On Fri, Jul 24, 2015 at 12:56:02PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 23, 2015 at 03:14:13PM -0400, Jason Baron wrote:
> > > +static enum jump_label_type jump_label_type(struct jump_entry *entry)
> > > +{
> > > + struct static_key *key = static_key_cast(iter->key);
> > > + bool true_branch = jump_label_get_branch_default(key);
> > > + bool state = static_key_enabled(key);
> > > + bool inv = static_key_inv(iter->key);
> > > +
> > > + return (true_branch ^ state) ^ inv;
> >
> > iiuc...this allows both the old style keys to co-exist with the
> > new ones. IE state wouldn't be set for the new ones. And inv
> > shouldn't be set for the old ones. Is that correct?
>
> @state is the dynamic variable here, the other two are compile time.
> @true_branch denotes the default (compile time) value, and @inv denotes
> the (compile time) branch preference.
Ha!, so that wasn't entirely correct, it turned out @inv means
arch_static_branch_jump().
That would let us remove the whole argument to the arch functions.
That said, I generated the logic table for @inv meaning the branch type
and then found a logic similar to what you outlined:
/*
* Combine the right initial value (type) with the right branch order
* to generate the desired result.
*
*
* type likely (1) unlikely (0)
* -----------+-----------------------+------------------
* | |
* true (1) | ... | ...
* | NOP | JMP L
* | <br-stmts> | 1: ...
* | L: ... |
* | |
* | | L: <br-stmts>
* | | jmp 1b
* | |
* -----------+-----------------------+------------------
* | |
* false (0) | ... | ...
* | JMP L | NOP
* | <br-stmts> | 1: ...
* | L: ... |
* | |
* | | L: <br-stmts>
* | | jmp 1b
* | |
* -----------+-----------------------+------------------
*
* The initial value is encoded in the LSB of static_key::entries,
* type: 0 = false, 1 = true.
*
* The branch type is encoded in the LSB of jump_entry::key,
* branch: 0 = unlikely, 1 = likely.
*
* This gives the following logic table:
*
* enabled type branch instuction
* -----------------------------+-----------
* 0 0 0 | NOP
* 0 0 1 | JMP
* 0 1 0 | NOP
* 0 1 1 | JMP
*
* 1 0 0 | JMP
* 1 0 1 | NOP
* 1 1 0 | JMP
* 1 1 1 | NOP
*
*/
This gives a map: ins = enabled ^ branch, which shows @type to be
redundant.
And we can trivially switch over the old static_key_{true,false}() to
emit the right branch type.
Whcih would mean we could remove the type encoding entirely, but I'll
leave that be for now, maybe it'll come in handy later or whatnot.
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