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Forum: ARM programming with GCC/GNU tools compiler optimization level


Author: Guy Vo (codefreakguy)
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Hi,

I recently started to use the winarm toolchain with the programmers
notepad editor. My target is a AT91SAMS256 board based on the same
drawings as the EK. The project import works pretty well with the editor
thanks to the guys from PN.

But if I build the atmel example (blinking leds) I encounter some
problems:

When I put the compiler OPT=s (size) my code isn't running at all the
leds ar on but never start to blink !
When I put the compiler OPT=0 (none) my code works just fine as I see
the led blinking !

In the makefile:

# Optimization level, can be [0, 1, 2, 3, s].
# 0 = turn off optimization. s = optimize for size.
# (Note: 3 is not always the best optimization level. See avr-libc FAQ.)
OPT = s
#OPT = 0

If I take a look in the main.lss file for both builds there's indeed a
difference in target main code maybe OPT=S is not recommended for this
target I don't know. nevertheless this cannot be the case, when changing
compiler options resulting in other execution behaviour.

Any one encountered the same kind of strange things ?
Solution ?

Author: Jonathan Dumaresq (dumarjo)
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>
> # Optimization level, can be [0, 1, 2, 3, s].
> # 0 = turn off optimization. s = optimize for size.
> # (Note: 3 is not always the best optimization level. See avr-libc FAQ.)
> OPT = s
> #OPT = 0
>
> If I take a look in the main.lss file for both builds there's indeed a
> difference in target main code maybe OPT=S is not recommended for this
> target I don't know. nevertheless this cannot be the case, when changing
> compiler options resulting in other execution behaviour.
>
> Any one encountered the same kind of strange things ?
> Solution ?

Have you try O1 or O2 ?

Jonathan

Author: Guy Vo (codefreakguy)
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Jonathan Dumaresq wrote:
> Have you try O1 or O2 ?

Yes I tried 1 and 2 same as s. Only the 0 is working correctly. I red
about some problems in thumb-interwork mode (bug in gcc) but this is not
used here.

My version  used:
arm-elf-gcc (GCC) 4.1.1 (WinARM)

# MCU name and submodel
MCU = arm7tdmi
SUBMDL = AT91SAM7S256
#THUMB    = -mthumb
#THUMB_IW = -mthumb-interwork

The only files in the project are build in arm-mode:

# List C source files here which must be compiled in ARM-Mode.
# use file-extension c for "c-only"-files
SRCARM = $(TARGET).c Cstartup_SAM7.c

# List Assembler source files here which must be assembled in ARM-Mode..
ASRCARM = Cstartup.S

Author: Guy Vo (codefreakguy)
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I downloaded the lastest version from the site of Martin Thomas of the
gamma project for AT91. And guess what no poblem at all with that for
the OPT possibilties !

I saw there were lots of improvements done in the linker .LD files. And
also some changes in the makefile (more targets)In this project there
are some files compiled even in Thumb mode.

Still don't know exactly what the error for the OPT=s was.But maybe
Martin can tel me a bit more what he has done to update the gamma
project. Because his latest version isn't included in the winarm
distribution.

Thnks&Cheers
Guy

Author: Clifford Slocombe (clifford)
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You did not mention which tool-chain versions you were using or exactly
which example code.

It is entirely possible that the change in behaviour is due to the code.
For example if hardware registers and data shared between an interrupt
context and the non-interrupt context, or between threads are not
declared volatile, then compiler my optimise out a read or write that
must be done for the code to work correctly.

Without the volatile keyword, the compiler may determine that it already
has a value from the last time it was read, or that it has not changed a
value since it was last written, and may then optimise out the access.

This is just one reason why apparently working code can fail after
optimization. Generally it will be dure to unsafe code rather than
optimizer bugs. That said, if the compiler has bugs, the optimizer is
also the most likely place.

Clifford

Author: Guy Vo (codefreakguy)
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Clifford Slocombe wrote:
> You did not mention which tool-chain versions you were using or exactly
> which example code.

the toolchain is winarm package prebuild for windows and the project was
the AT91SAM7S64 example from atmel (blinking leds for EK)

> Without the volatile keyword, the compiler may determine that it already
> has a value from the last time it was read, or that it has not changed a
> value since it was last written, and may then optimise out the access.

