bugAVR C Runtime Library - Bugs: bug #22163, Everytime...


bug #22163: Everytime ATOMIC_BLOCK(ATOMIC_RESTORESTATE) C++ compiler generates warning - unused variable 'sreg_save'

Submitted by:  Tomasz Francuz <tfrancuz>
Submitted on:  Wed 30 Jan 2008 05:17:06 AM UTC  
Category:  Header Severity:  4 - Important
Priority:  9 - Immediate Item Group:  libc code
Status:  None Percent Complete:  0%
Assigned to:  None Open/Closed:  Open
Release:  Any Fixed Release:  None

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Mon 03 Feb 2014 01:25:44 AM UTC, comment #5:

Can't reproduce this with:
- avr-g++ (GCC) 4.8.1
- avr-libc (debian) 1:1.8.0-4

This bug might have been fixed in avr-gcc as avr-libc haven't changed in time passed.

Just thought it might be good to clean obsolete bugs, thus tested and commented about my resulsts.

Jaakko Kantojärvi <raphendyr>
Thu 31 Jan 2008 10:16:58 AM UTC, comment #4:

I've just tried this code section, and there is a good reason for the "unused variable 'sreg_save'" warning - looking at the generated assembly, the variable is unused. In other words, the SREG is not restored after the block, as it is in the C version.

Could it be that the _attribute_((_cleanup_)) attribute is not working correctly for avr c++, at least when used in this way? If that's the case, then this is definitely a bug.

I've noticed another issue with the atomic.h functions - I don't think the memory blocks (asm("":::"memory")) are in the right places. It's important to ensure that any changes to memory are written out before interrupts are enabled - thus the block should appear before the sei() instructions, and before restoring SREG. The block is unnecessary before disabling interrupts. I could be wrong, but I think that's the correct ordering.

In C++ it is normally considered better to use a class and RAII for something like this. You should have a class such as:

class CriticalSection {
CriticalSection() : _sreg( SREG ) { cli(); }
~CriticalSection() { asm("" ::: "memory"); SREG = _sreg; }
uint8_t _sreg;

Then you simply declare a "CriticalSection" variable in the block you want to protect. Similar classes can be defined for non-critical sections, and for when you want to restore the old interrupt enable state, or to force the flag on or off.

As a workaround for the bug blocking cleanup functions, it's possible to use a RAII class within the same framework as the C-style atomic blocks. It's a little messy, but gives identical optimal assembly code (in my brief testing, anyway). I haven't gone through the details of patching this into atomic.h (I'm not sure what the policy of including c++ specific stuff is), but it can be added on at the end of atomic.h as it re-defines some macros:

#ifdef __cplusplus

// Override these macros:

class RestoreState {
RestoreState() : _x( SREG ), _restore( true ) {} // For keeping sreg
RestoreState(uint8_t x) : _x( x ), _restore( false ) {} // For __ToDo loop hack
~RestoreState() { if (_restore) SREG = _x; }
operator bool() const { return _x; }; // For testing __ToDo
uint8_t _x;
bool _restore;

class ForceOn {
ForceOn() : _x( 0 ), _restore( true ) {} // For ignoring sreg
ForceOn(uint8_t x) : _x( x ), _restore( false ) {} // For __ToDo loop hack
~ForceOn() { if (_restore) sei(); }
operator bool() const { return _x; }; // For testing __ToDo
uint8_t _x;
bool _restore;

class ForceOff {
ForceOff() : _x( 0 ), _restore( true ) {} // For ignoring sreg
ForceOff(uint8_t x) : _x( x ), _restore( false ) {} // For __ToDo loop hack
~ForceOff() { if (_restore) cli(); }
operator bool() const { return _x; }; // For testing __ToDo
uint8_t _x;
bool _restore;

#define ATOMIC_RESTORESTATE RestoreState sreg_save
#define ATOMIC_FORCEON ForceOn sreg_save
#define NONATOMIC_RESTORESTATE RestoreState sreg_save
#define NONATOMIC_FORCEOFF ForceOff sreg_save


David Brown <davidbrown>
Thu 31 Jan 2008 02:56:23 AM UTC, comment #3:

Sorry, but mistake I didn't mention that I'm using avr-g++ compiler. When using avr-gcc everything is working perfectly, but in g++ the following code produces warning:
#include <util/atomic.h>

int main()
volatile int b,a;
if(b==0) a++;

I don't know if you consider it as a bug, but it would be great if ATOMIC_BLOCK could be used in c++.

Tomasz Francuz <tfrancuz>
Wed 30 Jan 2008 04:14:44 PM UTC, comment #2:

I have not seen this warning despite having used this macro in several projects. Are you using C++ Tomasz (I only have C projects)?

Lars Jonsson <lajon>
Wed 30 Jan 2008 03:20:35 PM UTC, comment #1:

Could you please post a small test case that gives this warning?

Eric Weddington <arcanum>
Project Administrator
Wed 30 Jan 2008 05:17:06 AM UTC, original submission:

Everytime when using ATOMIC_BLOCK(ATOMIC_RESTORESTATE) macro I have got compiler warning - warning: unused variable 'sreg_save'. I'm using winavr-20071221.

Tomasz Francuz <tfrancuz>


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    Follow 9 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    Fri 16 Nov 2012 06:39:41 PM UTC arcanum Priority5 - Normal => 9 - Immediate
    Tue 22 Feb 2011 08:58:00 PM UTC arcanum Priority7 - High => 5 - Normal
        SummaryEverytime ATOMIC_BLOCK(ATOMIC_RESTORESTATE) compiler generates warning - unused variable \'sreg_save\' => Everytime ATOMIC_BLOCK(ATOMIC_RESTORESTATE) C++ compiler generates warning - unused variable 'sreg_save'
    Wed 09 Jun 2010 08:24:07 AM UTC joerg_wunsch Priority3 - Low => 7 - High
    Wed 09 Jun 2010 08:24:06 AM UTC joerg_wunsch Severity3 - Normal => 4 - Important
    Mon 11 Aug 2008 03:01:46 AM UTC arcanum Priority5 - Normal => 3 - Low
        ReleaseNone => Any
    Mon 17 Mar 2008 04:36:16 PM UTC arcanum CategoryNone => Header
    Wed 30 Jan 2008 05:17:07 AM UTC tfrancuz Carbon-Copy- => Added tfrancuz

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