Messages in this thread | | | Date | Thu, 22 Apr 2021 13:31:51 -0500 | From | Segher Boessenkool <> | Subject | Re: static_branch/jump_label vs branch merging |
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On Thu, Apr 22, 2021 at 07:49:23PM +0200, Peter Zijlstra wrote: > On Thu, Apr 22, 2021 at 12:08:20PM -0500, Segher Boessenkool wrote: > > On Thu, Apr 22, 2021 at 01:48:39PM +0200, Peter Zijlstra wrote: > > > Can we please have a __pure__ attribute that is prescriptive and not a > > > hint the compiler is free to ignore for $raisins ? > > > > What does that mean? What actual semantics do you want it to have? > > I want a function marked as pure to be treated as such, unconditionally. > > > The "pure" attribute means the compiler can assume this function does > > not have side effects. But in general (and in practice in many cases) > > there is no way the compiler can actually check that, if that is what > > you were asking for. > > Right, so currently the pure attribute gets ignored by the compiler > because of various reasons, one of them being an asm volatile ("") being > present somewhere inside it (AFAIU).
In general, the compiler only sees the *declaration* of the function, so it cannot do such a thing.
> Does this mean we can have invalid code generation when we faultily > mark things pure? Yes, but then it's our own damn fault for sticking on > pure in the first place.
Nope, you have undefined behaviour in that case, and you get to keep all N pieces, the compiler cannot do anything wrong in such a case :-)
> In short; I want pure (or really_pure if you want a second attribute) to
You cannot make the meaning of "pure" different from what it has been historically, because existing programs will no longer build (or worse, start behaving differently).
> be a do-what-I-tell-you-already and not a > only-if-you-think-you-can-prove-I-didn't-make-a-mistake kinda knob. A > little bit like inline vs always_inline.
It sounds like you want it to behave like attribute((pure)) already is documented as doing. Please open a PR? https://gcc.gnu.org/bugs.html (We need buildable stand-alone example code, with what flags to use, and something like what should happen and what did happen).
> > And any such checking will depend on optimisation level and related > > flags, making that a no-go anyway. > > Realistically I'm only bothered about -O2 and up since that's what we > build the kernel with. Obviously one doesn't care about optimizations > being lost when build with -O0.
GCC is used for other things as well, not just for building Linux ;-)
Segher
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