitted
         09-150    m187  Passed by J3 meeting
         09-187r2  m188  Failed J3 letter ballot #18 09-155
         11-nnn    m195  Revised answer.

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