link time analysis for the kernel.
Hi, There are some detaills about the current procedures for linking the kernel that I am unfamiliar with. My understanding is that GCC and Clang both have the ability to do link time analysis and transforms on code but is it possible to write link time passes that will run on the kernel since the linking phase is a bit different (i.e. doesnt produce an ELF file)? Regards, Carter.
Actually I have compiled and installed kernels before. I am wondering however if LTO still works for compiling kernel images on clang or gcc since my understanding is the kernel code includes a kernel loader which loads the ELF format but the image of an OS kernel is loaded either directly or via a bootloader which my understanding is cannot read ELF(is this correct?). On Fri, Oct 12, 2018 at 2:02 AM <o@goosey.org> wrote:
11.10.2018, 17:48, "Carter Cheng" <cartercheng@gmail.com>:
Hi,
Hello,
I want to ask pardon me and have you ever compiled a linux kernel? In my opinion you should first examine gcc ld and make process :)
The elf format executable format and the process after compiling the c code. Please read:
http://www.ntu.edu.sg/home/ehchua/programming/cpp/gcc_make.html
keep calm and go step by step and continue to learn c, gcc, ld, make, c code compile to machine code.
Ozgur
There are some detaills about the current procedures for linking the kernel that I am unfamiliar with. My understanding is that GCC and Clang both have the ability to do link time analysis and transforms on code but is it possible to write link time passes that will run on the kernel since the linking phase is a bit different (i.e. doesnt produce an ELF file)?
Regards,
Carter. ,
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
sorry not a kernel loader but an ELF loader. On Fri, Oct 12, 2018 at 2:55 AM Carter Cheng <cartercheng@gmail.com> wrote:
Actually I have compiled and installed kernels before. I am wondering however if LTO still works for compiling kernel images on clang or gcc since my understanding is the kernel code includes a kernel loader which loads the ELF format but the image of an OS kernel is loaded either directly or via a bootloader which my understanding is cannot read ELF(is this correct?).
On Fri, Oct 12, 2018 at 2:02 AM <o@goosey.org> wrote:
11.10.2018, 17:48, "Carter Cheng" <cartercheng@gmail.com>:
Hi,
Hello,
I want to ask pardon me and have you ever compiled a linux kernel? In my opinion you should first examine gcc ld and make process :)
The elf format executable format and the process after compiling the c code. Please read:
http://www.ntu.edu.sg/home/ehchua/programming/cpp/gcc_make.html
keep calm and go step by step and continue to learn c, gcc, ld, make, c code compile to machine code.
Ozgur
There are some detaills about the current procedures for linking the kernel that I am unfamiliar with. My understanding is that GCC and Clang both have the ability to do link time analysis and transforms on code but is it possible to write link time passes that will run on the kernel since the linking phase is a bit different (i.e. doesnt produce an ELF file)?
Regards,
Carter. ,
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
On Thu, 11 Oct 2018 21:45:16 +0800, Carter Cheng said:
There are some detaills about the current procedures for linking the kernel that I am unfamiliar with. My understanding is that GCC and Clang both have the ability to do link time analysis and transforms on code but is it possible to write link time passes that will run on the kernel since the linking phase is a bit different (i.e. doesnt produce an ELF file)?
The fact that the kernel gets linked is an existence proof that it is possible to do link time processing on the kernel. There's no LTO support in the stock 4.19 tree, but Andi Kleen did a patchset for 4.15, and there's another patchset to enable LTO when using Clang rather than gcc. (I haven't tried either one, don't use on a production machine, as the resulting kernel may crash, eat filesystems, and/or turn your dog green...) Note that 'vmlinux' is a statically linked ELF binary. That plus a bootstrap code gets merged to create a bzImage or similar thing that can be loaded by Grub2 or whatever boot loader.
I managed to find some information on this from Prof John Criswell who did something similar for his dissertation but I do wonder how complicated the make files for building the kernel are since the method he used would require using llvm-link to stitch together all the different object files(bitcode) into a single file and then convert them into machine code at one go. On Fri, Oct 12, 2018 at 10:20 AM <valdis.kletnieks@vt.edu> wrote:
On Thu, 11 Oct 2018 21:45:16 +0800, Carter Cheng said:
There are some detaills about the current procedures for linking the kernel that I am unfamiliar with. My understanding is that GCC and Clang both have the ability to do link time analysis and transforms on code but is it possible to write link time passes that will run on the kernel since the linking phase is a bit different (i.e. doesnt produce an ELF file)?
The fact that the kernel gets linked is an existence proof that it is possible to do link time processing on the kernel.
There's no LTO support in the stock 4.19 tree, but Andi Kleen did a patchset for 4.15, and there's another patchset to enable LTO when using Clang rather than gcc. (I haven't tried either one, don't use on a production machine, as the resulting kernel may crash, eat filesystems, and/or turn your dog green...)
Note that 'vmlinux' is a statically linked ELF binary. That plus a bootstrap code gets merged to create a bzImage or similar thing that can be loaded by Grub2 or whatever boot loader.
participants (5)
-
Carter Cheng -
o＠goosey.org -
Ozgur
-
Ozgur -
valdis.kletnieks＠vt.edu