Re: asm (lidt) question

["Randy.Dunlap" <rddunlap@osdl.org>]

On Thu, 17 Jul 2003 16:18:06 -0700 (PDT) Davide Libenzi <davidel@xmailserver.org> wrote:

| On Thu, 17 Jul 2003, Davide Libenzi wrote:
| 
| > On Thu, 17 Jul 2003, Randy.Dunlap wrote:
| >
| > >
| > > In arch/i386/kernel, inline asm for loading IDT (lidt) is used a few
| > > times, but with slightly different constraints and output/input
| > > definitions.  Are these OK, equivalent, or what?
| > >
| > > [rddunlap@dragon kernel]$ findc lidt
| > > ./cpu/common.c:484: __asm__ __volatile__("lidt %0": "=m" (idt_descr));
| > > ./traps.c:783:	__asm__ __volatile__("lidt %0": "=m" (idt_descr));
| > >
| > > vs.
| > >
| > > ./reboot.c:186:	__asm__ __volatile__ ("lidt %0" : : "m" (real_mode_idt));
| > > ./reboot.c:261:	__asm__ __volatile__("lidt %0": :"m" (no_idt));
| > > ./suspend.c:95:	asm volatile ("lidt %0" :: "m" (saved_context.idt_limit));
| >
| > I'd have said no looking at the syntax (input/output), but they indeed
| > generate the same code (just checked).
| 
| Randy, I'd say that this :
| 
| __asm__ __volatile__("lidt %0": "=m" (var));
| 
| is better then :
| 
| __asm__ __volatile__("lidt %0": :"m" (var));
| 
| IMHO, since "var" is really an output parameter.

Yes, I agree, var is output and should be listed after the first ':'
IMHO also.

Thanks for checking that they generate the same code, though.

I'll buy you a beer...  No, wait, I already did that.  :)

--
~Randy