Re: [PATCH nft 2/3] src: Add support for concatenated set ranges

[Phil Sutter <phil@nwl.cc>]

Hi,

On Thu, Nov 21, 2019 at 06:09:38PM +0100, Stefano Brivio wrote:
> On Wed, 20 Nov 2019 13:53:08 +0100
> Phil Sutter <phil@nwl.cc> wrote:
> 
> > On Wed, Nov 20, 2019 at 12:49:54PM +0100, Stefano Brivio wrote:
> >
> > > On Tue, 19 Nov 2019 23:12:38 +0100
> > > Phil Sutter <phil@nwl.cc> wrote:
> > [...]
> >
> > > So, the whole thing would look like (together with the other change
> > > you suggest):
> > > 
> > > --
> > > static int netlink_gen_concat_data_expr(const struct expr *i,
> > > 					unsigned char *data)
> > > {
> > > 	if (i->etype == EXPR_RANGE) {
> > > 		const struct expr *e;
> > > 
> > > 		if (i->flags & EXPR_F_INTERVAL_END)
> > > 			e = i->right;
> > > 		else
> > > 			e = i->left;
> > > 
> > > 		return netlink_export_pad(data, e->value, e);
> > > 	}
> > > 
> > > 	if (i->etype == EXPR_PREFIX) {
> > > 		if (i->flags & EXPR_F_INTERVAL_END) {
> > > 			int count;
> > > 			mpz_t v;
> > > 
> > > 			mpz_init_bitmask(v, i->len - i->prefix_len);
> > > 			mpz_add(v, i->prefix->value, v);
> > > 			count = netlink_export_pad(data, v, i);
> > > 			mpz_clear(v);
> > > 			return count;
> > > 		}
> > > 
> > > 		return netlink_export_pad(data, i->prefix->value, i);
> > > 	}
> > > 
> > > 	assert(i->etype == EXPR_VALUE);
> > > 
> > > 	return netlink_export_pad(data, i->value, i);
> > > }  
> > 
> > I would even:
> > 
> > | static int
> > | netlink_gen_concat_data_expr(const struct expr *i, unsigned char *data)
> > | {
> > | 	mpz_t *valp = NULL;
> > | 
> > | 	switch (i->etype) {
> > | 	case EXPR_RANGE:
> > | 		i = (i->flags & EXPR_F_INTERVAL_END) ? i->right : i->left;
> > | 		break;
> > | 	case EXPR_PREFIX:
> > | 		if (i->flags & EXPR_F_INTERVAL_END) {
> > | 			int count;
> > | 			mpz_t v;
> > | 
> > | 			mpz_init_bitmask(v, i->len - i->prefix_len);
> > | 			mpz_add(v, i->prefix->value, v);
> > | 			count = netlink_export_pad(data, v, i);
> > | 			mpz_clear(v);
> > | 			return count;
> > | 		}
> > | 		valp = &i->prefix->value;
> > | 		break;
> > | 	case EXPR_VALUE:
> > | 		break;
> > | 	default:
> > | 		BUG("invalid expression type '%s' in set", expr_ops(i)->name);
> > | 	}
> > | 
> > | 	return netlink_export_pad(data, valp ? *valp : i->value, i);
> > | }
> > 
> > But that's up to you. :)
> 
> I think it's nicer with a switch and that BUG() is more helpful than a
> random assert, but I personally find that ternary condition on valp a
> bit difficult to follow.

Yes, I overdid it a bit there trying to have a single call to
netlink_export_pad(). You found a good middle-ground!

[...]
> > > > So this is the fifth copy of the same piece of code. :(
> > > > 
> > > > Isn't there a better way to solve that?  
> > > 
> > > I couldn't think of any.  
> > 
> > I guess we would need an intermediate state which is 'multiton_rhs_expr
> > DOT multiton_rhs_expr'. Might turn into a mess as well. :)
> 
> I just tried to add that, and kept losing myself in the middle of it.
> It might be me, but it doesn't sound that promising when it comes to
> readability later.

I suspected that already. :)

> I guess we already have enough levels of indirection here -- I'd just
> go with the compound_expr_alloc_or_add() you suggested.
> 
> > > > Intuitively, I would just:
> > > > 
> > > > | int tmp = len;
> > > > | while (start != end && !(start & 1) && (end & 1) && tmp) {
> > > > |	start >>= 1;
> > > > |	end >>= 1;
> > > > |	tmp--;
> > > > | }
> > > > | return (tmp && start == end) ? len - tmp : -1;
> > > > 
> > > > Is that slow when dealing with gmp?  
> > > 
> > > I don't think so, it also avoid copies and allocations, while shifting
> > > and setting bits have comparable complexities. I'd go with the gmp
> > > version of this.  
> 
> Actually, I need to preserve the original elements, so the two copies
> are needed anyway -- other than that, I basically recycled your
> function.

*Obviously* my algorithm assumed pass-by-value. ;)

Thanks, Phil