Date: Mon, 15 Nov 1999 11:27:57 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Garrett Wollman <wollman@khavrinen.lcs.mit.edu> Cc: Pierre Beyssac <beyssac@enst.fr>, Sheldon Hearn <sheldonh@uunet.co.za>, freebsd-current@FreeBSD.ORG Subject: Re: egcs -O breaks ping.c:in_cksum() Message-ID: <199911151927.LAA44755@apollo.backplane.com> References: <19991115174831.B30139@enst.fr> <92805.942684743@axl.noc.iafrica.com> <19991115180145.A31542@enst.fr> <199911151835.NAA07676@khavrinen.lcs.mit.edu>
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:It's not a compiler bug, it's a source code bug.
:
:The C Language specifies that pointers to distinct types can be
:assumed, under certain conditions, never to alias one another. (This
:...
:Recent values of GCC make use of this obscure language feature to
:improve optimization. Essentially, the optimizer can assume that
:...
:-GAWollman
:
:--
:Garrett A. Wollman | O Siem / We are all family / O Siem / We're all the same
Someone try the patch below and tell me if it fixes the problem. If it
does then I'll commit it. If someone else wants to commit a 'better'
fix, be my guest! (but inform the list that you've done so). Otherwise
this is the one that will go in.
-Matt
Index: ping.c
===================================================================
RCS file: /home/ncvs/src/sbin/ping/ping.c,v
retrieving revision 1.45
diff -u -r1.45 ping.c
--- ping.c 1999/08/28 00:13:59 1.45
+++ ping.c 1999/11/15 19:26:23
@@ -920,6 +920,9 @@
/*
* in_cksum --
* Checksum routine for Internet Protocol family headers (C Version)
+ *
+ * note: volatilization of 'answer' is a bad hack to work around an
+ * aliasing problem.
*/
u_short
in_cksum(addr, len)
@@ -929,7 +932,7 @@
register int nleft = len;
register u_short *w = addr;
register int sum = 0;
- u_short answer = 0;
+ volatile u_short answer = 0;
/*
* Our algorithm is simple, using a 32 bit accumulator (sum), we add
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