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Message-ID: <d348799b-4404-b408-9c36-59885c811eed@solarflare.com> Date: Thu, 18 May 2017 15:10:03 +0100 From: Edward Cree <ecree@...arflare.com> To: David Miller <davem@...emloft.net> CC: <ast@...com>, <daniel@...earbox.net>, <alexei.starovoitov@...il.com>, <netdev@...r.kernel.org> Subject: Re: [PATCH v2 1/3] bpf: Use 1<<16 as ceiling for immediate alignment in verifier. On 18/05/17 01:16, David Miller wrote: > So, in C, addition (a += b) is something like: > > struct bpf_reg_bits { > u64 zero_bits; > u64 one_bits; > }; > > static void add_update_bits(struct bpf_reg_bits *a, struct bpf_reg_bits *b) > { > u64 m_zeros, m_ones, m_all; > > m_zeros = a->zero_bits ^ b->zero_bits; > m_ones = a->one_bits ^ b->one_bits; > m_all = m_zeros | m_ones; No, this should be u64 m_a, m_b, m_all; m_a = a->zero_bits ^ a->one_bits; /* unknown bits in a */ m_b = b->zero_bits ^ b->one_bits; /* unknown bits in b */ m_all = m_a | m_b; /* unknown bits in result */ > a->zero_bits = (a->zero_bits + b->zero_bits) | m_all; > a->one_bits = (a->one_bits + b->zero_bits) & ~m_all; > } > > Then, is subtraction merely: > > static void sub_update_bits(struct bpf_reg_bits *a, struct bpf_reg_bits *b) > { > u64 m_zeros, m_ones, m_all; > > m_zeros = a->zero_bits ^ b->zero_bits; > m_ones = a->one_bits ^ b->one_bits; > m_all = m_zeros | m_ones; > > a->zero_bits = (a->zero_bits - b->zero_bits) | m_all; > a->one_bits = (a->one_bits - b->zero_bits) & ~m_all; > } > > Or is something different needed? I suspect it's something different, just because I worry about what carries will do. But I think Alexei's idea (mask and value) is better anyway; at the least it's easier to think about. -Ed
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