[PATCH v3] RISC-V: Enable dead code elimination

[Jisheng Zhang]

On Sun, May 21, 2023 at 04:48:47PM +0800, Jisheng Zhang wrote:
> On Wed, May 17, 2023 at 04:29:36PM +0800, Zhangjin Wu wrote:
> > 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:
> 
> OOPS, I didn't noticed that you have sent out the patch and you are now
> in v3. I may read your patch sevral months ago, but I forget it.
> But your version missed an important preparation patch which is
> to fix the typo in commom vmlinux.lds.h in asm-generic, thus LD_ORPHAN_WARN
> will warn loudly. See[1] for more details.
> 
> Another missing point is we need to keep the .init.pi* and .init.bss*
> sections. See[1] for more details.
> 
> Since you are the first person to send out the patch of
> HAVE_LD_DEAD_CODE_DATA_ELIMINATION for riscv, do you mind if I update
> my series and take your patch in my series v2, make you as the commmit
> author, use your commit msg, and add my co-developped tag etc?

forget to add Link reference:

[1] Link: https://lore.kernel.org/linux-riscv/20230511141211.2418-1-jszhang@kernel.org/

> 
> Thanks
> 
> > 
> >           | 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 348c0fa1fc8c..d2eb1a862ea8 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -111,6 +111,7 @@ 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 e5f9f4677bbf..0f5dfbc113d4 100644
> > --- a/arch/riscv/kernel/vmlinux.lds.S
> > +++ b/arch/riscv/kernel/vmlinux.lds.S
> > @@ -112,7 +112,7 @@ SECTIONS
> >  	. = ALIGN(8);
> >  	.alternative : {
> >  		__alt_start = .;
> > -		*(.alternative)
> > +		KEEP(*(.alternative))
> >  		__alt_end = .;
> >  	}
> >  	__init_end = .;
> > -- 
> > 2.25.1
> > 
> > 
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