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SubjectRe: Compilation problem in ndisc.c / 2.5.1-pre2 : possible gcc bug?
On Wed, 28 Nov 2001, Brian Gerst wrote:

> "Richard B. Johnson" wrote:
> >
> > On Wed, 28 Nov 2001, Martin Eriksson wrote:
> >
> > > ----- Original Message -----
> > > From: "Richard B. Johnson" <root@chaos.analogic.com>
> > > To: <erik@hensema.net>
> > > Cc: <linux-kernel@vger.kernel.org>
> > > Sent: Wednesday, November 28, 2001 8:09 PM
> > > Subject: Re: Compilation problem in ndisc.c / 2.5.1-pre2 : possible gcc bug?
> > >
> > >
> > > > On 28 Nov 2001, Erik Hensema wrote:
> > > >
> > > > >
> > > > > I've been looking into the compile problems of net/ipv6/ndisc.c in
> > > > > 2.5.1-pre2 and I found that the asm generated by gcc (2.95.3) is wrong:
> > > > >
> > > > > This is a small part of a diff betweem two asm files generated by gcc,
> > > note
> > > > > the missing \n's in the wrong code:
> > > > >
> > > > > -
> > > > > - addl 0(%ebp), %edx
> > > > > - adcl 4(%ebp), %edx
> > > > > - adcl 8(%ebp), %edx
> > > > > - adcl 12(%ebp), %edx
> > > > > - adcl 0(%ecx), %edx
> > > > > - adcl 4(%ecx), %edx
> > > > > - adcl 8(%ecx), %edx
> > > > > - adcl 12(%ecx), %edx
> > > > > - adcl %edi, %edx
> > > > > - adcl %eax, %edx
> > > > > - adcl $0, %edx
> > > > > -
> > > > > + addl 0(%ebp), %edxadcl 4(%ebp), %edxadcl 8(%ebp), %edxadcl 12(%ebp),
> > > %edxadcl 0(%ecx), %edxadcl 4(%ecx), %edxadcl 8(%ecx), %edxadcl 12(%ecx),
> > > %edxadcl %edi, %edxadcl %eax, %edxadcl $0, %edx
> > > >
> > > >
> > > > This is probably just some loop unrolling, not some as you say "wrong
> > > > code".
> > >
> > > Correct me if I'm wrong, but I don't think "%edxadcl" really assembles....
> >
> > No, but an edited 'diff' of the assembly output of a 'C' compiler doesn't
> > really tell much. Note, no line numbers, no clue as to what the diff
> > was about. The actual defective section of code would be much more
> > instructive. For instance, is this as a result of an in-line macro
> > expansion; the result of a <CR><LF> 'dos' file; the true output of
> > a 'C' file with no __inline__ __asm__?
>
> It's from include/asm-i386/checksum.h, specifically csum_ipv6_magic().
> The asm statements in this header file need semicolons or explicit
> newlines after each opcode.
>

Yes. It isn't a 'C' compiler problem. Here is a patch. I remember
some comment about "multi-line string literals are depreciated..."
as a diagnostic message from some 'C' compiler version. Somebody
should check this out. This patch is "for the way we used to do it..."

SNIP
--- /usr/src/linux/include/asm-i386/checksum.h Tue Feb 1 02:41:14 2000
+++ checksum.h Thu Nov 29 08:56:11 2001
@@ -69,25 +69,24 @@
unsigned int ihl) {
unsigned int sum;

