When a while loop with the true condition (while (1)) contains
an if statement with a modulus evaluated on a counter variable, it
appears that gcc incorrectly optimizes out the modulus check as a
constant (even though the counter variable is updated after the if
statement). If the counter variable is updated _before_ the if
statement, it works properly. Example code is in the how-to-repeat section.
Environment:
System: Linux uktena64 2.6.10-5-amd64-k8 #1 Fri Jun 24 17:08:40 UTC 2005 x86_64
GNU/Linux
Architecture: x86_64
host: x86_64-pc-linux-gnu
build: x86_64-pc-linux-gnu
target: x86_64-pc-linux-gnu
configured with: ../src/configure -v
--enable-languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-gxx-include-dir=/usr/include/c++/3.3 --enable-shared --with-system-zlib
--enable-nls --without-included-gettext --enable-__cxa_atexit
--enable-clocale=gnu --enable-debug --enable-java-gc=boehm
--enable-java-awt=xlib --enable-objc-gc --disable-multilib x86_64-linux
How-To-Repeat:
The following code produces the bug (output is 1 1 1 1 ... instead
of 0 1 2 3 ...):
void main() {
int i=0;
while (1) {
if (i%100==0) printf("%d ",i);
i++;
}
}
------- Comment #1 from Nick at idontproperlyfilloutmyemailaddress dot com
2005-08-31 03:18 -------
Fix:
If i++; is above the if statement, the code works properly
(1 2 3 4 ...).
------- Comment #2 from pinskia at gcc dot gnu dot org 2005-08-31 04:55 -------
Fixed at least in 4.0.2 and above.
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Component|c |tree-optimization
Keywords| |wrong-code
Resolution| |FIXED
Summary|Bug w\ while (1) loop and |[4.0 Regression] Bug w\
|counter variable w\ |while (1) loop and counter
|optimization |variable w\ optimization
Target Milestone|--- |4.0.2
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23647
