Re: [PATCH v2 09/28] kbuild: add support for Clang LTO

On Sun, Sep 06, 2020 at 04:36:32AM +0900, Masahiro Yamada wrote:

On Fri, Sep 4, 2020 at 5:31 AM Sami Tolvanen [off-list ref] wrote:

quoted

This change adds build system support for Clang's Link Time
Optimization (LTO). With -flto, instead of ELF object files, Clang
produces LLVM bitcode, which is compiled into native code at link
time, allowing the final binary to be optimized globally. For more
details, see:

  https://llvm.org/docs/LinkTimeOptimization.html

The Kconfig option CONFIG_LTO_CLANG is implemented as a choice,
which defaults to LTO being disabled.

What is the reason for doing this in a choice?
To turn off LTO_CLANG for compile-testing?

I would rather want to give LTO_CLANG more chances
to be enabled/tested.

It's a choice to prevent LTO from being enabled by default with
allyesconfig and allmodconfig. It would take hours to build these even on a
fast computer, and probably days on older hardware.

quoted

+ifdef CONFIG_LTO_CLANG
+ifdef CONFIG_THINLTO
+CC_FLAGS_LTO_CLANG := -flto=thin -fsplit-lto-unit
+KBUILD_LDFLAGS += --thinlto-cache-dir=.thinlto-cache

I think this would break external module builds
because it would create cache files in the
kernel source tree.

External module builds should never ever touch
the kernel tree, which is usually located under
the read-only /usr/src/ in distros.


.thinlto-cache should be created in the module tree
when it is built with M=.

Thanks for pointing this out, I'll fix the path in v3.

quoted

 # Directories & files removed with 'make distclean'
-DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS
+DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS .thinlto-cache

This was suggested in v1, but I could not understand
why doing this in distclean was appropriate.

Is keeping cache files of kernel objects
useful for external module builds?

No, the cache only speeds up incremental kernel builds.

Also, please clean up .thinlto-cache for external module builds.

Ack.

Sami

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