Yes agree this can be the case. I changed two loop variables qualified
with volatile keyword.

global var:
volatile unsigned int LedSpeed = SPEED *50 ;

static void wait ( void )
{//* Begin
    volatile unsigned int waiting_time ;
    change_speed () ;
    for(waiting_time = 0; waiting_time < LedSpeed; waiting_time++) ;
}//* End

And now its running perfectly even with the OPT=s

I must study the differences in the lss file to see how the code is put
into assembler with or without the volatile keyword. But it's more or
like sure that the compiler optimized the looper vars so they did not
work properly. Also the question when to use the volatile to avoid these
kind of optimizations. Are there some rules in the world of GCC to
follow when you have a candidate variable to put into volatile?

Thanks
Guy

Author: A.K. (Guest)
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The "volatile" is an message to the compiler, to act literally as you
wrote it down, not as it thinks you might have intended.

Todays compilers are judged by speed/size of code produced. Without
"volatile", the loop is equivalent to the statement "waiting_time =
LedSpeed;" and since this variable is not used anywhere, even this
statement can be dropped. Adding "volatile" to the loop variable tells
the compiler that the variable has invisible side effects (like an I/O
register) and all intermediate results produced by the written code are
actually required.

Author: Guy Vo (codefreakguy)
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As it is in this code example for the purpose to test and view something
quickly on the board code like the wait loops which in fact do only an
INC(or DEC) and values of the loop are not used in the following
statements the compiler decide to optimize them out. This seems to me
very logic moreover since we have the volatile to tell the compiler that
they serve really in the program flow.

Indeed there is link when referring to I/O registers which are not
directly needed in the main program flow but must be proper set.

So loops which act as a delay or wait intention are perfect candidates
to use the volatile for the variables used.

Most of the time in real programs delays often will be made by a timer
interrupt routine and the caution for using "volatile" will be less
present there.

And yes the compilers these days are really pushed to the limit to
produce smaller (smalest) code in a world of small amount flash MCUs.

Guy

Author: Clifford Slocombe (clifford)
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Guy Vo wrote:
> the toolchain is winarm package prebuild for windows and the project was
> the AT91SAM7S64 example from atmel (blinking leds for EK)
>
WinARM has incorporated many versions of GCC over time. Run:

gcc --version

to determine the version. What I was interested in rather was the
difference in gcc version between the built that failed and the one that
apparently worked. This information may not help in tracking down the
problem, but it is useful information to the community should similar
problems occur.

With respect to the example, post a link so we can be sure we are
looking at exactly the same thing. If it is an example supplied with
WinARM, then I obviously have a different distribution to you, which is
why the release version is important. WinARM distributions are
identified by the date embedded into the distribution zip-file's file
name.

>
> And now its running perfectly even with the OPT=s
>
I am surprised, from the fragment you posted I am not sure how it would
make a difference.

> Also the question when to use the volatile to avoid these
> kind of optimizations. Are there some rules in the world of GCC to
> follow when you have a candidate variable to put into volatile?

As I said, normally, when data is shared between thread or interrupt
contexts, or when it is a hardware register that may be modified by the
hardware itself, or which does not have symmetrical read-write
properties (such as reset-on-read behaviour typical of  interrupt status
registers for example). In practice, it is simpler to declare all memory
mapped hardware registers and I/O as volatile, and you will see this in
the platform header files.


Clifford

Author: Guy Vo (codefreakguy)
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Clifford Slocombe wrote:

> gcc --version

arm-elf-gcc (GCC) 4.1.1 (WinARM)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is
NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.

>
> With respect to the example, post a link so we can be sure we are
> looking at exactly the same thing. If it is an example supplied with
> WinARM, then I obviously have a different distribution to you, which is
> why the release version is important. WinARM distributions are
> identified by the date embedded into the distribution zip-file's file
> name.
>

winarm20060606.zip I think it's the latest.

The example is in the directory:
c:\winarm\examples\at91sam7s64_Atmel_example

>>
>> And now its running perfectly even with the OPT=s
>>
> I am surprised, from the fragment you posted I am not sure how it would
> make a difference.

I removed the volatile once more to be sure and the leds just go on but
not blinking. Just for your information the same project is included
with the KEIL package and it seems to work without modifing any of these
variables.

