On Sat, 16 Feb 2008 18:58:49 +0100 > > If you think unlikely() means something else, we should fix what it > > maps to towards gcc ;) (to.. be empty ;) > > eventhough the gcc docs say it's just a hint to help the compiler > optimize the branch it takes by default, I too have noticed that it > more often does bad than good. Code gets completely reordered and > even sometimes partially duplicated (especially when the branch is a > return). > > Last but not least, gcc 4 tends to emit stupid checks, to the point > that I have replaced unlikely(x) with (x) in my code when gcc >= 4 is > detected. What I observe is that the following code : > > if (unlikely(p == NULL)) ...
this is pure bad since GCC already assumes that NULL checks are unlikely, and with the unlikely() code needing to normalize stuff... it will generate worse code for sure yes. > > often gets coded like this : > > reg1 = (p == NULL) > if (reg1 != 0) ... > > ... which clobbers reg1 for nothing and performs a double test. > > But yes, I assumed that the author considered its use to be > legitimate (I've not looked at the code). Maybe you're right and it > should be removed, but in this case we would need a large audit of > the abuses of unlikely()... no argument.. how about we start with all the cases where the author just got it entirely wrong ... like the ones from this patch ;) -- If you want to reach me at my work email, use [EMAIL PROTECTED] For development, discussion and tips for power savings, visit http://www.lesswatts.org -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
