]> git.baikalelectronics.ru Git - kernel.git/commit
nfp: bpf: optimize add/sub of a negative constant
authorJakub Kicinski <jakub.kicinski@netronome.com>
Wed, 25 Apr 2018 04:22:37 +0000 (21:22 -0700)
committerDaniel Borkmann <daniel@iogearbox.net>
Wed, 25 Apr 2018 07:56:10 +0000 (09:56 +0200)
commit2f2b2aa36f4b074bce5a23f6eeca4e1ca892b498
tree0ad0b3cf7ace13bc02338e6408b4bd45013f5b86
parent340b7b657423a7a4394e87b5f68c9d2d7e402666
nfp: bpf: optimize add/sub of a negative constant

NFP instruction set can fit small immediates into the instruction.
Negative integers, however, will never fit because they will have
highest bit set.  If we swap the ALU op between ADD and SUB and
negate the constant we have a better chance of fitting small negative
integers into the instruction itself and saving one or two cycles.

immed[gprB_21, 0xfffffffc]
alu[gprA_4, gprA_4, +, gprB_21], gpr_wrboth
immed[gprB_21, 0xffffffff]
alu[gprA_5, gprA_5, +carry, gprB_21], gpr_wrboth

now becomes:

alu[gprA_4, gprA_4, -, 4], gpr_wrboth
alu[gprA_5, gprA_5, -carry, 0], gpr_wrboth

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
drivers/net/ethernet/netronome/nfp/bpf/jit.c