Thread View: NET.v7bugs
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Started by ucbvaxmark
Sun, 18 May 1980 22:59
c2
Author: ucbvaxmark
Date: Sun, 18 May 1980 22:59
Date: Sun, 18 May 1980 22:59
23 lines
860 bytes
860 bytes
>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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