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Date: Fri, 11 Oct 1996 19:17:11 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199610110917.TAA20179@godzilla.zeta.org.au>
To: bugs@freebsd.org, jin@george.lbl.gov
Subject: Re: compiler bug in 2.2-961006-SNAP release
Sender: owner-bugs@freebsd.org
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>2.2-961006-SNAP introduces a bug in C compiler. The initialization uses
>memset which is only available in user space. So, if kernel has such code
>like:
>
>Mystruct V = {any#};
               ^^^^all zeros
>
>It will causes kernel linking failure:
>
>loading kernel
>znatm.Dro: Undefined symbol `_memset' referenced from text segment

This is best fixed by not using auto initializers for structs.  They tend
to be inefficient, and for all-zero initializes, gcc-2.7-2 only handles
ones of the following sizes efficiently: 1, 2, 4, 8.  It calls memset()
to for all other cases.  gcc-2.7.2 handles small nonzero auto struct
initializers better by copying a template using its builtin memcpy
(except for -O0 it calls memcpy).

I fixed this problem in ipx_usrreq.c.  The initializer was redundant.

Bruce

RCS file: /a/ncvs/src/sys/netipx/ipx_usrreq.c,v
----------------------------
revision 1.6
date: 1996/04/13 14:37:22;  author: jhay;  state: Exp;  lines: +6 -2
Don't use a newfangled auto initializer.  Initialize everything by
assignment to avoid one bug and several pessimizations.

In the old version, gcc-2.6.3 (i386 version) generates 16 bytes
of static data and copies it using 4 4-byte load-stores.  gcc-2.7.2
generates 2 1-byte stores and calls memset() to zero 14 bytes.
Linking fails because memset() doesn't exist in the kernel.

In both versions, the 2 bytes stored directly are all that is
actually used unless the null padding at the end is used, since
the 3 4-byte words in the middle are initialized again by struct
assignment.  These words are misaligned.  gcc generates misaligned
load-stores for (small) misaligned struct copies.

Submitted by:	Bruce Evans