There appears to be a problem with -O vs. -Wall on m68k machines running
gcc 2.7.2 and 2.7.2.1.  The following excerpt from calcru() in
kern_resource.c gets the point across pretty clearly, I think.

Anyone with gcc (or general compiler) clue care to take a stab at this?

--scott

# This is a shell archive.  Save it in a file, remove anything before
# this line, and then unpack it by entering "sh file".  Note, it may
# create directories; files and directories will be owned by you and
# have default permissions.
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# This archive contains:
#
#	moof.c
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echo x - moof.c
sed 's/^X//' >moof.c << 'END-of-moof.c'
X/*
X * What follows is a snippet of the NetBSD-current kernel file
X * src/sys/kern/kern_resource.c that I've found causes gcc lossage.
X *
X * Compile me on an m68k-based NetBSD system using gcc 2.7.2 or 2.7.2.1
X * with the following command:
X *
X *	cc -c -O -Wall moof.c
X *
X * Then, be surprised to see:
X *
X *	moof.c: In function `calcru':
X *	moof.c:37: warning: `u' might be used uninitialized in this function
X *
X * This is clearly a bug.  (A similar problem turns up in
X * src/sys/nfs/nfs_vnops.c in nfs_writebp() wrt the `off' variable.)
X */
X
X#define _KERNEL
X#include <sys/param.h>
X#include <sys/types.h>
X#include <sys/kernel.h>
X#include <sys/time.h>
X#include <sys/proc.h>
X
X/*
X * Transform the running time and tick information in proc p into user,
X * system, and interrupt time usage.
X */
Xvoid
Xcalcru(p, up, sp, ip)
X	register struct proc *p;
X	register struct timeval *up;
X	register struct timeval *sp;
X	register struct timeval *ip;
X{
X	register u_quad_t u, st;
X	register long sec, usec;
X
X	st = p->p_sticks;
X	sec = p->p_rtime.tv_sec;
X	usec = p->p_rtime.tv_usec;
X
X	u = (u_quad_t) sec * 1000000 + usec;
X	st = (u * st);
X	sp->tv_sec = st / 1000000;
X}
END-of-moof.c
exit