On 26/06/2018 21:59, Jakub Kicinski wrote:
On Sun, 24 Jun 2018 20:54:21 -0700, Jakub Kicinski wrote:
+ * NOTE: because we are using "reciprocal_value_adv" which doesn't
+ * support dividend with MSB set, so we need to JIT separate NFP
+ * sequence to handle such case. It could be a simple sequence if there
+ * is conditional move, however there isn't for NFP. So, we don't bother
+ * generating compare-if-set-branch sequence by rejecting the program
+ * straight away when the u32 dividend has MSB set. Divide by such a
+ * large constant would be rare in practice. Also, the programmer could
+ * simply rewrite it as "result = divisor >= the_const".
Thinking about this again, can we just use carry bit?
Good catch, yes we can.
The code may end
up shorter than the explanation why we don't support that case :P
immed[c, 0]
alu[--, a, -, b]
alu[c, c, +carry, 0]
eBPF input will be "a = a / b", given "immed" doesn't affect carry bit,
I'd reorder the sequence so we only need one tmp register for holding
"b" who is constant.
alu[--, a, -, b]
immed[b, 0]
alu[a, b, +carry, 0]
Thanks.
Regards,
Jiong
Should be equivalent to:
c = a >= b
(Thanks to Edwin for double-checking the carry semantics.)
