netdev - Re: [PATCH bpf-next 7/7] nfp: bpf: migrate to advanced reciprocal divide in reciprocal

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Date:   Tue, 26 Jun 2018 13:59:24 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Jiong Wang <jiong.wang@...ronome.com>
Cc:     alexei.starovoitov@...il.com, daniel@...earbox.net,
        oss-drivers@...ronome.com, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next 7/7] nfp: bpf: migrate to advanced reciprocal
 divide in reciprocal_div.h

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?  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]

Should be equivalent to:

c = a >= b

(Thanks to Edwin for double-checking the carry semantics.)