On Mon, Jul 18, 2016 at 5:36 PM, Bin.Cheng <amker.ch...@gmail.com> wrote: > On Mon, Jul 18, 2016 at 4:28 PM, NightStrike <nightstr...@gmail.com> wrote: >> 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? > Hmm, good point, I will iterate the patch to see if I can only remove > -funsafe-loop-optimizations, while keep -Wunsafe-loop-optimizations.
Of course the naming of -Wunsafe-loop-optimizations is misleading then. Maybe provide an alias -Wmissed-loop-optimizations and re-word it to say "disable _some_ loop optimizations" as I hope more loop optimizations get aware of "assumptions" and deal with them. Richard. > Thanks, > bin
