http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49169
Summary: ARM: optimisations strip the Thumb/ARM mode bit off
function pointers
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: michael.h...@linaro.org
ARM devices encode the instruction set mode in the LSB of the function address.
Functions are word aligned on ARM. If you try to test the LSB of a function
pointer then GCC assumes that the two least significant bits are zero and
optimises away the test.
This problem is seen in Mono and was originally reported at:
https://bugs.launchpad.net/ubuntu/+source/gcc-4.5/+bug/721531
A reduced test case is:
void main() {
void *p = main;
if ((int)p & 1) printf ("HIT!\n");
}
When compiled with -march=armv7-a -mthumb -O0 then the word 'HIT!' will show.
When compiled with -O2, the branch is not taken.
The problem does not occur in 4.4.5. It does occur in 4.5.2, 4.6.0, and trunk
r174044.
