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Re: ARM Cortex Mx vs the rest of the gang

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On 12.6.2017 г. 17:21, StateMachineCOM wrote:
> I said that "... the FPU integration in the Cortex-M4F/M7 is horrible",
> because it adds tons of overhead and a lot of headache for the
> system-level software.

> The problem is that the ARM Vector Floating-Point (VFP) coprocessor
> comes with a big context of 32 32-bit registers (S0-S31). These
> registers need to be saved and restored as part of every context switch,
> just like the CPU registers.

They have to be saved during every context switch if the OS is broken.
What stops it from saving the FPU context only for tasks which use
the FPU? It is a single bit of information in the task descriptor.
Then what stops the programmer from declaring the FPU is in use or
not within a task? Normally what is done for this purpose is
- enable FPU for tha task, saving the state the previous FPU_in_use bit
state,
- do the FPU work,
- restore the previous state of the FPU_in_use bit.
Obviously for tasks which use the FPU intensively this needs not
be done, one just leaves the FPU_in_use on.

>.. ARM has come up with some hardware
> optimizations called "lazy stacking and context switching" (see ARM
> AppNote 298 at
> http://infocenter.arm.com/help/topic/com.arm.doc.dai0298a/DAI0298A_cortex_m4f_lazy_stacking_and_context_switching.pdf
> ).

I once had a look at that only to confirm my suspicion that it is just
piling problems over existing problems. Things like this are done
by software, or rather software is written such that there is no
need for that sort of thing.

> Anyway, does it have to be that hard? Apparently not. For example the
> Renesas RX CPU comes also with single precision FPU, which is much
> better integrated with the CPU and does not have its own register
> context. Compared to the ARM VFP it is a pleasure to work with.


Not having a separate FPU register set is a disadvantage, not an
advantage (now if the FPU is a useless 32 bit one having it at all
is a disadvantage but this is another matter :-).
Nothing prevents software from using what it wants and from saving
only what has to be saved; often having an entire FPU and saving its
entire context in addition to that of the integer unit makes things more
efficient, sometimes a lot more efficient (e.g. implementing a FIR
on a plain FPU where data dependencies do not allow you to do it
at a speed more than speed/(pipeline length), you just need many
registers).
The e500 cores from freescale were never popular with me exactly
because they had the integer and FP register sets in one, just 32
registers in total. Not a good idea on a load/store machine.

Dimiter


======================================================
Dimiter Popoff, TGI             http://www.tgi-sci.com
======================================================
http://www.flickr.com/photos/didi_tgi/

