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@gmail.com>
Cc: Vasiliy Kulikov <segoon@openwall.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions