Re: Short blocking delay in Cortex-M0+

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.arch.embedded
Subject	Re: Short blocking delay in Cortex-M0+
Date	2017-03-25 01:11 +0100
Organization	A noiseless patient Spider
Message-ID	<ob4cdt$3ga$1@dont-email.me> (permalink)
References	(1 earlier) <ob2vem$37q$1@dont-email.me> <ob31ei$9k7$1@dont-email.me> <ob34pd$lfg$1@dont-email.me> <ob39rm$7pk$1@dont-email.me> <ob3rnm$82s$1@dont-email.me>

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On 24/03/17 20:26, Tauno Voipio wrote:
> On 24.3.17 16:21, David Brown wrote:

>>
>> Finally, they might like to note that:
>>
>> void portable_delay_cycles(unsigned long n) {
>>     while (n--) {
>>         asm volatile ("");
>>     }
>> }
>>
>> gives a faster loop, as well as actually being portable!
>>
>> The best code for accurate delays is usually made by having static
>> inline functions, so that the compiler can calculate and compensate for
>> loop entry/exit directly, and use whatever registers make most sense for
>> efficient code.
>>
>>
>>
>
> I would delete the cmp instruction and change the sub instruction
> to a subs instruction.

That is what gcc generates for the C loop above (I said it gave a faster 
loop, but did not post the generated code).

>
> One more vote to the C version of David.
>

Yes.

The " asm volatile("");  " trick might be new to some people - it tells 
the compiler "pretend something important is happening here, even though 
there is no code".  It is cheaper than the traditional idea of making 
the loop variable volatile to force the compiler to keep the loop, or 
the alternative of using an assembly "nop" instruction.

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Short blocking delay in Cortex-M0+ pozz <pozzugno@gmail.com> - 2017-02-23 17:15 +0100
  Re: Short blocking delay in Cortex-M0+ John Speth <johnspeth@yahoo.com> - 2017-02-23 09:00 -0800
    Re: Short blocking delay in Cortex-M0+ Tim Wescott <tim@seemywebsite.com> - 2017-02-23 13:15 -0600
    Re: Short blocking delay in Cortex-M0+ pozz <pozzugno@gmail.com> - 2017-02-24 09:51 +0100
      Re: Short blocking delay in Cortex-M0+ rickman <gnuarm@gmail.com> - 2017-02-24 04:26 -0500
      Re: Short blocking delay in Cortex-M0+ Tim Wescott <seemywebsite@myfooter.really> - 2017-02-24 14:22 -0600
    Re: Short blocking delay in Cortex-M0+ Jack <jack4747@gmail.com> - 2017-02-24 02:22 -0800
    Re: Short blocking delay in Cortex-M0+ Don Y <blockedofcourse@foo.invalid> - 2017-02-24 10:51 -0700
    Re: Short blocking delay in Cortex-M0+ Stefan Reuther <stefan.news@arcor.de> - 2017-02-24 18:29 +0100
      Re: Short blocking delay in Cortex-M0+ Dave Nadler <drn@nadler.com> - 2017-02-24 13:03 -0800
    Re: Short blocking delay in Cortex-M0+ mrfirmware <mrfirmware@gmail.com> - 2017-03-22 09:29 -0700
      Re: Short blocking delay in Cortex-M0+ mrfirmware <mrfirmware@gmail.com> - 2017-03-22 09:48 -0700
        Re: Short blocking delay in Cortex-M0+ Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2017-03-22 22:02 +0100
        Re: Short blocking delay in Cortex-M0+ David Brown <david.brown@hesbynett.no> - 2017-03-23 10:05 +0100
          Re: Short blocking delay in Cortex-M0+ Tim Wescott <seemywebsite@myfooter.really> - 2017-03-23 12:54 -0500
            Re: Short blocking delay in Cortex-M0+ mrfirmware <mrfirmware@gmail.com> - 2017-03-23 12:52 -0700
  Re: Short blocking delay in Cortex-M0+ Don Y <blockedofcourse@foo.invalid> - 2017-02-23 13:54 -0700
  Re: Short blocking delay in Cortex-M0+ Paul <paul@pcserviceselectronics.co.uk> - 2017-02-24 10:36 +0000
  Re: Short blocking delay in Cortex-M0+ pozz <pozzugno@gmail.com> - 2017-03-24 12:23 +0100
    Re: Short blocking delay in Cortex-M0+ David Brown <david.brown@hesbynett.no> - 2017-03-24 12:57 +0100
      Re: Short blocking delay in Cortex-M0+ pozz <pozzugno@gmail.com> - 2017-03-24 13:54 +0100
        Re: Short blocking delay in Cortex-M0+ David Brown <david.brown@hesbynett.no> - 2017-03-24 15:21 +0100
          Re: Short blocking delay in Cortex-M0+ Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2017-03-24 21:26 +0200
            Re: Short blocking delay in Cortex-M0+ David Brown <david.brown@hesbynett.no> - 2017-03-25 01:11 +0100

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