On Thu, Sep 24, 2020 at 11:45 AM John Fastabend
<john.fastabend@xxxxxxxxx> wrote:
>
> In BPF_AND and BPF_OR alu cases we have this pattern when the src and dst
> tnum is a constant.
>
> 1 dst_reg->var_off = tnum_[op](dst_reg->var_off, src_reg.var_off)
> 2 scalar32_min_max_[op]
> 3 if (known) return
> 4 scalar_min_max_[op]
> 5 if (known)
> 6 __mark_reg_known(dst_reg,
> dst_reg->var_off.value [op] src_reg.var_off.value)
>
> The result is in 1 we calculate the var_off value and store it in the
> dst_reg. Then in 6 we duplicate this logic doing the op again on the
> value.
>
> The duplication comes from the the tnum_[op] handlers because they have
> already done the value calcuation. For example this is tnum_and().
>
> struct tnum tnum_and(struct tnum a, struct tnum b)
> {
> u64 alpha, beta, v;
>
> alpha = a.value | a.mask;
> beta = b.value | b.mask;
> v = a.value & b.value;
> return TNUM(v, alpha & beta & ~v);
> }
>
> So lets remove the redundant op calculation. Its confusing for readers
> and unnecessary. Its also not harmful because those ops have the
> property, r1 & r1 = r1 and r1 | r1 = r1.
>
> Signed-off-by: John Fastabend <john.fastabend@xxxxxxxxx>
Applied. Thanks for the follow up.
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