> As I said, normally, when data is shared between thread or interrupt
> contexts, or when it is a hardware register that may be modified by the
> hardware itself, or which does not have symmetrical read-write
> properties (such as reset-on-read behaviour typical of  interrupt status
> registers for example). In practice, it is simpler to declare all memory
> mapped hardware registers and I/O as volatile, and you will see this in
> the platform header files.

In this program there are no such things as thread or IR switches. It
just a simple toggling of leds. I just adapted the code a bit because my
board has no PLL and only two leds and two switches like

/*-----------------*/
/* Leds Definition */
/*-----------------*/
#define LED1            (1<<0)
#define LED2            (1<<1)
#define NB_LEB      2

#define LED_MASK        (LED1|LED2)

/*-------------------------*/
/* Push Buttons Definition */
/*-------------------------*/
#define SW1_MASK        (1<<15)
#define SW2_MASK        (1<<16)
#define SW_MASK         (SW1_MASK|SW2_MASK)

And I modified Cstartup.s for the Main clock to take no PLL:

                LDR     R0, =PMC_BASE
    MOV  R1,  #1
                STR     R1, [R0, #PMC_MOR]
    STR     R1, [R0, #PMC_MCKR]


//*--------------------------------------------------------------------- 
-------
//*         ATMEL Microcontroller Software Support  -  ROUSSET  -
//*--------------------------------------------------------------------- 
-------
//* The software is delivered "AS IS" without warranty or condition of
any
//* kind, either express, implied or statutory. This includes without
//* limitation any warranty or condition with respect to merchantability
or
//* fitness for any particular purpose, or against the infringements of
//* intellectual property rights of others.
//*--------------------------------------------------------------------- 
-------
//* File Name           : main.c
//* Object              : main application written in C
//* Creation            : JPP   16/Jun/2004
//*--------------------------------------------------------------------- 
-------

// Include Standard files
#include "AT91SAM7S64.h"                       /* AT91SAMT7S64
definitions  */
#include "lib_AT91SAM7S64.h"

#include "Board.h"

/* Global variables */
#define SPEED     (MCKKHz/10)

unsigned int LedSpeed = SPEED *50 ;
//volatile unsigned int LedSpeed = SPEED *50 ;
const int led_mask[]= {LED1, LED2};

//*--------------------------------------------------------------------- 
-----------------
//* Function Name       : change_speed
//* Object              : Adjust "LedSpeed" value depending on SW1 and
SW2 are pressed or not
//* Input Parameters    : none
//* Output Parameters   : Update of LedSpeed value.
//*--------------------------------------------------------------------- 
-----------------
static void change_speed ( void )
{//* Begin
    if ( (AT91F_PIO_GetInput(AT91C_BASE_PIOA) & SW1_MASK) == 0 )
    {
        if ( LedSpeed > SPEED ) LedSpeed -=SPEED ;
    }
    if ( (AT91F_PIO_GetInput(AT91C_BASE_PIOA) & SW2_MASK) == 0 )
    {
        if ( LedSpeed < MCK ) LedSpeed +=SPEED ;
    }
}//* End

//*--------------------------------------------------------------------- 
-----------------
//* Function Name       : wait
//* Object              : Software waiting loop
//* Input Parameters    : none. Waiting time is defined by the global
variable LedSpeed.
//* Output Parameters   : none
//*--------------------------------------------------------------------- 
-----------------
static void wait ( void )
{//* Begin
    unsigned int waiting_time ;
//volatile unsigned int waiting_time ;
    change_speed () ;
    for(waiting_time = 0; waiting_time < LedSpeed; waiting_time++) ;
}//* End

//*--------------------------------------------------------------------- 
-----------------
//* Function Name       : Main
//* Object              : Software entry point
//* Input Parameters    : none.
//* Output Parameters   : none.
//*--------------------------------------------------------------------- 
-----------------
int main(void)
{//* Begin


    // First, enable the clock of the PIO
       AT91F_PMC_EnablePeriphClock ( AT91C_BASE_PMC, 1 << AT91C_ID_PIOA
) ;

    // then, we configure the PIO Lines corresponding to LED1 to LED4
    // to be outputs. No need to set these pins to be driven by the PIO
because it is GPIO pins only.
       AT91F_PIO_CfgOutput( AT91C_BASE_PIOA, LED_MASK ) ;

