Messages in this thread | | | Subject | Re: Building Kernel with -O0 | From | "Keith A. Prickett" <> | Date | Tue, 09 Sep 2008 10:10:40 -0700 |
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On Tue, 2008-09-09 at 19:04 +0300, Adrian Bunk wrote: > On Thu, Sep 04, 2008 at 10:50:53AM -0700, Keith A. Prickett wrote: > >... > > It seems building with optimization > > level 0 should be a natural thing for kernel driver developers to do for > > debugging purposes. > > > > In summary: I want to compile with optimizations off and the compile is > > failing now when I try this. How can this be resolved? > >... > > There are several places in the kernel where we e.g. rely on gcc > removing dead code, and otherwise the linking of the kernel fails. > > Compiling with -O0 never worked, and is not likely to ever work. I am currently running the kernel compiled with -O0. The final problem I ran into was some NULL pointer dereference and compiling ONLY files in the mm directory with -O2 fixed the issue.
To build with -O0 I had to change a few things (for my ARM architecture): 1. Ensure the __attribute__((always_inline)) was turned off in the "inline" #define (compiler.h)
2. Fix some inline assembly in kprobes (ARM only) that, due to no optimization, needed more registers than the compiler had available. (Similar to this issue: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13850)
3. Implement a fix as described here: (http://lists.arm.linux.org.uk/lurker/message/20050201.210458.5cc93c10.en.html)
4. Change -O2 to -O0 in the root Makefile
5. Add extra cflags in the mm Makefile so it builds with -O2 only in that directory.
All of this could, potentially be changed by a configuration parameter and added into the kernel release. Does someone have a primer on why this kind of behavior is not desired or "possible"?
Doing this has given me SO much visibility into stack variables data values while single-stepping the code. I feel that the time invested to make this work is well worth the return. In the book "Writing Solid Code" (http://www.amazon.com/Writing-Solid-Code-Microsofts-Programming/dp/1556155514) one technique discussed is: stepping through your code. In order to be able to do this effectively you need optimizations turned off.
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