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Date: Tue, 8 Dec 2020 17:56:13 +0100
From: Arnd Bergmann <>
To: Sami Tolvanen <>
Cc: Masahiro Yamada <>, Steven Rostedt <>, 
	Will Deacon <>, Josh Poimboeuf <>, 
	Peter Zijlstra <>, Greg Kroah-Hartman <>, 
	"Paul E. McKenney" <>, Kees Cook <>, 
	Nick Desaulniers <>, 
	clang-built-linux <>, 
	Kernel Hardening <>, 
	linux-arch <>, 
	Linux ARM <>, 
	Linux Kbuild mailing list <>, 
	"" <>, linux-pci <>
Subject: Re: [PATCH v8 00/16] Add support for Clang LTO

On Tue, Dec 8, 2020 at 5:53 PM 'Sami Tolvanen' via Clang Built Linux
<> wrote:
> > A small update here: I see this behavior with every single module
> > build, including 'tinyconfig' with one module enabled, and 'defconfig'.
> The .o file here is a thin archive of the bitcode files for the
> module. We compile .lto.o from that before modpost, because we need an
> ELF binary to process, and then reuse the .lto.o file when linking the
> final module.
> At no point should we link the .o file again, especially not with
> .lto.o, because that would clearly cause every symbol to be
> duplicated, so I'm not sure what goes wrong here. Here's the relevant
> part of scripts/Makefile.modfinal:
> # With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to
> # avoid a second slow LTO link
> prelink-ext := .lto
> ...
> $(modules): %.ko: %$(prelink-ext).o %.mod.o scripts/ FORCE
>         +$(call if_changed,ld_ko_o)

Ah, it's probably a local problem now, as I had a merge conflict against
linux-next in this Makefile and I must have resolved the conflict incorrectly.


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