Date: Mon, 9 Jul 2007 10:55:47 +0800 From: zhouyi zhou <zhouzhouyi@FreeBSD.org> To: freebsd-hackers@FreeBSD.org Cc: freebsd-current@FreeBSD.org Subject: rewrite src/sys/i386/i386/in_cksum.c Message-ID: <20070709105547.71827eb8.zhouzhouyi@FreeBSD.org>
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Hey,
Since FreeBSD update gcc from 3.4.x to 4.2.0, the FreeBSD gdb remote debugger in i386 platform
will find the TCP/UPD checksum will not be computed right, when calling macro in_cksum
which calls in_cksum_skip in src/sys/i386/i386/in_cksum.c in case of getting rid of -O flag
supplied to gcc when compiling(The optimize will interfere with comfortable gdb debugging).
The reason is for example in src/sys/i386/i386/in_cksum.c:
126 if (mlen >= 16) {
127 __asm volatile ("addl %1, %0" : "+r" (sum) : "g" (((const unsigned int *)w)[0 / 4]));
128 __asm volatile ("adcl %1, %0" : "+r" (sum) : "g" (((const unsigned int *)w)[4 / 4]));
129 __asm volatile ("adcl %1, %0" : "+r" (sum) : "g" (((const unsigned int *)w)[8 / 4]));
130 __asm volatile ("adcl %1, %0" : "+r" (sum) : "g" (((const unsigned int *)w)[12 / 4]));
131 __asm volatile ("adcl $0, %0" : "+r" (sum));
132 w += 8;
133 mlen -= 16;
134 }
This is OK with gcc-3.4.x, because the compiler will not insert add instructions between the _asm expressions,
but in gcc-4.x.x case, the compiler will insert add instruction between the _asm expressions which will let carrage bit in eflags cleared.
0x0804877c <in_cksum_skip+780>: cmpl $0xf,0xffffffec(%ebp)
0x08048780 <in_cksum_skip+784>: jle 0x80487d0 <in_cksum_skip+864>
0x08048782 <in_cksum_skip+786>: mov 0xffffffe4(%ebp),%eax
0x08048785 <in_cksum_skip+789>: mov (%eax),%eax
0x08048787 <in_cksum_skip+791>: mov 0xffffffe8(%ebp),%edx
0x0804878a <in_cksum_skip+794>: add %eax,%edx
0x0804878c <in_cksum_skip+796>: mov %edx,0xffffffe8(%ebp)
0x0804878f <in_cksum_skip+799>: mov 0xffffffe4(%ebp),%eax
---Type <return> to continue, or q <return> to quit---
0x08048792 <in_cksum_skip+802>: add $0x4,%eax
0x08048795 <in_cksum_skip+805>: mov (%eax),%eax
0x08048797 <in_cksum_skip+807>: mov 0xffffffe8(%ebp),%edx
0x0804879a <in_cksum_skip+810>: adc %eax,%edx
0x0804879c <in_cksum_skip+812>: mov %edx,0xffffffe8(%ebp)
0x0804879f <in_cksum_skip+815>: mov 0xffffffe4(%ebp),%eax
0x080487a2 <in_cksum_skip+818>: add $0x8,%eax
0x080487a5 <in_cksum_skip+821>: mov (%eax),%eax
0x080487a7 <in_cksum_skip+823>: mov 0xffffffe8(%ebp),%edx
0x080487aa <in_cksum_skip+826>: adc %eax,%edx
0x080487ac <in_cksum_skip+828>: mov %edx,0xffffffe8(%ebp)
0x080487af <in_cksum_skip+831>: mov 0xffffffe4(%ebp),%eax
0x080487b2 <in_cksum_skip+834>: add $0xc,%eax
0x080487b5 <in_cksum_skip+837>: mov (%eax),%eax
0x080487b7 <in_cksum_skip+839>: mov 0xffffffe8(%ebp),%edx
0x080487ba <in_cksum_skip+842>: adc %eax,%edx
0x080487bc <in_cksum_skip+844>: mov %edx,0xffffffe8(%ebp)
So, I suggest rewrite src/sys/i386/i386/in_cksum.c in following style, for example:
--- in_cksum.c 2007-07-06 22:45:52.000000000 +0000
+++ in_cksum1.c 2007-07-07 00:18:25.000000000 +0000
@@ -124,11 +124,14 @@
mlen -= 32;
}
if (mlen >= 16) {
- __asm volatile ("addl %1, %0" : "+r" (sum) : "g" (((const unsigned int *)w)[0 / 4]));
- __asm volatile ("adcl %1, %0" : "+r" (sum) : "g" (((const unsigned int *)w)[4 / 4]));
- __asm volatile ("adcl %1, %0" : "+r" (sum) : "g" (((const unsigned int *)w)[8 / 4]));
- __asm volatile ("adcl %1, %0" : "+r" (sum) : "g" (((const unsigned int *)w)[12 / 4]));
- __asm volatile ("adcl $0, %0" : "+r" (sum));
+ __asm volatile ("addl %1, %0 \n"
+ "adcl %2, %0 \n"
+ "adcl %3, %0 \n"
+ "adcl %4, %0 \n"
+ "adcl $0, %0": "+r"(sum): "g" (((const unsigned int *)w)[0 / 4]),
+ "g" (((const unsigned int *)w)[4 / 4]),
+ "g" (((const unsigned int *)w)[8 / 4]),
+ "g" (((const unsigned int *)w)[12 / 4]));
w += 8;
mlen -= 16;
}
If someone is interested in it, he can lend me a hand to rewrite all of the similiar cases.
Sincerely yours
Zhouyi Zhou
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