Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!wuarchive!mit-eddie!uw-beaver!sumax!amc-gw!pilchuck!dataio!bright
From: bright@Data-IO.COM (Walter Bright)
Newsgroups: comp.lang.c
Subject: Re: Array bounds checking with C????
Message-ID: <2694@dataio.Data-IO.COM>
Date: 7 Sep 90 18:40:53 GMT
References: <DAVEG.90Aug31224630@near.cs.caltech.edu> <1589@redsox.bsw.com> <1990Sep6.203031.29254@laguna.ccsf.caltech.edu>
Organization: Data I/O Corporation; Redmond, WA
Lines: 42

In article <1990Sep6.203031.29254@laguna.ccsf.caltech.edu> jimmy@tybalt.caltech.edu (Jimmy Hu) writes:
<In article <1589@redsox.bsw.com> campbell@redsox.bsw.com (Larry Campbell) writes:
<<Are there actually any current compilers out there that are so stupid that
<<they generate substantially different code for the following two code fragments?
<<
<<    /* Fragment 1 */
<<    for (p = array; p < &array[ARRAY_SIZE]; p++)
<<	*p = '\0';
<<
<<    /* Fragment 2 */
<<    for (i = 0; i < ARRAY_SIZE; i++)
<<	array[i] = '\0';
<<
<The final result will still be the
<same, in that the array is cleared to 0, but I don't think that the compiler
<will "convert" or "optimize" the second to the first or vice versa.

Zortech's optimizer will do this conversion.

<After all,
<assuming it is trying to convert the second to the first, where is it going to
<get a pointer from? Create one?

Yes.

<How will i become ARRAYSIZE after exiting the
<loop? There has to be SOME code difference. I don't think that a compiler
<should have to be sophisticated enough to "guess" at the intent of your code.

If i is used after the exit of the loop, the expression
	i = p - array;
is added to each branch that exits the loop.

<(Another article stated that a MIPS compiler actually generated the SAME
<code for the two fragments. I find that hard to believe unless index i was
<never used anywhere else. Even so, that must have been a sophisticated
<compiler.) 

Thanks for the compliment!

These kinds of optimizations are fairly well understood. Check out
Aho and Ullman's "Compilers" book (the dragon book).