nfp: bpf: optimize add/sub of a negative constant

Browse source

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>

...

This commit is contained in:

Jakub Kicinski 2018-04-24 21:22:37 -07:00 • committed by Daniel Borkmann

parent 9c9e53233c

commit 6c59500c2d

1 changed files with 35 additions and 0 deletions

Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL

Download patch file Download diff file Expand all files Collapse all files

35

drivers/net/ethernet/netronome/nfp/bpf/jit.c

View file