On Wed, 2010-04-14 at 23:55 -0700, David Miller wrote: > From: Ingo Molnar <mingo@xxxxxxx> > Date: Thu, 15 Apr 2010 08:49:40 +0200 > > > Btw., WARN_ON trapping on PowerPC is clearly a PowerPC bug - there's a good > > reason we have WARN_ON versus BUG_ON - it should be fixed. > > I disagree, an implementation should be allowed to use the most > efficient implementation possible for both interfaces. Right, and I don't think the reason why we have WARN_ON vs. BUG_ON ever had anything to do with whether it's implemented with a trap or not :-) It's purely related to whether it's supposed to be fatal or not. Now, there is indeed the potential problem you mention of WARN_ON being called in places where such a trap is unsafe, but so far, this is the first time I can remember we hit that problem so we've been getting away with it for quite a while :-) Now, whether the trap is or is not more efficient than an explicit test is something that is still being debated on powerpc. It has the advantage of not un-leafing functions (and thus not creating stack frames, adding register reloads, etc... when not needed), basically putting the burden of saving/restoring registers to the (hopefully) rare path of actually taking the WARN/BUG. We could probably manufacture something similar with careful use of inline asm and an out of line asm trampoline. The benefit of the trap instruction vs. conditional branches per-se is probably nil. It's really more about the codegen impact, register clobber due to the added function call, etc.. at least for us. Cheers, Ben. > I would be using traps for both on sparc64 if that were really > feasible on sparc64 (and actually with gcc-4.5's "asm goto" it might > actually be now) > > The WARN and BUG macros, when implemented without traps, have serious > implications for overall code size and register pressure. > _______________________________________________ > Linuxppc-dev mailing list > Linuxppc-dev@xxxxxxxxxxxxxxxx > https://lists.ozlabs.org/listinfo/linuxppc-dev -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
