Messages in this thread |  | | Date | Mon, 17 Sep 2001 18:16:48 +0200 (CEST) | From | Jean-Marc Saffroy <> | Subject | [Q] Implementation of spin_lock on i386: why "rep;nop" ? |
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Hi all,
One of my coworkers directed my attention to the implementation of spinlocks on IA-32. In spin_lock_string, we can read:
"cmpb $0,%0\n\t" \ "rep;nop\n\t" \ "jle 2b\n\t" \
The "rep;nop" line looks dubious, since the IA-32 programmer's manual from Intel (year 2001) mentions that the behaviour of REP is undefined when it is not used with string opcodes. BTW, according to the same manual, REP is supposed to modify ecx, but it looks like is is not the case here... which is fortunate, since ecx is never saved. :-)
What is the intent behind this "rep;nop" ? Does it really rely on an undocumented behaviour ?
Regards,
-- Jean-Marc Saffroy - Research Engineer - Silicomp Research Institute mailto:saffroy@ri.silicomp.fr
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