Re: [PATCH v8 00/16] Add support for Clang LTO

[Sami Tolvanen <samitolvanen@google.com>]

On Tue, Dec 8, 2020 at 5:55 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, Dec 8, 2020 at 1:15 PM Arnd Bergmann <arnd@kernel.org> wrote:
> > On Tue, Dec 1, 2020 at 10:37 PM 'Sami Tolvanen' via Clang Built Linux <clang-built-linux@googlegroups.com> wrote:
> >
> > - many builds complain about thousands of duplicate symbols in the kernel, e.g.
> >   ld.lld: error: duplicate symbol: qrtr_endpoint_post
> >  >>> defined in net/qrtr/qrtr.lto.o
> >  >>> defined in net/qrtr/qrtr.o
> >  ld.lld: error: duplicate symbol: init_module
> >  >>> defined in crypto/842.lto.o
> >  >>> defined in crypto/842.o
> >  ld.lld: error: duplicate symbol: init_module
> >  >>> defined in net/netfilter/nfnetlink_log.lto.o
> >  >>> defined in net/netfilter/nfnetlink_log.o
> >  ld.lld: error: duplicate symbol: vli_from_be64
> >  >>> defined in crypto/ecc.lto.o
> >  >>> defined in crypto/ecc.o
> >  ld.lld: error: duplicate symbol: __mod_of__plldig_clk_id_device_table
> >  >>> defined in drivers/clk/clk-plldig.lto.o
> >  >>> defined in drivers/clk/clk-plldig.o
>
> 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:

ifdef CONFIG_LTO_CLANG
# 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/module.lds FORCE
        +$(call if_changed,ld_ko_o)

Sami