No option to build gentoo-sources with LTO

[causality]

This is concerning gentoo-sources-6.13.4 version. I have enabled the "experimental" USE flag.

I notice that when I run "make menuconfig", there is an entry under "General architecture-dependent options" that is called "Link Time Optimization (LTO)". I can select this to bring up a sub-menu, but the only available option is "None" or "CONFIG_LTO_NONE". I have GCC version 14.2.1_p20241221 and Clang 19.1.7, both of which support LTO and often build packages with it enabled.

Please correct me if I have this wrong, but my understanding is that building the kernel with Clang+LTO has been supported for some time now and GCC+LTO is more recently supported. So, why can't I select these options through standard configuration? Usually, the menuconfig "<HELP>" option will tell you what selects or de-selects a given option, but that information is notably absent.

I keep a few previous kernels around and configured in my bootloader, so there is very little risk for me if an LTO kernel doesn't boot or wants to crash.

Further, I perused https://wiki.gentoo.org/wiki/Kernel/Optimization#Link_Time_Optimization but it appears to be dated. Specifically, that site mentions kernel version 6.6 and I could find nothing more recent on that site. I also tried https://github.com/CachyOS/kernel-patches/ but none of the patches there under 6.13 indicate LTO.

I would rather not use a patch if that's not needed. I would much prefer to figure out why a nice, standard configuration option is unavailable than force the issue by manually trying to add KCFLAGS and such. From what I hear, apparently some other users get to select LTO and then select whether that'll be Clang or GCC. I'm unclear on how they are doing that and recent information on this topic is quite difficult to find (most of it is about 5.12 series kernels), but I would like to try that.

Standard output of "emerge --info":

[Hu]

As I read arch/Kconfig, this prompt has 3 options: None (always available), LTO_CLANG_FULL (depends on HAS_LTO_CLANG), and LTO_CLANG_THIN (depends on HAS_LTO_CLANG and ARCH_SUPPORTS_LTO_CLANG_THIN). I then looked above and found the implementation of HAS_LTO_CLANG:

[sam_]

kernel.org upstream still doesn't support GCC LTO because they don't want to adapt their asm (or add workarounds, depending on your perspective) to mark them up with attributes.

As for using LTO with Clang, you need to make sure you pass LLVM=1 on all make command lines (including for e.g. menuconfig).

[causality]

Yes, adding "LLVM=1" with each invocation of "make" opened up the appropriate Clang options including Full LTO and ThinLTO. Thank you, that's nice and simple.

The kernel built just fine. Then x11-drivers/nvidia-drivers automatically detected that the kernel was built with Clang and built itself with Clang as well (usually both would be GCC). nvidia-drivers built and installed fine, but I had a suspicion that LTO might be a little too aggressive at removing symbols that an out-of-tree module would need. Sure enough, that's what happened, with both full LTO and thin LTO. The kernel boots fine and functions, except that the nvidia driver fails to load. From kern.log: