[Bug c/57813] Change of global variable ignored
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57813 --- Comment #9 from Manuel López-Ibáñez --- (In reply to Daniel Oertwig from comment #8) > Ok, so: Is this behaviour "part" of the language or a bug in gcc? > I could not find any documentation specifying that a sequence point should > be inbetween the (side-effect) evaluation of the function call and the > (value) evaluation of the assignment. http://c-faq.com/expr/seqpoints.html http://en.wikipedia.org/wiki/Sequence_point
[Bug c/57813] Change of global variable ignored
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57813 --- Comment #8 from Daniel Oertwig --- Ok, so: Is this behaviour "part" of the language or a bug in gcc? I could not find any documentation specifying that a sequence point should be inbetween the (side-effect) evaluation of the function call and the (value) evaluation of the assignment.
[Bug c/57813] Change of global variable ignored
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57813 Manuel López-Ibáñez changed: What|Removed |Added CC||manu at gcc dot gnu.org --- Comment #7 from Manuel López-Ibáñez --- > (In reply to Daniel Oertwig from comment #5) > > (In reply to Andrew Pinski from comment #3) > > > taskInfo.ready[priority]->wakeTime = Task_enforceTimeslice(priority); > > > > > > This is the same as: > > > (*taskInfo.ready[priority]).wakeTime = Task_enforceTimeslice(priority); > > > > > > Which means either the compiler can read the value of > > > taskInfo.ready[priority] before or after the function call to > > > Task_enforceTimeslice because there is no sequence point between them. > > > > Shouldn't the compiler recognize that enforceTimeslice is altering the > > global variable? The GCC code that could warn about this does not look within functions being called, so GCC does not know what enforeTimeslice is doing. Implementing this would require a substantial amount of work, and the current devs have their hands pretty full. Perhaps cppcheck or clang-analyzer can detect this case, but I wouldn't be surprised if they can't.
[Bug c/57813] Change of global variable ignored
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57813 Andrew Pinski changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #6 from Andrew Pinski --- (In reply to Daniel Oertwig from comment #5) > (In reply to Andrew Pinski from comment #3) > > taskInfo.ready[priority]->wakeTime = Task_enforceTimeslice(priority); > > > > This is the same as: > > (*taskInfo.ready[priority]).wakeTime = Task_enforceTimeslice(priority); > > > > Which means either the compiler can read the value of > > taskInfo.ready[priority] before or after the function call to > > Task_enforceTimeslice because there is no sequence point between them. > > Shouldn't the compiler recognize that enforceTimeslice is altering the > global variable? No because without sequence point the left hand side or right hand side can be done first.
[Bug c/57813] Change of global variable ignored
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57813 --- Comment #5 from Daniel Oertwig --- (In reply to Andrew Pinski from comment #3) > taskInfo.ready[priority]->wakeTime = Task_enforceTimeslice(priority); > > This is the same as: > (*taskInfo.ready[priority]).wakeTime = Task_enforceTimeslice(priority); > > Which means either the compiler can read the value of > taskInfo.ready[priority] before or after the function call to > Task_enforceTimeslice because there is no sequence point between them. Shouldn't the compiler recognize that enforceTimeslice is altering the global variable?
[Bug c/57813] Change of global variable ignored
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57813
Daniel Oertwig changed:
What|Removed |Added
Status|RESOLVED|UNCONFIRMED
Resolution|INVALID |---
--- Comment #4 from Daniel Oertwig ---
Task * Task_getNextReady()
{
uint8_t priority;
if (taskInfo.ready[0]) priority = 0;
else if (taskInfo.ready[1]) priority = 1;
else if (taskInfo.ready[2]) priority = 2;
else if (taskInfo.ready[3]) priority = 3;
else if (taskInfo.ready[4]) priority = 4;
else if (taskInfo.ready[5]) priority = 5;
else if (taskInfo.ready[6]) priority = 6;
else priority = 7;
time_t delay = Task_enforceTimeslice(priority);
Task * next = taskInfo.ready[priority];
next->wakeTime = delay;
return next;
}
This code solves the issue, using the correct value of the global variable.
Please tell me if I am wrong, but I think this code should be equivalent to the
original code.
(Task_enforceTimeslice is not declared as pure, I checked just to be sure)
[Bug c/57813] Change of global variable ignored
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57813 Andrew Pinski changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #3 from Andrew Pinski --- taskInfo.ready[priority]->wakeTime = Task_enforceTimeslice(priority); This is the same as: (*taskInfo.ready[priority]).wakeTime = Task_enforceTimeslice(priority); Which means either the compiler can read the value of taskInfo.ready[priority] before or after the function call to Task_enforceTimeslice because there is no sequence point between them.
[Bug c/57813] Change of global variable ignored
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57813 --- Comment #2 from Daniel Oertwig --- Oh, I am compiling with: CFLAGS := CFLAGS += -Os CFLAGS += -funsigned-char CFLAGS += -funsigned-bitfields CFLAGS += -fpack-struct CFLAGS += -fshort-enums CFLAGS += -ffreestanding CFLAGS += -Wall -Wextra CFLAGS += -Wstrict-prototypes CFLAGS += -Wundef CFLAGS += -Wsign-compare CFLAGS += -std=gnu99 #-std=c99 CFLAGS += -g3 -gdwarf-2 -pg CFLAGS += -fstack-usage LDFLAGS := LDFLAGS += -Wl,--relax using avr-gcc. There are no warnings.
[Bug c/57813] Change of global variable ignored
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57813 --- Comment #1 from Daniel Oertwig --- Created attachment 30451 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30451&action=edit Disassembly of the relating parts (from *.elf file, i.e. after linking)
