From owner-freebsd-current  Wed Apr 21  0:55:43 1999
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Date: Wed, 21 Apr 1999 03:53:13 -0400 (EDT)
From: Luoqi Chen <luoqi@watermarkgroup.com>
Message-Id: <199904210753.DAA22828@lor.watermarkgroup.com>
To: current@freebsd.org
Subject: egcs bug caused ftp hang problem
Sender: owner-freebsd-current@FreeBSD.ORG
Precedence: bulk
X-Loop: FreeBSD.ORG

An egcs optimizer bug caused incorrect tcp checksum recalculation in libalias
for the rewritten PORT command packet and the server subsequently discard the
packet.

The following piece of C code (from TcpChecksum() in alias_util.c)

	u_short *ptr;
	int sum, oddbyte;

	oddbyte = 0; 
	*((u_char *) &oddbyte) = *(u_char *) ptr;
	sum += oddbyte;

is compiled into (%esi = ptr, %ecx = sum)

0x28067038 <TcpChecksum+80>:    movb   (%esi),%al
0x2806703a <TcpChecksum+82>:    addl   %eax,%ecx

egcs should have zeroed %eax before the movb.

libalias compiled without -O works correctly.

-lq


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