Messages in this thread | | | Date | Sat, 20 Mar 2010 05:51:58 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH RFC 00/11] lock monitor: Separate features related to lock |
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On Fri, Mar 19, 2010 at 12:00:41PM -0400, Jason Baron wrote: > On Fri, Mar 19, 2010 at 08:56:00AM -0400, Mathieu Desnoyers wrote: > > * Frederic Weisbecker (fweisbec@gmail.com) wrote: > > > On Thu, Mar 18, 2010 at 10:40:42PM -0400, Mathieu Desnoyers wrote: > > > > Well, the use-case that drove the asm goto implementation _is_ the tracepoints. > > > > ;) > > > > > > > > > > > > > > But, looking at __DO_TRACE: > > > > > > > > > > if (it_func) { \ > > > > > do { \ > > > > > ((void(*)(proto))(*it_func))(args); \ > > > > > } while (*(++it_func)); \ > > > > > } > > > > > > > > > > I would expect the compiler not to load the parameters in the stack > > > > > before first checking the branch. > > > > > > > > Note that you have to put that in its full context. It's a macro expanded within > > > > a static inline function. The initial parameters are passed to the static > > > > inline, not directly as "args" here. So parameters with side-effects have to be > > > > evaluated before their result can be passed to the static inline function, so in > > > > that sense their evaluation cannot be moved into the conditional branch. > > > > > > > > > Evaluation yeah, I agree. A function passed as an argument is > > > going to be evaluated indeed, or whatever thing that has a side effect. > > > But there is nothing here that need to setup the parameters to the stack > > > right before the true tracepoint call, not until we passed the branch check > > > once. > > > > > > > > > > > So, the fact that parameters are not loaded before we know we'll call > > > > > the tracepoint is something we already have or is it something that the jump > > > > > label brings in the package somehow? > > > > > > > > It's standard compiler optimization behavior. > > > > > > > > > Sure. My doubt is: currently with the upstream version, does the > > > compiler tend to load the parameters to the stack before the branch is > > > checked? Or is this a magic that jmp labels bring for whatever reason? > > > > Even without the static jump patching, the compiler takes care of putting the > > stack setup after the branch is checked. That worked with a standard test on a > > variable, with immediate values and should still work with asm gotos. > > right. stack setup happens after the branch is checked for asm gotos as > well. However, as mentioned functions as parameters, which have side-effects > need to be evaluated in the off case, there's nothing to be done about > that as its a correctness issue. > > Hoever, constructs like a->b, do evaluated even in the disabled case. > This could be solved via macros, see my proposed patch set: > http://marc.info/?l=linux-kernel&m=124276710606872&w=2 > > However, the conclusion of the thread was that this should be done in > the compiler, and as such I filed a bug with gcc about this issue. > > I'll re-post an updated jump label series shortly.
Ok, thanks guys for these informations.
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