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

On Tue, Sep 08, 2020 at 12:25:54AM +0900, Masahiro Yamada wrote:

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

quoted

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

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