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Date:   Tue, 5 May 2020 11:36:25 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, rostedt@goodmis.org,
        mhiramat@kernel.org, bristot@redhat.com, jbaron@akamai.com,
        torvalds@linux-foundation.org, tglx@linutronix.de,
        mingo@kernel.org, namit@vmware.com, hpa@zytor.com, luto@kernel.org,
        ard.biesheuvel@linaro.org, jpoimboe@redhat.com,
        pbonzini@redhat.com, mathieu.desnoyers@efficios.com,
        hjl.tools@gmail.com, ndesaulniers@google.com
Subject: Re: [PATCH v4 14/18] static_call: Add static_cond_call()
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HJ, Nick,

Any chance any of you can see a way to make your respective compilers
not emit utter junk for this?

On Mon, May 04, 2020 at 10:14:45PM +0200, Peter Zijlstra wrote:

> https://godbolt.org/z/SDRG2q
> 
> ---
> #include <stddef.h>
> 
> 
> #define READ_ONCE(var)		(*((volatile typeof(var) *)&(var)))
> #define WRITE_ONCE(var, val)	(*((volatile typeof(var) *)&(var)) = (val))
> 
> struct static_call_key {
> 	void *func;
> };
> 
> #define DECLARE_STATIC_CALL(name, func)	\
> 	extern struct static_call_key name; \
> 	extern typeof(func) __SCT__##name;
> 
> #define DEFINE_STATIC_COND_CALL(name, _func) \
> 	DECLARE_STATIC_CALL(name, _func) \
> 	struct static_call_key name = { \
> 		.func = NULL, \
> 	}
> 
> static void __static_call_nop(void)
> {
> }
> 
> #define __static_cond_call(name) \
> ({ \
> 	void *func = READ_ONCE(name.func); \
> 	if (!func) \
> 		func = &__static_call_nop; \
> 	(typeof(__SCT__##name)*)func; \
> })
> 
> #define static_cond_call(name) (void)__static_cond_call(name)
> 
> static void inline static_call_update(struct static_call_key *call, void *func)
> {
> 	WRITE_ONCE(call->func, func);
> }
> 
> volatile int _x;
> 
> void bar(int x)
> {
> 	_x = x;
> }
> 
> DEFINE_STATIC_COND_CALL(foo, bar);
> 
> void ponies(int x)
> {
> 	static_cond_call(foo)(x);
> }