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ARM Cortex Mx vs the rest of the gang Klaus Kragelund <klauskvik@hotmail.com> - 2017-05-30 05:11 -0700
  ARM Cortex Mx vs the rest of the gang klaus.kragelund@gmail.com - 2017-05-30 05:48 -0700
    Re: ARM Cortex Mx vs the rest of the gang Uwe Bonnes <bon@hertz.tu-darmstadt.de> - 2017-05-30 14:32 +0000
      Re: ARM Cortex Mx vs the rest of the gang Tim Wescott <seemywebsite@myfooter.really> - 2017-05-30 22:07 -0500
        Re: ARM Cortex Mx vs the rest of the gang rickman <gnuarm@gmail.com> - 2017-05-30 23:57 -0400
          Re: ARM Cortex Mx vs the rest of the gang David Brown <david.brown@hesbynett.no> - 2017-05-31 12:08 +0200
            Re: ARM Cortex Mx vs the rest of the gang rickman <gnuarm@gmail.com> - 2017-05-31 09:11 -0400
              Re: ARM Cortex Mx vs the rest of the gang Tim Wescott <seemywebsite@myfooter.really> - 2017-06-01 15:17 -0500
                Re: ARM Cortex Mx vs the rest of the gang rickman <gnuarm@gmail.com> - 2017-06-01 16:27 -0400
                Re: ARM Cortex Mx vs the rest of the gang rickman <gnuarm@gmail.com> - 2017-06-01 16:30 -0400
            Re: ARM Cortex Mx vs the rest of the gang Tim Wescott <seemywebsite@myfooter.really> - 2017-06-01 15:17 -0500
  Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-05-30 07:31 -0700
    Re: ARM Cortex Mx vs the rest of the gang Klaus Kragelund <klauskvik@hotmail.com> - 2017-05-30 08:07 -0700
      Re: ARM Cortex Mx vs the rest of the gang George Neuner <gneuner2@comcast.net> - 2017-05-31 00:38 -0400
      Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-05-30 22:14 -0700
  Re: ARM Cortex Mx vs the rest of the gang rickman <gnuarm@gmail.com> - 2017-05-30 10:53 -0400
  Re: ARM Cortex Mx vs the rest of the gang StateMachineCOM <statemachineguru@gmail.com> - 2017-06-07 12:58 -0700
    Re: ARM Cortex Mx vs the rest of the gang Klaus Kragelund <klauskvik@hotmail.com> - 2017-06-07 13:27 -0700
    Re: ARM Cortex Mx vs the rest of the gang Jack <jack4747@gmail.com> - 2017-06-07 23:24 -0700
      Re: ARM Cortex Mx vs the rest of the gang StateMachineCOM <statemachineguru@gmail.com> - 2017-06-12 07:21 -0700
        Re: ARM Cortex Mx vs the rest of the gang Dimiter_Popoff <dp@tgi-sci.com> - 2017-06-12 17:57 +0300
          Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-06-12 10:10 -0700
            Re: ARM Cortex Mx vs the rest of the gang Dimiter_Popoff <dp@tgi-sci.com> - 2017-06-12 22:06 +0300
              Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-06-12 12:19 -0700
                Re: ARM Cortex Mx vs the rest of the gang Dimiter_Popoff <dp@tgi-sci.com> - 2017-06-12 23:04 +0300
                Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-06-12 14:34 -0700
                Re: ARM Cortex Mx vs the rest of the gang Dimiter_Popoff <dp@tgi-sci.com> - 2017-06-13 02:44 +0300
                Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-06-12 17:31 -0700
        Re: ARM Cortex Mx vs the rest of the gang Jack <jack4747@gmail.com> - 2017-06-12 08:04 -0700
        Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-06-12 09:54 -0700
          Re: ARM Cortex Mx vs the rest of the gang Chris <xxx.syseng.yyy@gfsys.co.uk> - 2017-07-10 13:30 +0000
            Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-07-10 16:03 -0700
              Re: ARM Cortex Mx vs the rest of the gang Chris <xxx.syseng.yyy@gfsys.co.uk> - 2017-07-11 16:24 +0000
                Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-07-11 12:15 -0700
                Re: ARM Cortex Mx vs the rest of the gang Chris <xxx.syseng.yyy@gfsys.co.uk> - 2017-07-11 23:16 +0000
                Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-07-12 11:44 -0700
                Re: ARM Cortex Mx vs the rest of the gang George Neuner <gneuner2@comcast.net> - 2017-07-12 17:46 -0400
                Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-07-12 18:12 -0700
                Re: ARM Cortex Mx vs the rest of the gang Anders.Montonen@kapsi.spam.stop.fi.invalid - 2017-07-13 11:42 +0000
                Re: ARM Cortex Mx vs the rest of the gang Don Y <blockedofcourse@foo.invalid> - 2017-07-13 21:38 -0700
                Re: ARM Cortex Mx vs the rest of the gang David Brown <david.brown@hesbynett.no> - 2017-07-13 23:31 +0200
                Re: ARM Cortex Mx vs the rest of the gang Chris <xxx.syseng.yyy@gfsys.co.uk> - 2017-07-14 15:27 +0000
                Re: ARM Cortex Mx vs the rest of the gang David Brown <david.brown@hesbynett.no> - 2017-07-14 17:38 +0200
                Re: ARM Cortex Mx vs the rest of the gang Chris <xxx.syseng.yyy@gfsys.co.uk> - 2017-07-14 16:03 +0000
                Re: ARM Cortex Mx vs the rest of the gang David Brown <david.brown@hesbynett.no> - 2017-07-14 20:20 +0200
                Re: ARM Cortex Mx vs the rest of the gang Chris <xxx.syseng.yyy@gfsys.co.uk> - 2017-07-14 18:59 +0000
                Re: ARM Cortex Mx vs the rest of the gang Chris <xxx.syseng.yyy@gfsys.co.uk> - 2017-07-14 17:17 +0000
                Re: ARM Cortex Mx vs the rest of the gang Dimiter_Popoff <dp@tgi-sci.com> - 2017-07-15 00:45 +0300
                Re: ARM Cortex Mx vs the rest of the gang Chris <xxx.syseng.yyy@gfsys.co.uk> - 2017-07-14 23:00 +0000
                Re: ARM Cortex Mx vs the rest of the gang Dimiter_Popoff <dp@tgi-sci.com> - 2017-07-15 04:04 +0300
                Re: ARM Cortex Mx vs the rest of the gang Chris <xxx.syseng.yyy@gfsys.co.uk> - 2017-07-16 16:58 +0000
                Re: ARM Cortex Mx vs the rest of the gang Richard Damon <Richard@Damon-Family.org> - 2017-07-14 20:05 -0400
                Re: ARM Cortex Mx vs the rest of the gang Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2017-07-15 11:53 +0300
                Re: ARM Cortex Mx vs the rest of the gang David Brown <david.brown@hesbynett.no> - 2017-07-15 12:05 +0200
                Re: ARM Cortex Mx vs the rest of the gang Anders.Montonen@kapsi.spam.stop.fi.invalid - 2017-07-20 11:11 +0000
                Re: ARM Cortex Mx vs the rest of the gang Anders.Montonen@kapsi.spam.stop.fi.invalid - 2017-07-20 10:29 +0000
        Re: ARM Cortex Mx vs the rest of the gang "Boudewijn Dijkstra" <sp4mtr4p.boudewijn@indes.com> - 2017-06-14 09:54 +0200

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