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NNTP-Posting-Date: Fri, 04 Nov 2005 17:27:52 -0600
From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Default rep specs for record types - documented??
References: <plMaf.40$w_6.36@trndny09>
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Date: Fri, 04 Nov 2005 17:27:52 -0600
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Date: 2005-11-04T17:27:52-06:00
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> The raison d'�tre behind record representation clauses is to make sure
> that a record layout matches some external data format. For internal
> data, you would be better off forgoing the representation clauses and
> letting the compiler choose the optimal record layout.
    Absolutely.  Alignment can make a substantial performance difference
on some machines, so re-ordering or inserting padding will be an
optimization.  If you insert a rep spec where it really isn't needed
(whether a hand or compiler or tool generated one) it may prevent
valuable, and non-obvious, optimization when the program is recompiled
for a different CPU.  IIRC, at least one Ada compiler, given
  type R is record
    A : integer;
    B : integer range 40 .. 40 := 40;
    C : integer;
  end record;
allocated zero bits for B since its value was known to be a constant 40.
Underspecifying can be erroneous, but overspecifying can also have bad
results.