    // Clear the LED's. On the EB55 we must apply a "1" to turn off LEDs
       AT91F_PIO_SetOutput( AT91C_BASE_PIOA, LED_MASK ) ;

    // Loop forever
    for (;;)
    {
       AT91F_PIO_ClearOutput( AT91C_BASE_PIOA, LED_MASK) ;
       AT91F_PIO_ClearOutput( AT91C_BASE_PIOA, LED_MASK) ;
  wait();
       AT91F_PIO_SetOutput( AT91C_BASE_PIOA, LED_MASK ) ;
       AT91F_PIO_SetOutput( AT91C_BASE_PIOA, LED_MASK ) ;
  wait();

    }// End for

}//* End

Author: Clifford Slocombe (clifford)
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The odd thing is that when a variable is not decclared volatile when it
should be, the worst that should happen is that its value never changes.
So this is very odd behaviour, and I still cannot see why it needs to be
volatile.

However just from a 'good-design' point of view, I would make LedSpeed a
local static of change_speed(), and have change speed return the value.

static int change_speed ( void )
{
    static unsigned int LedSpeed = SPEED *50 ;

    if ( (AT91F_PIO_GetInput(AT91C_BASE_PIOA) & SW1_MASK) == 0 )
    {
        if ( LedSpeed > SPEED ) LedSpeed -=SPEED ;
    }
    if ( (AT91F_PIO_GetInput(AT91C_BASE_PIOA) & SW2_MASK) == 0 )
    {
        if ( LedSpeed < MCK ) LedSpeed +=SPEED ;
    }

    return LedSpeed ;
}

static void wait ( void )
{
    unsigned int waiting_time ;
    int led_speed = change_speed() ;
    for(waiting_time = 0; waiting_time < led_speed; waiting_time++)
    {
        /* do nothing */
    }
}

Author: A.K. (Guest)
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void wait ( void )
{
    unsigned int waiting_time ;
    change_speed () ;
    for(waiting_time = 0; waiting_time < 1000; waiting_time++) ;
}

=> compiled with -Os

wait:
        b       change_speed

So there is nothing left of the delayloop and the variable, because as
seen by the compiler it does not have any effect and is not seen by
anyone else outside of the compiler's scope, thus can be dopped
completely unless the variable is volatile.

Volatile tells the compiler that there is someone outside of the
compiler's scope which tracks the value of the variable.

This is the expected behaviour for a compiler not specifically
targetting microcontrollers (those which do usually know that such as
loop does make sense, but this is knowledge beyond any C rules) and a
common surprise for programmers, as such behavior may change between
compiler versions.

Author: A.K. (Guest)
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> static void wait ( void )
> {
>     unsigned int waiting_time ;
>     int led_speed = change_speed() ;
>     for(waiting_time = 0; waiting_time < led_speed; waiting_time++)
>     {
>         /* do nothing */
>     }
> }

When you ask a machine, you usually get the same result for the same
answer. It doesn't matter where LedSpeed comes from, because the
compiler knows that there is absolutely no visible effect beyond the
call of change_speed(). Keeping a constant runtime is not a compiler's
job.

And once you find a way to fool the compiler by using code too
complicated to be optimized away, just wait for some future update.

Author: Guy Vo (codefreakguy)
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A.K. wrote:
>> static void wait ( void )
>> {
>>     unsigned int waiting_time ;
>>     int led_speed = change_speed() ;
>>     for(waiting_time = 0; waiting_time < led_speed; waiting_time++)
>>     {
>>         /* do nothing */
>>     }
>> }

I changed my code for change_speed() and wait() into your code snippets.
still doesn't work. I must make the waiting_time volatile ! Here are
some parts out of the lss files:
______________________________________________________________________ 
____
Without making the waiting_time volatile this is the generated asm:

bl  100f24 <wait>

00100f24 <wait>:
  100f24:  e3e03c0b   mvn  r3, #2816  ; 0xb00
  100f28:  e51330c3   ldr  r3, [r3, #-195]
    static unsigned int LedSpeed = SPEED *50 ;
===>  this is the only thing generated for wait !

