[Bug rtl-optimization/24922] New: gcc -O2 removes important instruction for ASM

[gvaxon at gmail dot com]

I found that when I compile this code with -O2: 

#include <stdio.h>

#define bit_swap(v, count, res)                 \
        do {                                    \
        asm volatile (                          \
                "1:     sarb %1\n"              \
                "       rclb %0\n"              \
                "       sarb %1\n"              \
                "       rclb %0\n"              \
                "       loop 1b\n"              \
                : "=a" (res)                    \
                : "d" (v), "c" (count)          \
        );                                      \
        } while (0)


int main()
{
        char u1, u2;
        u1=0xA;
        bit_swap(u1, 4, u2);
        bit_swap(u2, 4, u1);
        printf("u1=0x%x u2=0x%x\n", u1, u2);
}

, gcc removes loop counter cx register between the first bit_swap() and the
second bit_swap() statements, which results in the problem that the second
bit_swap() is running 2^32 loops instead of 4 loops also it corrupts a final
result in u1.

#gcc -v

Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../gcc/configure --enable-languages=c,c++
--prefix=/home/dinar/tmp/gcc-i686-pc-linux-gnu/
Thread model: posix
gcc version 4.1.0 20051026 (experimental)


-- 
           Summary: gcc -O2 removes important instruction for ASM
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gvaxon at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24922