[PATCH v2] RISC-V: Enable dead code elimination

[Zhangjin Wu]

Select CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION for RISC-V, allowing
the user to enable dead code elimination. In order for this to work,
ensure that we keep the alternative table by annotating them with KEEP.

This boots well on qemu with both rv32_defconfig & rv64 defconfig, but
it only shrinks their builds by ~1%, a smaller config is thereforce
customized to test this feature:

          | rv32                   | rv64
  --------|------------------------|---------------------
   No DCE | 4460684                | 4893488
      DCE | 3986716                | 4376400
   Shrink |  473968 (~10.6%)       |  517088 (~10.5%)

The config used above only reserves necessary options to boot on qemu
with serial console, more like the size-critical embedded scenes:

  - rv64 config: https://pastebin.com/crz82T0s
  - rv32 config: rv64 config + 32-bit.config

Signed-off-by: Zhangjin Wu <falcon at tinylab.org>
---
 arch/riscv/Kconfig              | 1 +
 arch/riscv/kernel/vmlinux.lds.S | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index e2b656043abf..8a73d7180cb8 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -103,6 +103,7 @@ config RISCV
 	select HAVE_KPROBES_ON_FTRACE if !XIP_KERNEL
 	select HAVE_KRETPROBES if !XIP_KERNEL
 	select HAVE_RETHOOK if !XIP_KERNEL
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	select HAVE_MOVE_PMD
 	select HAVE_MOVE_PUD
 	select HAVE_PCI
diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
index 4e6c88aa4d87..51218cfe1ee4 100644
--- a/arch/riscv/kernel/vmlinux.lds.S
+++ b/arch/riscv/kernel/vmlinux.lds.S
@@ -101,7 +101,7 @@ SECTIONS
 	. = ALIGN(8);
 	.alternative : {
 		__alt_start = .;
-		*(.alternative)
+		KEEP(*(.alternative))
 		__alt_end = .;
 	}
 	__init_end = .;
-- 
2.25.1