  if ( (AT91F_PIO_GetInput(AT91C_BASE_PIOA) & SW1_MASK) == 0 )
    {
        if ( LedSpeed > SPEED ) LedSpeed -=SPEED ;
  100f2c:  e59f0048   ldr  r0, [pc, #72]  ; 100f7c <.text+0xf7c>
  100f30:  e3130902   tst  r3, #32768  ; 0x8000
    }
______________________________________________________________________ 
____
With making the waiting_time volatile this is the generated asm:

    volatile unsigned int waiting_time ;
    int led_speed = change_speed() ;
    for(waiting_time = 0; waiting_time < led_speed; waiting_time++)
  100f84:  e3a03000   mov  r3, #0  ; 0x0
  100f88:  ea000001   b  100f94 <wait+0x70>
  100f8c:  e59d3000   ldr  r3, [sp]
  100f90:  e2833001   add  r3, r3, #1  ; 0x1
  100f94:  e58d3000   str  r3, [sp]
  100f98:  e59d3000   ldr  r3, [sp]
  100f9c:  e1530002   cmp  r3, r2
  100fa0:  3afffff9   bcc  100f8c <wait+0x68>

This sounds like loop code to me don't we think ? ;-)

R3 = init zero of loop
R2 = ledspeed
______________________________________________________________________ 
____

So my conclusion is indeed that empty loops with no further use in other
parts of the program will be optimized out in arm-elf-gcc. I agree that
in normal cases you don't write code like that but it's given as an
example in the winarm distribution. I'd worked with other compilers
(intel,paradigm,VC,borland,watcom) and they did not such a thing as
agressivly as that. But I think the guys who adapt gcc to work for the
embedded world must make tradeoffs in their algorithms to optimize the
most out of it when you put the OPT=s. In my opinion this can be better
not optimized why ? Wel, I count 4 instructions for the loop more on the
arm7(32b mode) target which means only 16 bytes ! And as i said before
these are code snippets you write quick and dirty to test something out
and are not supposed to be stay in in the final release. At last if you
intend to write a line of code it must be useful after all and you can
always chose OPT=0 in other cases ;-)

Guy

Author: A.K. (Guest)
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> I'd worked with other compilers (intel,paradigm,VC,borland,watcom)
> and they did not such a thing as agressivly as that.

I don't know Paradigm, but both the Borland and the Watcom compilers are
no longer up current code generation and optimization standards since at
least a decade.

And for Intel's compiler - well, IIRC Intel was caught cheating because
their compiler detected SPECmark hotspots and replaced them by hand
crafted code or simply by the well known result. Which either resulted
in wrong results, when it did not recognize a minor change in source
code and still used the know wrong replacement. Or resulted in a large
performance drop when it did recognize it and had to do it the intended
way. Well, that's what I'd call agressive optimization... (I'd guess
they are not alone in this game though, GCC however is open source,
you'd see it).

> But I think the guys who adapt gcc to work for the
> embedded world must make tradeoffs

It is the same compiler for all. This kind of optimization is part of
the common GCC code and unaffected by the target specification. So those
who maintain the ARM code generator have no control over such
optimizations.

Sorry, but do not expect other people to do your job, especially when
they are not paid for it. An empty loop without any side effects is just
that - void. It is your job to tell the compiler that it is not. If you
need a compiler specifically designed for the embedded world, then buy
one. GCC is not.

Author: Guy Vo (codefreakguy)
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And yet another thing I encounterd for the same project:

In board.h there are following includes

#ifndef Board_h
#define Board_h

#include "AT91SAM7S64.h"
#define __inline extern inline
#include "lib_AT91SAM7S64.h"

In main.c there is

#include "Board.h"

But also in the startup file Cstartup_SAM7.c there

#include "Board.h"

If this file comes first in the compile pass which is the case board.h
will no more included in the main.c

The results I saw are different on the OPT parameter.

