[PATCH v2 4/4] riscv: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Jisheng Zhang]

From: Zhangjin Wu <falcon at tinylab.org>

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

Here is Jisheng's original commit-msg:
When trying to run linux with various opensource riscv core on
resource limited FPGA platforms, for example, those FPGAs with less
than 16MB SDRAM, I want to save mem as much as possible. One of the
major technologies is kernel size optimizations, I found that riscv
does not currently support HAVE_LD_DEAD_CODE_DATA_ELIMINATION, which
passes -fdata-sections, -ffunction-sections to CFLAGS and passes the
--gc-sections flag to the linker.

This not only benefits my case on FPGA but also benefits defconfigs.
Here are some notable improvements from enabling this with defconfigs:

nommu_k210_defconfig:
   text    data     bss     dec     hex
1112009  410288   59837 1582134  182436     before
 962838  376656   51285 1390779  1538bb     after

rv32_defconfig:
   text    data     bss     dec     hex
8804455 2816544  290577 11911576 b5c198     before
8692295 2779872  288977 11761144 b375f8     after

defconfig:
   text    data     bss     dec     hex
9438267 3391332  485333 13314932 cb2b74     before
9285914 3350052  483349 13119315 c82f53     after

Signed-off-by: Zhangjin Wu <falcon at tinylab.org>
Co-developed-by: Jisheng Zhang <jszhang at kernel.org>
Signed-off-by: Jisheng Zhang <jszhang at kernel.org>
Reviewed-by: Guo Ren <guoren at kernel.org>
Tested-by: Bin Meng <bmeng at tinylab.org>
---
 arch/riscv/Kconfig              | 1 +
 arch/riscv/kernel/vmlinux.lds.S | 6 +++---
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 8f55aa4aae34..62e84fee2cfd 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -115,6 +115,7 @@ config RISCV
 	select HAVE_KPROBES if !XIP_KERNEL
 	select HAVE_KPROBES_ON_FTRACE if !XIP_KERNEL
 	select HAVE_KRETPROBES 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 e5f9f4677bbf..492dd4b8f3d6 100644
--- a/arch/riscv/kernel/vmlinux.lds.S
+++ b/arch/riscv/kernel/vmlinux.lds.S
@@ -85,11 +85,11 @@ SECTIONS
 	INIT_DATA_SECTION(16)
 
 	.init.pi : {
-		*(.init.pi*)
+		KEEP(*(.init.pi*))
 	}
 
 	.init.bss : {
-		*(.init.bss)	/* from the EFI stub */
+		KEEP(*(.init.bss*))	/* from the EFI stub */
 	}
 	.exit.data :
 	{
@@ -112,7 +112,7 @@ SECTIONS
 	. = ALIGN(8);
 	.alternative : {
 		__alt_start = .;
-		*(.alternative)
+		KEEP(*(.alternative))
 		__alt_end = .;
 	}
 	__init_end = .;
-- 
2.40.1