- __asm__ __volatile__("
- movl (%1), %0
- subl $4, %2
- jbe 2f
- addl 4(%1), %0
- adcl 8(%1), %0
- adcl 12(%1), %0
-1: adcl 16(%1), %0
- lea 4(%1), %1
- decl %2
- jne 1b
- adcl $0, %0
- movl %0, %2
- shrl $16, %0
- addw %w2, %w0
- adcl $0, %0
- notl %0
-2:
- "
+ __asm__ __volatile__(
+ "\n\tmovl (%1), %0\n"
+ "\tsubl $4, %2\n"
+ "\tjbe 2f\n"
+ "\taddl 4(%1), %0\n"
+ "\tadcl 8(%1), %0\n"
+ "\tadcl 12(%1), %0\n"
+ "1:\tadcl 16(%1), %0\n"
+ "\tlea 4(%1), %1\n"
+ "\tdecl %2\n"
+ "\tjne 1b\n"
+ "\tadcl $0, %0\n"
+ "\tmovl %0, %2\n"
+ "\tshrl $16, %0\n"
+ "\taddw %w2, %w0\n"
+ "\tadcl $0, %0\n"
+ "\tnotl %0\n"
+ "2:\n"
/* Since the input registers which are loaded with iph and ipl
are modified, we must also specify them as outputs, or gcc
will assume they contain their original values. */
@@ -102,10 +101,9 @@

static inline unsigned int csum_fold(unsigned int sum)
{
- __asm__("
- addl %1, %0
- adcl $0xffff, %0
- "
+ __asm__(
+ "\n\taddl %1, %0\n"
+ "\tadcl $0xffff, %0\n"
: "=r" (sum)
: "r" (sum << 16), "0" (sum & 0xffff0000)
);
@@ -118,12 +116,11 @@
unsigned short proto,
unsigned int sum)
{
- __asm__("
- addl %1, %0
- adcl %2, %0
- adcl %3, %0
- adcl $0, %0
- "
+ __asm__(
+ "\n\taddl %1, %0\n"
+ "\tadcl %2, %0\n"
+ "\tadcl %3, %0\n"
+ "\tadcl $0, %0\n"
: "=r" (sum)
: "g" (daddr), "g"(saddr), "g"((ntohs(len)<<16)+proto*256), "0"(sum));
return sum;
@@ -158,19 +155,18 @@
unsigned short proto,
unsigned int sum)
{
- __asm__("
- addl 0(%1), %0
- adcl 4(%1), %0
- adcl 8(%1), %0
- adcl 12(%1), %0
- adcl 0(%2), %0
- adcl 4(%2), %0
- adcl 8(%2), %0
- adcl 12(%2), %0
- adcl %3, %0
- adcl %4, %0
- adcl $0, %0
- "
+ __asm__(
+ "\n\taddl 0(%1), %0\n"
+ "\tadcl 4(%1), %0\n"
+ "\tadcl 8(%1), %0\n"
+ "\tadcl 12(%1), %0\n"
+ "\tadcl 0(%2), %0\n"
+ "\tadcl 4(%2), %0\n"
+ "\tadcl 8(%2), %0\n"
+ "\tadcl 12(%2), %0\n"
+ "\tadcl %3, %0\n"
+ "\tadcl %4, %0\n"
+ "\tadcl $0, %0\n"
: "=&r" (sum)
: "r" (saddr), "r" (daddr),
"r"(htonl(len)), "r"(htonl(proto)), "0"(sum));
SNIP

Here is something to test it with:

#define asmlinkage
#define NULL (void *)0
#define DUMMY 0
#define VERIFY_WRITE DUMMY
#define EFAULT DUMMY
typedef unsigned int __u32;
struct in6_addr{ int dummy; };
#include "checksum.h"
void foo()
{
char bar[0x100];
ip_fast_csum(bar, sizeof(bar));
csum_ipv6_magic(NULL, NULL, sizeof(bar), DUMMY, DUMMY);
csum_tcpudp_nofold(DUMMY, DUMMY, sizeof(bar), DUMMY, DUMMY);
csum_fold(DUMMY);
}

Just do `gcc -S -o xxx xxx.c` to see the code generated and verify
that the stuborn 'C' compilers like these strings okay.


Cheers,
Dick Johnson

Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).

I was going to compile a list of innovations that could be
attributed to Microsoft. Once I realized that Ctrl-Alt-Del
was handled in the BIOS, I found that there aren't any.


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