[jba...@redhat.com: [PATCH 0/3] tracepoints: delay argument evaluation]
hi,
While working on some Linux kernel code, I've found that functions that
are declared as 'static inline' are having their arguments evaluated
well before they are used. For example I have a function:
static inline void trace(arg1, arg2)
{
if (unlikely(enabled)) {
}
}
If i call 'trace(ptr->arg1, ptr->arg2)', then the pointers are
dereferenced before the 'if' is executed. Is there any way to delay the
argument evaluation until they are used? Am I missing a compiler option?
I am used gcc 4.3.0.
thanks,
-Jason
Re: [RFC] gcc feature request: Moving blocks into sections
On 08/05/2013 03:40 PM, Marek Polacek wrote: On Mon, Aug 05, 2013 at 11:34:55AM -0700, Linus Torvalds wrote: On Mon, Aug 5, 2013 at 11:24 AM, Linus Torvalds wrote: Ugh. I can see the attraction of your section thing for that case, I just get the feeling that we should be able to do better somehow. Hmm.. Quite frankly, Steven, for your use case I think you actually want the C goto *labels* associated with a section. Which sounds like it might be a cleaner syntax than making it about the basic block anyway. FWIW, we also support hot/cold attributes for labels, thus e.g. if (bar ()) goto A; /* ... */ A: __attribute__((cold)) /* ... */ I don't know whether that might be useful for what you want or not though... Marek It certainly would be. That was how I wanted to the 'static_key' stuff to work, but unfortunately the last time I tried it, it didn't move the text out-of-line any further than it was already doing. Would that be expected? The change for us, if it worked would be quite simple. Something like: --- a/arch/x86/include/asm/jump_label.h +++ b/arch/x86/include/asm/jump_label.h @@ -21,7 +21,7 @@ static __always_inline bool arch_static_branch(struct static_key *key) ".popsection \n\t" : : "i" (key) : : l_yes); return false; -l_yes: +l_yes: __attribute__((cold)) return true; }
Re: [RFC] gcc feature request: Moving blocks into sections
On 08/05/2013 02:39 PM, Steven Rostedt wrote: On Mon, 2013-08-05 at 11:20 -0700, Linus Torvalds wrote: Of course, it would be good to optimize static_key_false() itself - right now those static key jumps are always five bytes, and while they get nopped out, it would still be nice if there was some way to have just a two-byte nop (turning into a short branch) *if* we can reach another jump that way..For small functions that would be lovely. Oh well. I had patches that did exactly this: https://lkml.org/lkml/2012/3/8/461 But it got dropped for some reason. I don't remember why. Maybe because of the complexity? -- Steve Hi Steve, I recall testing your patches and the text size increased unexpectedly. I believe I correctly accounted for changes to the text size *outside* of branch points. If you do re-visit the series that is one thing I'd like to double check/understand. Thanks, -Jason
Re: [RFC] gcc feature request: Moving blocks into sections
On 08/05/2013 04:35 PM, Richard Henderson wrote: On 08/05/2013 09:57 AM, Jason Baron wrote: On 08/05/2013 03:40 PM, Marek Polacek wrote: On Mon, Aug 05, 2013 at 11:34:55AM -0700, Linus Torvalds wrote: On Mon, Aug 5, 2013 at 11:24 AM, Linus Torvalds wrote: Ugh. I can see the attraction of your section thing for that case, I just get the feeling that we should be able to do better somehow. Hmm.. Quite frankly, Steven, for your use case I think you actually want the C goto *labels* associated with a section. Which sounds like it might be a cleaner syntax than making it about the basic block anyway. FWIW, we also support hot/cold attributes for labels, thus e.g. if (bar ()) goto A; /* ... */ A: __attribute__((cold)) /* ... */ I don't know whether that might be useful for what you want or not though... Marek It certainly would be. That was how I wanted to the 'static_key' stuff to work, but unfortunately the last time I tried it, it didn't move the text out-of-line any further than it was already doing. Would that be expected? The change for us, if it worked would be quite simple. Something like: It is expected. One must use -freorder-blocks-and-partition, and use real profile feedback to get blocks moved completely out-of-line. Whether that's a sensible default or not is debatable. Hi Steve, I think if the 'cold' attribute on the default disabled static_key branch moved the text completely out-of-line, it would satisfy your requirement here? If you like this approach, perhaps we can make something like this work within gcc. As its already supported, but doesn't quite go far enough for our purposes. Also, if we go down this path, it means the 2-byte jump sequence is probably not going to be too useful. Thanks, -Jason
