Aucbvax.114
NET.v7bugs
utzoo!duke!vax135!ucbvax!mark
Sun May 18 22:59:08 1980
c2
>From mark Sat Apr 19 01:18:51 1980
To: v7bugs
Subject: bug in pdp-11 C optimizer
The following program demonstrates a bug in the V7 pdp-11 /lib/c2.
#include <STDIO.H>
main () {
	int x;
	int t1;

	x = 0;
	x += 1;
	t1 = x != 1;
	printf("x=%d, t1=%d\n", x, t1);
	exit(0);
}
It should print 1, 0, but it prints 1, 1.  Seems that the optimizer has
correctly noticed that the value of x can be predicted at compile time
and hence the comparison can be precomputed, but it does the precomputation
wrong.  Changing the 2 assignments to x to "x = 1" works properly.

I don't have a fix for this, but dmr's (more recent than v7) version of
c2 seems to handle it correctly.
	Mark


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