2011/9/18 Henrik Nordström <henrik@xxxxxxxxxxxxxxxxxxx>: > The armv7 distro bootstrap have run into a number of packages failing to > compile in Thumb mode (default enabled in rpmrc) failing on ARM assembly > parts with conditional branch instructions. In thumb mode one apparently > need to use the IT instruction to hint to the CPU about the conditions > of following instructions in addition to have the conditions on the > instructions themselves. While investing these I saw rumors about GCC > eventually adding an automatic handling of these adding IT instructions > as needed, and even projects being very reluctant about touching any of > this until GCC have made up it's mind, and finally in an upstream bug > report on pulseaudio there is a reference to the -mimplicit-it=thumb GAS > option doing exacly this. > > Is there any drawbacks from enabling implicit IT instruction generation > letting GAS automatically deal with this as required by Thumb mode? If > not, should we add this to rpmrc? > > -Wa, -mimplicit-it=thumb Hi Henrik. We discussed this on gcc-patches and decided against it. The problem is that GCC does range calculations to figure out where to put the constant pool. If the assembler inserts instructions after GCC has run then these calculations are wrong and you could fail with a link error. Ubuntu used to turn on -mimplicit-it by default. They don't now and have fixed up many of the upstream packages. We've been fixing up any left overs during the weekly Linaro porting jam. The gcc-patches discussion is here: http://gcc.gnu.org/ml/gcc-patches/2010-07/msg01339.html http://gcc.gnu.org/ml/gcc-patches/2010-07/msg01351.html http://gcc.gnu.org/ml/gcc-patches/2010-07/msg01353.html http://gcc.gnu.org/ml/gcc-patches/2010-07/msg01354.html http://gcc.gnu.org/ml/gcc-patches/2010-07/msg01355.html http://gcc.gnu.org/ml/gcc-patches/2010-07/msg01356.html http://gcc.gnu.org/ml/gcc-patches/2010-07/msg01357.html The review list is here: https://wiki.ubuntu.com/ARM/Thumb2PackageReviewList You can view the porting jam bugs here: https://bugs.launchpad.net/ubuntu/+bugs?field.tag=arm-porting-queue&orderby=-id > from what I can understand it should be quite safe. And I am even of the > opinion that GAS is way better suited at adding these instructions than > the average programmer as it carries redundant information which need to > match the conditions of the instructions following after. GAS does validate the instructions and will error out if the IT instruction doesn't match the following. Note that while Thumb-2 has all of the same condition codes as ARM mode, the IT block can only do a condition and its negative. This covers 90 % of the cases I've seen though. -- Michael _______________________________________________ arm mailing list arm@xxxxxxxxxxxxxxxxxxxxxxx https://admin.fedoraproject.org/mailman/listinfo/arm
