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: unassig...@gcc.gnu.org
ReportedBy: t...@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
