On Mon, Jul 18, 2016 at 3:55 AM, Bin.Cheng <amker.ch...@gmail.com> wrote: > On Sat, Jul 16, 2016 at 6:28 PM, NightStrike <nightstr...@gmail.com> wrote: >> On Fri, Jul 15, 2016 at 1:07 PM, Bin Cheng <bin.ch...@arm.com> wrote: >>> Hi, >>> This patch removes support for -funsafe-loop-optimizations, as well as >>> -Wunsafe-loop-optimizations. By its name, this option does unsafe >>> optimizations by assuming all loops must terminate and doesn't wrap. >>> Unfortunately, it's not as useful as expected because: >>> 1) Simply assuming loop must terminate isn't enough. What we really want >>> is to analyze scalar evolution and loop niter bound under such assumptions. >>> This option does nothing in this aspect. >>> 2) IIRC, this option generates bogus code for some common programs, that's >>> why it's disabled by default even at Ofast level. >>> >>> After I sent patches handling possible infinite loops in both (scev/niter) >>> analyzer and vectorizer, it's a natural step to remove such options in GCC. >>> This patch does so by deleting code for -funsafe-loop-optimizations, as >>> well as -Wunsafe-loop-optimizations. It also deletes the two now useless >>> tests, while the option interface is preserved for backward compatibility >>> purpose. >> >> There are a number of bugs opened against those options, including one >> that I just opened rather recently: >> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71769 >> >> but some go back far, in this case 9 years: >> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=34114 >> >> If you are going to remove the options, you should address open bugs >> related to those options. > Hi, > Thanks for pointing me to these PRs, I will have a look at them.
I only highlighted two PRs, I was suggesting that you look for all of them. > IMHO, the old one reports weakness in loop niter analyzer, the issue > exists whether I remove unsafe-loop-optimization or not. The new one > is a little bit trickier, I will put some comments on PR, and again, > the issue (if it is) is in niter analyzer which has nothing to do with > the option really. Well, one thing to note is that the warning is an easy way to get a notice of a possible missed optimization (and I have many more occurrences of it in a particular code base that I use). If the warning is highlighted potential issues that aren't due to the -f option but are issues nonetheless, and we remove the warning, then how should I go about finding these missed opportunities in the future? Is there a different mechanism that does the same thing?
