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From: Falk Hueffner <falk.hueffner@student.uni-tuebingen.de> To: nobody@gcc.gnu.org Cc: gcc-prs@gcc.gnu.org, Subject: Re: c/10339: strncmp generates imPure code Date: Mon, 07 Apr 2003 19:06:00 -0000 [thread overview] Message-ID: <20030407190601.24608.qmail@sources.redhat.com> (raw) The following reply was made to PR optimization/10339; it has been noted by GNATS. From: Falk Hueffner <falk.hueffner@student.uni-tuebingen.de> To: Wolfgang Bangerth <bangerth@ices.utexas.edu> Cc: Michael Ubell <ubell@mindspring.com>, gcc-bugs@gcc.gnu.org, <gcc-gnats@gcc.gnu.org> Subject: Re: c/10339: strncmp generates imPure code Date: 07 Apr 2003 20:56:15 +0200 Wolfgang Bangerth <bangerth@ices.utexas.edu> writes: > > It generates a memcmp of 4 bytes when one of the arguments > > is 3 bytes long. > > You're a little terse in your descriptions of the problem :-] > > I think I understand now what you mean: with -O, gcc optimizes the strncmp > away and replaces it by a call to memcmp. However, it doesn't take into > account that strncmp compares _at most_ n characters, but that the actual > number may be less than n if one of the two strings is shorter. memcmp on > the other hand always does n bytes. Well, just like in real life, we can do anything we like as long as nobody notices. This optimiziation would only be invalid if * it gives a different result, * or the memory read straddles a page boundary and faults. I suspect the extra byte read is actually not relevant for the result, and because of alignment, gcc knows the second problem cannot occur, but I have neither a SPARC nor SPARC knowledge to test that. -- Falk
next reply other threads:[~2003-04-07 19:06 UTC|newest] Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top 2003-04-07 19:06 Falk Hueffner [this message] -- strict thread matches above, loose matches on Subject: below -- 2003-04-08 1:56 Hans-Peter Nilsson 2003-04-07 21:46 Michael Ubell 2003-04-07 21:46 Andreas Schwab 2003-04-07 21:36 Michael Ubell 2003-04-07 21:26 Andreas Schwab 2003-04-07 20:56 Michael Ubell 2003-04-07 20:06 Andreas Schwab 2003-04-07 20:06 Michael Ubell 2003-04-07 19:56 Michael Ubell 2003-04-07 19:36 Falk Hueffner 2003-04-07 19:36 Andreas Schwab 2003-04-07 19:06 Michael Ubell 2003-04-07 18:16 Wolfgang Bangerth 2003-04-07 17:06 Michael Ubell 2003-04-07 16:48 bangerth 2003-04-07 16:16 ubell
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