______________________________________________________________________ 
____
If I take a OPT=0 than the linker gives me:

linking: main.elf
arm-elf-gcc  -mcpu=arm7tdmi  -I. -gstabs -DROM_RUN  -O0 -Wall
-Wcast-align -Wimplicit  -Wpointer-arith -Wswitch -Wredundant-decls
-Wreturn-type -Wshadow -Wunused -Wa,-adhlns=Cstartup.lst   -MD -MP -MF
.dep/main.elf.d Cstartup.o   main.o Cstartup_SAM7.o      --output
main.elf -nostartfiles -Wl,-Map=main.map,--cref -lc  -lm -lc -lgcc
-TAT91SAM7S256-ROM.ld
main.o: In function `change_speed':
main.c:30: undefined reference to `AT91F_PIO_GetInput'
main.c:34: undefined reference to `AT91F_PIO_GetInput'
main.o: In function `main':
main.c:63: undefined reference to `AT91F_PMC_EnablePeriphClock'
main.c:67: undefined reference to `AT91F_PIO_CfgOutput'
main.c:70: undefined reference to `AT91F_PIO_SetOutput'
main.c:75: undefined reference to `AT91F_PIO_ClearOutput'
main.c:76: undefined reference to `AT91F_PIO_ClearOutput'
main.c:78: undefined reference to `AT91F_PIO_SetOutput'
main.c:79: undefined reference to `AT91F_PIO_SetOutput'
______________________________________________________________________ 
____

If I take a OPT=s:

Size after:
   text     data      bss      dec      hex  filename
      0      628        0      628      274  main.hex

Errors: none
______________________________________________________________________ 
____

But in the case of OPT=s there is no code compiled for the above
undefined function which are as extern inlines as the result the program
isn't working regardless of volatile here. This is a seriuos malfunction
of the linker/locater because I get no undefined warnings at all and
moreover the functions are not present in the final bin file !

If I include these files explicitly like

#include "AT91SAM7S64.h"                       /* AT91SAMT7S64
definitions  */
#include "lib_AT91SAM7S64.h"

in the main.c the output of the build is like:

Size after:
   text     data      bss      dec      hex  filename
      0     6488        0     6488     1958  main.hex

Errors: none

The function are now in the hex file as you can see the size is 10 times
more.
______________________________________________________________________ 
____

My general conclusion on this example is that it is not working as it
should be in using arm-elf- toolchain. I tried this in the KEIL
environment and there wer no problems at all.

If you start working and learning this toolchain it's better to take the
gamma project from Martin Thomas on follwing link:

http://gandalf.arubi.uni-kl.de/avr_projects/arm_pr...

Take the gamma example you get pretty much on testing even a small
monitor which works great on DBGU port.

So for the guys who make the winarm distributions please take a look at
your basic examples for AT91.

Cheers
Guy

Author: A.K. (Guest)
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It's just a wild guess because I have never used the SAM7 lib, but IMHO
you currently are the process detected one optimization pitfall after
the other, this time regarding an optimization strategy called
"inlining". The keywork "volatile" won't help you out this time.

Author: A.K. (Guest)
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small fix:
... but IMHO you currently are the in process detecting ...

Author: Guy Vo (codefreakguy)
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Yes by taking that one particular example I became the optimization
pittfal guy ;-) But I 'm learning from it that's the good thing.

I'm also rather new in this ARM buisness. I'm just looking out and try
some IDEs to get me of programming for it.

I checked both IAR and KEIL and now GNU. A friend of mine is using the
codevision IDE which is cheaper than KEIL or IAR.

But I red some benchmarking reports that GNU is one of the best in
size/speed so I take a look at now ;-)

Author: Clifford Slocombe (clifford)
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A.K. wrote:
> void wait ( void )
> {
>     unsigned int waiting_time ;
>     change_speed () ;
>     for(waiting_time = 0; waiting_time < 1000; waiting_time++) ;
> }
>
> => compiled with -Os
>
> wait:
>         b       change_speed
>
> So there is nothing left of the delayloop and the variable, ...

Well spotted A.K. I fell so foolish since I have seen this before (while
benchmarking compilers!). I was confused by Guy's setting LedSpeed
volatile, which I am presuming makes no difference. Because waiting time
is not used outside the loop, it can be optimised out, sinve teh aim of
the optimiser is to produce functionally equivalent code, but it is not
aware of 'real-time' constraints of the code. Declaring volatile forces
all values to be calculated.

So to guy, that is one other 'rule' about volatime I omitted. You need
it if the coded must be forced to generate all intermediate 'states'
even if they are apparently 'ignored'. Timing loops and benchmarking
being two cases in-point.

There is a more general point here - using 'delay loops' is a bad idea,
all sorts of things could break this code - a faster processor, caching,
optimisation. It is far better to configure a hardware timer and either
poll its value, or use its interrupt to increment a system counter that
is polled.

Clifford

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