Re: [PATCH v2 13/28] kbuild: lto: merge module sections

Sami Tolvanen
2020-09-08T21:08:15Z

On Tue, Sep 08, 2020 at 12:25:54AM +0900, Masahiro Yamada wrote:
> On Fri, Sep 4, 2020 at 5:31 AM Sami Tolvanen <samitolvanen@google.com> wrote:
> >
> > LLD always splits sections with LTO, which increases module sizes. This
> > change adds a linker script that merges the split sections in the final
> > module.
> >
> > Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
> > Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> > ---
> > Makefile | 2 ++
> > scripts/module-lto.lds | 26 ++++++++++++++++++++++++++
> > 2 files changed, 28 insertions(+)
> > create mode 100644 scripts/module-lto.lds
> >
> > diff --git a/Makefile b/Makefile
> > index c69e07bd506a..bb82a4323f1d 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -921,6 +921,8 @@ CC_FLAGS_LTO_CLANG += -fvisibility=default
> > # Limit inlining across translation units to reduce binary size
> > LD_FLAGS_LTO_CLANG := -mllvm -import-instr-limit=5
> > KBUILD_LDFLAGS += $(LD_FLAGS_LTO_CLANG)
> > +
> > +KBUILD_LDS_MODULE += $(srctree)/scripts/module-lto.lds
> > endif
> >
> > ifdef CONFIG_LTO
> > diff --git a/scripts/module-lto.lds b/scripts/module-lto.lds
> > new file mode 100644
> > index 000000000000..cbb11dc3639a
> > --- /dev/null
> > +++ b/scripts/module-lto.lds
> > @@ -0,0 +1,26 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * With CONFIG_LTO_CLANG, LLD always enables -fdata-sections and
> > + * -ffunction-sections, which increases the size of the final module.
> > + * Merge the split sections in the final binary.
> > + */
> > +SECTIONS {
> > + __patchable_function_entries : { *(__patchable_function_entries) }
> > +
> > + .bss : {
> > + *(.bss .bss.[0-9a-zA-Z_]*)
> > + *(.bss..L*)
> > + }
> > +
> > + .data : {
> > + *(.data .data.[0-9a-zA-Z_]*)
> > + *(.data..L*)
> > + }
> > +
> > + .rodata : {
> > + *(.rodata .rodata.[0-9a-zA-Z_]*)
> > + *(.rodata..L*)
> > + }
> > +
> > + .text : { *(.text .text.[0-9a-zA-Z_]*) }
> > +}
> > --
> > 2.28.0.402.g5ffc5be6b7-goog
> >
>
>
> After I apply https://patchwork.kernel.org/patch/11757323/,
> is it possible to do like this ?
>
>
> #ifdef CONFIG_LTO
> SECTIONS {
> ...
> };
> #endif
>
> in scripts/module.lds.S

Yes, that should work. I'll change this in v3 after your change is
applied.

Sami