[PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Arnd Bergmann]

From: Arnd Bergmann <arnd at arndb.de>

When looking at kernel size optimizations, I found that arm64
does not currently support HAVE_LD_DEAD_CODE_DATA_ELIMINATION,
which enables the --gc-sections flag to the linker.

I see that for a defconfig build with llvm, there are some
notable improvements from enabling this, in particular when
combined with the recently added CONFIG_LTO_CLANG_THIN
and CONFIG_TRIM_UNUSED_KSYMS:

   text    data     bss     dec     hex filename
16570322 10998617 506468 28075407 1ac658f defconfig/vmlinux
16318793 10569913 506468 27395174 1a20466 trim_defconfig/vmlinux
16281234 10984848 504291 27770373 1a7be05 gc_defconfig/vmlinux
16029705 10556880 504355 27090940 19d5ffc gc+trim_defconfig/vmlinux
17040142 11102945 504196 28647283 1b51f73 thinlto_defconfig/vmlinux
16788613 10663201 504196 27956010 1aa932a thinlto+trim_defconfig/vmlinux
16347062 11043384 502499 27892945 1a99cd1 gc+thinlto_defconfig/vmlinux
15759453 10532792 502395 26794640 198da90 gc+thinlto+trim_defconfig/vmlinux

I needed a small change to the linker script to get clean randconfig
builds, but I have not done any meaningful boot testing on it to
see if it works. If there are no regressions, I wonder whether this
should be autmatically done for LTO builds, given that it improves
both kernel size and compile speed.

Link: https://lore.kernel.org/lkml/CAK8P3a05VZ9hSKRzVTxTn+1nf9E+gqebJWTj6N23nfm+ELHt9A@mail.gmail.com/
Signed-off-by: Arnd Bergmann <arnd at arndb.de>
---
 arch/arm64/Kconfig              | 1 +
 arch/arm64/kernel/vmlinux.lds.S | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b94a678afce4..75e13cc52928 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -2,6 +2,7 @@
 config ARM64
 	def_bool y
 	select ACPI_CCA_REQUIRED if ACPI
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	select ACPI_GENERIC_GSI if ACPI
 	select ACPI_GTDT if ACPI
 	select ACPI_IORT if ACPI
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index bad2b9eaab22..926cdb597a45 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -217,7 +217,7 @@ SECTIONS
 		INIT_CALLS
 		CON_INITCALL
 		INIT_RAM_FS
-		*(.init.altinstructions .init.bss .init.bss.*)	/* from the EFI stub */
+		*(.init.altinstructions .init.data.* .init.bss .init.bss.*)	/* from the EFI stub */
 	}
 	.exit.data : {
 		EXIT_DATA
-- 
2.29.2