Adding new build target

Adding new build target

[Derrick McKee]

Hi all,

Now that the kernel can be built with clang, I'd like to implement a
new build target to generate LLVM bytecode for all C source files
involved for a specific build configuration.  I am thinking that this
should be very simple if I know the make variables that captures the
needed information.  Specifically, I think I just need to perform the
following command for all C files:

clang -emit-llvm -c <appropriate CFLAGS and include paths> <source
file> -o <corresponding object file location>.bc

My problem is that I don't know how to get the info in the brackets,
but I am thinking that I should make a new build target `bytecode`.
Any help would be appreciated.  Thanks.

Derrick McKee

Re: Adding new build target

[Tom Roeder]

On Fri, Jun 07, 2019 at 12:58:33PM -0400, Derrick McKee wrote:
> Hi all,
> 
> Now that the kernel can be built with clang, I'd like to implement a
> new build target to generate LLVM bytecode for all C source files
> involved for a specific build configuration.  I am thinking that this
> should be very simple if I know the make variables that captures the
> needed information.  Specifically, I think I just need to perform the
> following command for all C files:
> 
> clang -emit-llvm -c <appropriate CFLAGS and include paths> <source
> file> -o <corresponding object file location>.bc
> 
> My problem is that I don't know how to get the info in the brackets,
> but I am thinking that I should make a new build target `bytecode`.
> Any help would be appreciated.  Thanks.

I know this isn't exactly what you're asking for, but one way to get
kernel object files as bitcode is to add -save-temps=obj to KCFLAGS.
That gives you bitcode for each object file, and you can pull them
together into a single object using llvm-link.

Note that at least when I was experimenting with this, I also needed to
turn off some warnings, because setting -save-temps=obj changes the way
that clang builds the objects. In particular, I needed to set

	-Wno-constant-logical-operand
	-Wno-parentheses-equality
	-Wno-pointer-bool-conversion
	-Wno-self-assign

> 
> Derrick McKee

Re: Adding new build target

[Masahiro Yamada]

On Sat, Jun 8, 2019 at 1:58 AM Derrick McKee <derrick.mckee@gmail.com> wrote:
>
> Hi all,
>
> Now that the kernel can be built with clang, I'd like to implement a
> new build target to generate LLVM bytecode for all C source files
> involved for a specific build configuration.  I am thinking that this
> should be very simple if I know the make variables that captures the
> needed information.  Specifically, I think I just need to perform the
> following command for all C files:
>
> clang -emit-llvm -c <appropriate CFLAGS and include paths> <source
> file> -o <corresponding object file location>.bc
>
> My problem is that I don't know how to get the info in the brackets,
> but I am thinking that I should make a new build target `bytecode`.
> Any help would be appreciated.  Thanks.
>
> Derrick McKee

Commit 433db3e260bc supported a single target to
generate llvm bytecode,
but it is not used for the final vmlinux or modules.

For what reason,
do you need to generate llvm bytecode for every file?

-- 
Best Regards
Masahiro Yamada