Date: Mon, 15 Nov 1999 19:43:57 +0100 From: Pierre Beyssac <beyssac@enst.fr> To: Garrett Wollman <wollman@khavrinen.lcs.mit.edu> Cc: Sheldon Hearn <sheldonh@uunet.co.za>, freebsd-current@FreeBSD.ORG Subject: Re: egcs -O breaks ping.c:in_cksum() Message-ID: <19991115194357.T28348@enst.fr> In-Reply-To: <199911151835.NAA07676@khavrinen.lcs.mit.edu>; from Garrett Wollman on Mon, Nov 15, 1999 at 01:35:15PM -0500 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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On Mon, Nov 15, 1999 at 01:35:15PM -0500, Garrett Wollman wrote:
> If, rather than casting pointers, the code used a union (containing
> one u_int16_t and one array[2] of u_int8_t), the compiler would have
> enough information to know about the aliases.
You're right, this seems to work even with optimization turned on.
If nobody objects, I'll commit it.
--- ck.c.old Mon Nov 15 19:41:35 1999
+++ ck.c Mon Nov 15 19:39:43 1999
@@ -13,7 +13,10 @@
register int nleft = len;
register u_short *w = addr;
int sum = 0;
- volatile u_short answer = 0;
+ union {
+ u_int16_t us;
+ u_int8_t uc[2];
+ } answer;
/*
* Our algorithm is simple, using a 32 bit accumulator (sum), we add
@@ -27,15 +30,16 @@
/* mop up an odd byte, if necessary */
if (nleft == 1) {
- *(u_char *)(&answer) = *(u_char *)w ;
- sum += answer;
+ answer.uc[0] = *(u_char *)w ;
+ answer.uc[1] = 0;
+ sum += answer.us;
}
/* add back carry outs from top 16 bits to low 16 bits */
sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */
sum += (sum >> 16); /* add carry */
- answer = ~sum; /* truncate to 16 bits */
- return(answer);
+ answer.us = ~sum; /* truncate to 16 bits */
+ return(answer.us);
}
int main()
--
Pierre Beyssac pb@enst.fr
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