Re: [PATCH] arm64: enable dead code elimination

From: Kefeng Wang
Date: Mon Jul 17 2023 - 07:20:48 EST




On 2023/7/17 17:24, Will Deacon wrote:
On Mon, Jul 17, 2023 at 04:07:39PM +0800, Kefeng Wang wrote:
Select CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION for arm64, allowing the
user to enable dead code elimination. In order for this to work, ensure
that we keep the necessary tables by annotating them with KEEP, also it
requires further changes to linker script to KEEP some tables and wildcard
compiler generated sections into the right place.

The following comparison is based 6.5-rc2 with defconfig,

$ ./scripts/bloat-o-meter vmlinux-base vmlinux-new
add/remove: 3/1106 grow/shrink: 4102/6964 up/down: 35704/-99980 (-64276)
Function old new delta
...
Total: Before=17888959, After=17824683, chg -0.36%

add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-44 (-44)
Data old new delta
...
Total: Before=4820808, After=4820764, chg -0.00%

add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-1096 (-1096)
RO Data old new delta
...
Total: Before=5179123, After=5178027, chg -0.02%

$ size vmlinux-base vmlinux
text data bss dec hex filename
25433734 15385766 630656 41450156 2787aac vmlinux-base
24756738 15360870 629888 40747496 26dc1e8 vmlinux-new

Memory available after booting, saving 704k on qemu,
base: 8084532K/8388608K
new: 8085236K/8388608K

Is that a 0.009% improvement? Is it really worth the hassle?

x86 doesn't select this and risc-v had to turn it off for LLD, so it feels
like we're just creating a rod for our own back by selecting it.


The LD_DEAD_CODE_DATA_ELIMINATION is particularly used for small configs
on small systems, risc-v is aimed to resource limited board platforms,
maybe x86 has no strong requirement, and we will try to use it on some
embedded board, if no one try it, this feature will never become stable :)


Will