[PATCH v2 11/11] static_call: Remove DEFINE_STATIC_CALL_RET0()

[Peter Zijlstra]

On Tue, Mar 21, 2023 at 09:00:17PM -0700, Josh Poimboeuf wrote:
> NULL and RET0 static calls are both slightly different ways of nopping a
> static call.  A not-insignificant amount of code and complexity is spent
> maintaining them separately.  It's also somewhat tricky for the user who
> has to try to remember to use the correct one for the given function
> type.

Well, I have very little sympathy for that argument. The return type
should be a big frigging clue.

> Simplify things all around by just combining them, such that NULL static
> calls always return 0.
> 
> While it doesn't necessarily make sense for void-return functions to
> return 0, it's pretty much harmless.  The return value register is
> already callee-clobbered, and an extra "xor %eax, %eax" shouldn't affect
> performance (knock on wood).

Urgh.. OTOH I do like the lines removes.

> This "do nothing return 0" default should work for the vast majority of
> NULL cases.  Otherwise it can be easily overridden with a user-specified
> function which panics or returns 0xdeadbeef or does whatever one wants.
> 
> This simplifies the static call code and also tends to help simplify
> users' code as well.

Can we at least keep the DEFINE_STATIC_CALL_RET0() and
__static_call_return0 as aliases? It reads really daft to use _NULL or
__static_call_nop for non-void functions.