From: dewar@gnat.com
To: egcs@tantalophile.demon.co.uk, schwab@suse.de
Cc: aoliva@redhat.com, denisc@overta.ru, dkorn@pixelpower.com,
gcc@gcc.gnu.org, peter.osterlund@mailbox.swipnet.se
Subject: Re: Bug in loop optimize (invalid postinc to preinc transformation)
Date: Sun, 31 Dec 2000 08:30:00 -0000 [thread overview]
Message-ID: <20001231163045.5B3B534D81@nile.gnat.com> (raw)
<<Comparing pointers that don't point to the same (part of) object is
undefined.
>>
Indeed. The important thing to remember in C is that the semantic
model of a pointer is a pair (pointer-to-object, offset). Address
arithmetic just modifies the offset, comparison compares only the
offsets (and is undefined if the pointer-to-object values are
different).
Of course in practice the implementation is typically a single pointer
from a flat space, representing the sum of the two components, but the
proper semantic model needs to be kept in mind, and code that does not
respect this semantic model has undefined effects.
As C compilers get more clever, they are likely to be less permissive
to abuses of the semantics :-)
next reply other threads:[~2000-12-31 8:30 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2000-12-31 8:30 dewar [this message]
2001-01-03 0:52 ` Jamie Lokier
-- strict thread matches above, loose matches on Subject: below --
2000-12-29 18:43 dewar
2000-12-30 15:18 ` Toon Moene
2000-12-28 20:33 dewar
2000-12-28 5:40 Robert Dewar
2000-12-28 12:21 ` Alexandre Oliva
2000-12-28 14:32 ` Geoff Keating
2000-12-28 15:22 ` Tim Hollebeek
2000-12-28 22:20 ` Dave Korn
2000-12-29 0:52 ` Richard Henderson
2000-12-29 1:51 ` Tim Hollebeek
2000-12-28 1:14 Richard Kenner
2000-12-27 22:40 Robert Dewar
2000-12-29 18:36 ` Jamie Lokier
2000-12-27 3:04 Bug in loop optimize (TREE stage) David Korn
2000-12-27 15:42 ` Bug in loop optimize (invalid postinc to preinc transformation) Peter Osterlund
2000-12-27 19:55 ` Alexandre Oliva
2000-12-27 21:09 ` Torbjorn Granlund
2000-12-27 21:20 ` Alexandre Oliva
2000-12-27 22:24 ` Jamie Lokier
2000-12-31 8:25 ` Andreas Schwab
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