ld-linux.so.2 segmentation fault with new memcpy

[Johan Evenhuis]

Dear glib-maintainer,

After have learned the basics of assembler, I decided to practice a 
little by writing a memcpy function, because I new the original one 
(glibc-2.3.2/sysdeps/i386/i686glibc-2.3.2/sysdeps/i386/i686/memcpy.S) 
had a performance problem on my system (PIII). It was caused by lack of 
making memory addresses dividable by 8, which is needed by the used 
"cld; rep movsl" combination to perform fast. Another problem of this 
combination was that it used quite some time to start, so it shouldn't 
be used to copy just a few bytes.

While rebuilding libc with a rewritten memcpy, execution of 
ld-linux.so.2 (which uses memcpy) failed with a segmentation fault. So I 
tried to narrow this problem and found the next code sample, inserted in 
the original memcpy code, causes it:

   jmp 2f
   movl 1f, %eax
1:    .long 3
2:

It seems that moving a labeled address or the value it points to is 
forbidden in ld-linux.so.2, even if this instruction is only present but 
not executed. Do you know how to do what seems to be forbidden here?

Best regards,

J. Evenhuis

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