[llvmlinux] Compilation errors android msm-caf-nexus5 and oneplus one

[Khem Raj]

> On Aug 24, 2015, at 6:14 AM, Fürst Stefan <dieb.stefan.96 at gmail.com> wrote:
> 
> drivers/built-in.o: In function `__video_do_ioctl':
> /home/stefan/llvmlinux/targets/nexus5/src/android_kernel_nexus5/drivers/media/video/v4l2-ioctl.c:1604: undefined reference to `__aeabi_memclr4'
> make[1]: *** [.tmp_vmlinux1] Error 1
> make: *** [sub-make] Error 2

This means that compiler is finding a place where it can rely on builtins function implemented in compiler runtime which is compiler-rt or libgcc depending upon which runtime you use. if it was a user application, then simply linking with compiler-rt would have been sufficient but in kernel it does not require any C runtime and whatever is needed is implemented in kernel itself. So kernel has a small helper library which implements some of these builtins, you have two options,

1. Implement this builtin in kernel
2. Alter the code in question so that compiler does not generate the call to builtin
3. Add -fno-builtin-<value> to compiler flags to disable compiler from emitting it

Thanks
-Khem

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