08-171
To: J3
From: Malcolm Cohen
Subject: Interp F95/0102
Date: 2008 May 09

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NUMBER: F95/0102
TITLE: mask-expr evaluated only once
KEYWORDS: WHERE
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION:

Is a mask-expr required to be evaluated in a WHERE such as:

      WHERE ( (/ .TRUE., .TRUE. /) )
        A = A + 1
      ELSE WHERE (MASKF())
        A = MAX
      END WHERE

where an optimizing compiler might notice that the ELSEWHERE
branch will never be executed ?

Page 112, line 38 says of the mask expression in ELSEWHERE statements

"The mask-expression is evaluated only once."

ANSWER:

The above quote is from the Fortran 95 standard.  In Fortran 2003 the
mask expression in an ELSEWHERE is not required to be evaluated, and
this is made clear by the fourth paragraph of 7.4.3.2 (at [147:7])
which, referring to a masked ELSEWHERE statement, states
  "The <mask-expr> is evaluated at most once."

This also applies to the mask expressions of nested WHERE construct
statements; this is made clear by the last sentence of the seventh
paragraph of 7.4.3.2 (at [147:16]) which also states that it is
evaluated "at most once".

However, this does not apply to the mask expression of the outermost
WHERE construct statement, which must be evaluated exactly once; this
is made clear by the last statement of the first paragraph of 7.4.3.2
(at [147:1]) which states that this mask expression is evaluated "only
once".  That does not guarantee invocation of functions in that
expression if they are not required to determine its value.

EDITS:

None.

SUBMITTED BY: Matthijs van Waveren

HISTORY: 03-239    m165  F95/0102 Passed by J3 meeting vote
         04-417r1  m170  Passed by J3 letter ballot #8
         05-180    m172  Failed WG5 ballot N1617
         08-171    m184  Revised answer.

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