Messages in this thread | | | Date | Tue, 9 Nov 2021 19:38:20 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH 1/7] static_call: get rid of static_call_cond() |
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On Tue, Nov 09, 2021 at 05:45:43PM +0100, Ard Biesheuvel wrote: > The main reason for the existence of static_call_cond() seems to be that > in theory, when using the generic implementation of static calls, it may > be possible for a compiler to elide the indirect call entirely if the > target is NULL, while still guaranteeing that all side effects of > argument evaluation occur as expected. > > This is rather optimistic: as documented by an existing code comment, > both GCC and Clang (version 10) get this wrong, and even if they ever > get it right, this is far too subtle to rely on for a code path that is > expected to be used only by the 'remaining' architectures once all the > best supported ones implement either the out-of-line or inline optimized > variety of static calls. > > Given that having static_call_cond() clutters up the API, and puts the > burden on the caller to go and check what kind of static call they are > dealing with, let's just get rid of the distinction.
No, static_call_cond() signifies the function can be NULL. Both gcc and clang generate correct (but wildly ineffecient) code for this. Without static_call_cond() the generic implementation will do a NULL deref.
That is, static_call_cond() does properly encapuslate:
func = READ_ONCE(key.func); if (func) func(ARGS);
You can't take that out.
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