[Bug c/54500] New: While(TRUE) loop optimization of global struct variables

[Bug c/54500] New: While(TRUE) loop optimization of global struct variables

[tma at email dot cz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54500

             Bug #: 54500
           Summary: While(TRUE) loop optimization of global struct
                    variables
    Classification: Unclassified
           Product: gcc
           Version: 4.3.2
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tma@email.cz
            Target: avr-*
             Build: WinAVR 20081205


I have large global static structure with substructers, longs etc.

struct {

  longs ...
  structs ...
  struct {
    boolean isEnd;
  } ss;
  etc.


} gs;

void procedure() {

  while(true) {
    if (gs.ss.isEnd) {
      if (an_cond1) {
        break;
      }
    }
    if (an_cond2) {
      break;
    }
    do_access_and_modify_gs_fields;
    gs.ss.isEnd = TRUE;
    do_access_and_modify_gs_fields;
  }

  if (gs.ss.IsEnd) {
  }

when used in "neverending" while with breaks then gcc fails to optimize
correctly changed fields of global structure. Gcc tries to cache field used in
loop into registers and when no more registers on the stack (what is benefit 
of copying vars from global variable onto stack ???). Changed values are
flushed back into global variable when loop terminates. There are several
breaks how to exit the loop. Suprisingly every loop has "copy&pasted" the same
exit code (why don't reuse one to save some memory ??). Respectively they
should be at least equal but in my case one of them did not flush back one
field (gs.isEnd after loop get unupdated value). I tried somehow persuade gcc
to know that isEnd is really modified by asm inline code. Sucessfully but any
change of code may change optimization and it appears again in another form.

Currently is problem that "gs.isEnd = TRUE;" updates optimized value in
register but "if (gs.ss.isEnd)" in loop check again global memory.

Seems gcc is totally confused with this construction. I hope setting struct as
volatile should help. 

avr-g++ -MM -mmcu=atmega2560 -g -Os -Wall -Wextra -w -ffunct
ion-sections -fdata-sections -fno-exceptions my.cpp -MF my.d -MT my.o

[Bug c/54500] While(TRUE) loop optimization of global struct variables

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54500

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2012-09-06
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-09-06 09:51:13 UTC ---
This is not a testcase that can be compiled, please provide one so we can
reproduce the issue and analyze it.  Please also try a more recent compiler,
4.3.2 is out of maintainance since several years - try at least GCC 4.6.3.

[Bug other/54500] While(TRUE) loop optimization of global struct variables

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54500

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|avr-*                       |avr
   Last reconfirmed|2012-09-06 00:00:00         |
          Component|c                           |other
              Build|WinAVR 20081205             |
           Severity|major                       |normal

[Bug other/54500] While(TRUE) loop optimization of global struct variables

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54500

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
                 CC|                            |gjl at gcc dot gnu.org
         Resolution|                            |INVALID

--- Comment #2 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2013-03-03 16:55:28 UTC ---
No answer and no valid test case for 6 months now.  Closed as invalid.