gcc inline asm - short question

[Carl Spalletta <cspalletta@yahoo.com>]

  In studying the kernel I find many gcc inline 'asm' statements,
like so.
	
        asm ( assembler template
	    : output operands			 	(optional)
	    : input operands			 	(optional)
	    : list of clobbered registers 	  	(optional)
	    );	

  I have read all the docs and I still can't clearly understand when it 
is required to specify a clobberlist - a register or memory that will
be modified and must be preserved by gcc.

  In searching the kernel source there were very few clobbers given
and most of those were for memory.

  For example in arch/i386/lib/delay.c:

71         __asm__("mull %0"
72         :"=d" (xloops), "=&a" (d0)
73         :"1" (xloops),"" (current_cpu_data.loops_per_jiffy));
74         __delay(xloops * HZ);

  It's pretty obvious that eax gets clobbered, as I know clobber.  Why is
it not listed as such? Does the answer to this question apply in all cases? 
What about memory clobbers - how do they happen?


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