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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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