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Date: Fri, 10 Aug 2018 21:10:04 -0700 From: Richard Henderson <rth@...ddle.net> To: Al Viro <viro@...IV.linux.org.uk> Cc: Firoz Khan <firoz.khan@...aro.org>, linux-alpha@...r.kernel.org, ink@...assic.park.msu.ru, mattst88@...il.com, y2038@...ts.linaro.org, linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org, arnd@...db.de, deepa.kernel@...il.com Subject: Re: [PATCH 3/6] alpha: Unify the not-implemented system call entry name On 08/10/2018 07:45 PM, Al Viro wrote: > BTW, seeing that it's your code - why was unop used in > alpha_ni_syscall? I don't remember the rules re pipeline > stalls; is it that some earlier variants prefer unop to > nop in such places? It's not that microoptimizing that > one makes any difference, but just out of curiosity - > would something like > lda $0, -ENOSYS > stq $sp, 0($sp) /* sp != 0 */ > ret > do just as well there? Oh that. Well, unop is a "load" and pairs with everything on all cpus, where nop is an integer and doesn't always pair. So I got into the habit of using unop for everything. The extra nop was so that lda + unop // stq + ret paired up in in two cycles as opposed to lda /stall/ stq // ret in three cycles on ev4+ev5. ev6 didn't care. FWIW. r~
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