https://sourceware.org/bugzilla/show_bug.cgi?id=19661
H.J. Lu <hjl.tools at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|WAITING |RESOLVED
Resolution|--- |WONTFIX
--- Comment #3 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to njholcomb from comment #2)
> Created attachment 9202 [details]
> File displays locking without a memory operand.
>
> objdump -d lock_no_mem.o
>
> lock_no_mem.o: file format elf64-x86-64
>
>
> Disassembly of section .text:
>
> 0000000000000000 <main>:
> 0: 55 push %rbp
> 1: 48 89 e5 mov %rsp,%rbp
> 4: f0 b7 9f lock mov $0x9f,%bh
> 7: 96 xchg %eax,%esi
> 8: 00 00 add %al,(%rax)
> a: 00 b8 00 00 00 00 add %bh,0x0(%rax)
> 10: 5d pop %rbp
> 11: c3 retq
It looks OK to me. You get UD because LOCK prefix is used on non-memory
operand:
(gdb) disass
Dump of assembler code for function main:
0x0000000000400488 <+0>: push %rbp
0x0000000000400489 <+1>: mov %rsp,%rbp
=> 0x000000000040048c <+4>: lock mov $0x9f,%bh
0x000000000040048f <+7>: xchg %eax,%esi
0x0000000000400490 <+8>: add %al,(%rax)
0x0000000000400492 <+10>: add %bh,0x0(%rax)
0x0000000000400498 <+16>: pop %rbp
0x0000000000400499 <+17>: retq
End of assembler dump.
(gdb)
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