[PATCH v2] x86/patches-5.4: enable DCDE for x86(-64)

[Rui Salvaterra]

Port and adapt Nick Piggin's original patch [1]. This enables dead code and
data elimination at linking time (gc-sections) on x86(-64).

openwrt-x86-64-generic-kernel.bin size, with my config:

Before:	3138048 bytes
After:	2937344 bytes

In other words, we save about 200 kB.

[1] https://lore.kernel.org/lkml/20170709031333.29443-1-npiggin@gmail.com/

Signed-off-by: Rui Salvaterra <rsalvaterra at gmail.com>
---
v2: move the patch to x86/patches-5.4.

 ...nable-dead-code-and-data-elimination.patch | 127 ++++++++++++++++++
 1 file changed, 127 insertions(+)
 create mode 100644 target/linux/x86/patches-5.4/350-x86-enable-dead-code-and-data-elimination.patch

diff --git a/target/linux/x86/patches-5.4/350-x86-enable-dead-code-and-data-elimination.patch b/target/linux/x86/patches-5.4/350-x86-enable-dead-code-and-data-elimination.patch
new file mode 100644
index 0000000000..392ddd71ce
--- /dev/null
+++ b/target/linux/x86/patches-5.4/350-x86-enable-dead-code-and-data-elimination.patch
@@ -0,0 +1,127 @@
+From f08a0e4e59f92b4a88501653761cbca08935b9b6 Mon Sep 17 00:00:00 2001
+From: Rui Salvaterra <rsalvaterra at gmail.com>
+Date: Wed, 4 Nov 2020 19:45:04 +0000
+Subject: [PATCH] x86: enable dead code and data elimination
+
+Adapt Nick Piggin's original patch [1]. This saves nearly 300 kiB on the final
+vmlinuz (zstd-compressed).
+
+[1] https://lore.kernel.org/lkml/20170709031333.29443-1-npiggin@gmail.com/
+
+Signed-off-by: Rui Salvaterra <rsalvaterra at gmail.com>
+---
+ arch/x86/Kconfig              |  1 +
+ arch/x86/kernel/vmlinux.lds.S | 24 ++++++++++++------------
+ 2 files changed, 13 insertions(+), 12 deletions(-)
+
+--- a/arch/x86/Kconfig
++++ b/arch/x86/Kconfig
+@@ -184,6 +184,7 @@ config X86
+ 	select HAVE_FUNCTION_ERROR_INJECTION
+ 	select HAVE_KRETPROBES
+ 	select HAVE_KVM
++	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
+ 	select HAVE_LIVEPATCH			if X86_64
+ 	select HAVE_MEMBLOCK_NODE_MAP
+ 	select HAVE_MIXED_BREAKPOINTS_REGS
+--- a/arch/x86/kernel/vmlinux.lds.S
++++ b/arch/x86/kernel/vmlinux.lds.S
+@@ -242,14 +242,14 @@ SECTIONS
+ 	 * See static_cpu_has() for an example.
+ 	 */
+ 	.altinstr_aux : AT(ADDR(.altinstr_aux) - LOAD_OFFSET) {
+-		*(.altinstr_aux)
++		KEEP(*(.altinstr_aux))
+ 	}
+ 
+ 	INIT_DATA_SECTION(16)
+ 
+ 	.x86_cpu_dev.init : AT(ADDR(.x86_cpu_dev.init) - LOAD_OFFSET) {
+ 		__x86_cpu_dev_start = .;
+-		*(.x86_cpu_dev.init)
++		KEEP(*(.x86_cpu_dev.init))
+ 		__x86_cpu_dev_end = .;
+ 	}
+ 
+@@ -257,7 +257,7 @@ SECTIONS
+ 	.x86_intel_mid_dev.init : AT(ADDR(.x86_intel_mid_dev.init) - \
+ 								LOAD_OFFSET) {
+ 		__x86_intel_mid_dev_start = .;
+-		*(.x86_intel_mid_dev.init)
++		KEEP(*(.x86_intel_mid_dev.init))
+ 		__x86_intel_mid_dev_end = .;
+ 	}
+ #endif
+@@ -271,7 +271,7 @@ SECTIONS
+ 	. = ALIGN(8);
+ 	.parainstructions : AT(ADDR(.parainstructions) - LOAD_OFFSET) {
+ 		__parainstructions = .;
+-		*(.parainstructions)
++		KEEP(*(.parainstructions))
+ 		__parainstructions_end = .;
+ 	}
+ 
+@@ -283,7 +283,7 @@ SECTIONS
+ 	. = ALIGN(8);
+ 	.altinstructions : AT(ADDR(.altinstructions) - LOAD_OFFSET) {
+ 		__alt_instructions = .;
+-		*(.altinstructions)
++		KEEP(*(.altinstructions))
+ 		__alt_instructions_end = .;
+ 	}
+ 
+@@ -293,7 +293,7 @@ SECTIONS
+ 	 * get the address and the length of them to patch the kernel safely.
+ 	 */
+ 	.altinstr_replacement : AT(ADDR(.altinstr_replacement) - LOAD_OFFSET) {
+-		*(.altinstr_replacement)
++		KEEP(*(.altinstr_replacement))
+ 	}
+ 
+ 	/*
+@@ -304,14 +304,14 @@ SECTIONS
+ 	 */
+ 	.iommu_table : AT(ADDR(.iommu_table) - LOAD_OFFSET) {
+ 		__iommu_table = .;
+-		*(.iommu_table)
++		KEEP(*(.iommu_table))
+ 		__iommu_table_end = .;
+ 	}
+ 
+ 	. = ALIGN(8);
+ 	.apicdrivers : AT(ADDR(.apicdrivers) - LOAD_OFFSET) {
+ 		__apicdrivers = .;
+-		*(.apicdrivers);
++		KEEP(*(.apicdrivers))
+ 		__apicdrivers_end = .;
+ 	}
+ 
+@@ -346,7 +346,7 @@ SECTIONS
+ 	. = ALIGN(PAGE_SIZE);
+ 	.smp_locks : AT(ADDR(.smp_locks) - LOAD_OFFSET) {
+ 		__smp_locks = .;
+-		*(.smp_locks)
++		KEEP(*(.smp_locks))
+ 		. = ALIGN(PAGE_SIZE);
+ 		__smp_locks_end = .;
+ 	}
+@@ -380,8 +380,8 @@ SECTIONS
+ 	. = ALIGN(PAGE_SIZE);
+ 	.brk : AT(ADDR(.brk) - LOAD_OFFSET) {
+ 		__brk_base = .;
+-		. += 64 * 1024;		/* 64k alignment slop space */
+-		*(.brk_reservation)	/* areas brk users have reserved */
++		. += 64 * 1024;			/* 64k alignment slop space */
++		KEEP(*(.brk_reservation))	/* areas brk users have reserved */
+ 		__brk_limit = .;
+ 	}
+ 
+@@ -407,7 +407,7 @@ SECTIONS
+ 	. = ALIGN(HPAGE_SIZE);
+ 	.init.scratch : AT(ADDR(.init.scratch) - LOAD_OFFSET) {
+ 		__init_scratch_begin = .;
+-		*(.init.scratch)
++		KEEP(*(.init.scratch))
+ 		. = ALIGN(HPAGE_SIZE);
+ 		__init_scratch_end = .;
+ 	}
-- 
2.30.0