From: Alexey Dobriyan <adobriyan@xxxxxxxxx> Subject: lib/list: tweak LIST_POISON2 for better code generation on x86_64 list_del() poisoning can generate 2 64-bit immediate loads but it also can generate one 64-bit immediate load and an addition: 48 b8 00 01 00 00 00 00 ad de movabs rax,0xdead000000000100 48 89 47 58 mov QWORD PTR [rdi+0x58],rax 48 05 00 01 00 00 <=====> add rax,0x100 48 89 47 60 mov QWORD PTR [rdi+0x60],rax However on x86_64 not all constants are equal: those within [-128, 127] range can be added with shorter "add r64, imm32" instruction: 48 b8 00 01 00 00 00 00 ad de movabs rax,0xdead000000000100 48 89 47 58 mov QWORD PTR [rdi+0x58],rax 48 83 c0 22 <======> add rax,0x22 48 89 47 60 mov QWORD PTR [rdi+0x60],rax Patch saves 2 bytes per some LIST_POISON2 usage. (Slightly disappointing) space savings on F29 x86_64 config: add/remove: 0/0 grow/shrink: 0/2164 up/down: 0/-5184 (-5184) Function old new delta zstd_get_workspace 548 546 -2 ... mlx4_delete_all_resources_for_slave 4826 4804 -22 Total: Before=83304131, After=83298947, chg -0.01% New constants are: 0xdead000000000100 0xdead000000000122 Note: LIST_POISON1 can't be changed to ...11 because something in page allocator requires low bit unset. Link: http://lkml.kernel.org/r/20190513191502.GA8492@avx2 Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx> Cc: Vasiliy Kulikov <segoon@xxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/poison.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/include/linux/poison.h~tweak-list_poison2-for-better-code-generation-on-x86_64 +++ a/include/linux/poison.h @@ -21,7 +21,7 @@ * non-initialized list entries. */ #define LIST_POISON1 ((void *) 0x100 + POISON_POINTER_DELTA) -#define LIST_POISON2 ((void *) 0x200 + POISON_POINTER_DELTA) +#define LIST_POISON2 ((void *) 0x122 + POISON_POINTER_DELTA) /********** include/linux/timer.h **********/ /* _
