Is it possible to turn off the gcc optimization when compiling kernel?
Hi, When I am debugging the linux kernel, I find that the execution sequence of some code is very strange. I think I need to turn off gcc optimization by changing "-O2" to "-O0". But I encounter many errors. So, I want to know is it possible to turn off the gcc optimization or how can I compile some functions without optimization. Thanks.
On Sun, Mar 20, 2016 at 02:46:41PM +0800, Hao Lee wrote:
Hi, When I am debugging the linux kernel, I find that the execution sequence of some code is very strange. I think I need to turn off gcc optimization by changing "-O2" to "-O0". But I encounter many errors. So, I want to know is it possible to turn off the gcc optimization or how can I compile some functions without optimization.
You can not turn it off in all functions as some need particluar optimization flags to comile at all, but you can pass individual CFLAGS per file via the Makefile CFLAGS_target.o = -O0 or -flags-to-use aswell as remove specific CFLAGS with CFLAGS_REMOVE_target.o = -flags-to-remove but if you want to debug the kernel it is most likely not a good idea to try and disable optimization as the code you then are debugging might not have that much to do with the final code once optimization is on again. So simply generate the .lst file of the target you are trying to debug e.g. for kernel/sched/core.c: make kernel/sched/core.lst and then use that .lst file to understand the output of gdb you are inspecting. thx! hofrat
On Mon, Mar 21, 2016 at 5:51 PM, Nicholas Mc Guire <der.herr@hofr.at> wrote:
You can not turn it off in all functions as some need particluar optimization flags to comile at all, but you can pass individual CFLAGS per file via the Makefile
CFLAGS_target.o = -O0 or -flags-to-use
aswell as remove specific CFLAGS with
CFLAGS_REMOVE_target.o = -flags-to-remove
but if you want to debug the kernel it is most likely not a good idea to try and disable optimization as the code you then are debugging might not have that much to do with the final code once optimization is on again. So simply generate the .lst file of the target you are trying to debug e.g. for kernel/sched/core.c:
make kernel/sched/core.lst
and then use that .lst file to understand the output of gdb you are inspecting.
Thanks for your reply! Besides,I also find that use "gcc -c -Q -O1 --help=optimizers" can print the exact set of optimizations. regards, Hao Lee
On Tue, Mar 22, 2016 at 07:24:38PM +0800, Hao Lee wrote:
On Mon, Mar 21, 2016 at 5:51 PM, Nicholas Mc Guire <der.herr@hofr.at> wrote:
You can not turn it off in all functions as some need particluar optimization flags to comile at all, but you can pass individual CFLAGS per file via the Makefile
CFLAGS_target.o = -O0 or -flags-to-use
aswell as remove specific CFLAGS with
CFLAGS_REMOVE_target.o = -flags-to-remove
but if you want to debug the kernel it is most likely not a good idea to try and disable optimization as the code you then are debugging might not have that much to do with the final code once optimization is on again. So simply generate the .lst file of the target you are trying to debug e.g. for kernel/sched/core.c:
make kernel/sched/core.lst
and then use that .lst file to understand the output of gdb you are inspecting.
Thanks for your reply! Besides,I also find that use "gcc -c -Q -O1 --help=optimizers" can print the exact set of optimizations.
yes - but for any given code you will find that many of those options actually have no effect for the particular code blob. The set of flags effectively impacting the generated object file is generally much smaller than the ones reported by gcc -c -Q -O1 --help=optimizers. thx! hofrat
On Tue, Mar 22, 2016 at 7:35 PM, Nicholas Mc Guire <der.herr@hofr.at> wrote:
yes - but for any given code you will find that many of those options actually have no effect for the particular code blob. The set of flags effectively impacting the generated object file is generally much smaller than the ones reported by gcc -c -Q -O1 --help=optimizers.
thx! hofrat
yeah,only several options take effect. Because "-O0" will break the link progress,I just want use "-O1" and some other options to approximate the effects of "-O0". regards, Hao Lee
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Hao Lee -
Nicholas Mc Guire