I've ventured into the g++ front end a couple of times recently (I usually only touch libstdc++ code) trying to fix some bugs. I haven't managed to get my patches reviewed, but as stage 3 has ended I'll have to wait until 4.6 is branched and then try to get them into 4.7
More clang bangin'
One of the goals/features of Clang/LLVM is faster compile times than GCC, so I think something's very wrong with my build (which I configured as a "Release+Asserts build"):
$ wc -l src/Order.cc 4107 src/Order.cc $ time make CXX=g++ objs/Order.o ... real 0m8.993s user 0m8.483s sys 0m0.339s $ time make CXX=g++ CXXFLAGS="-O2 -DNDEBUG" objs/Order.o ... real 0m12.111s user 0m11.999s sys 0m0.309s $ time make CXX=clang++ objs/Order.o ... real 2m11.608s user 2m7.475s sys 0m0.260s $ time make CXX=clang++ CXXFLAGS="-O2 -DNDEBUG" objs/Order.o ... real 17m22.577s user 17m21.589s sys 0m0.377s
Yes, that really did take 17 minutes at 100% CPU (on a 2.93GHz Xeon X5570) compared to 12 seconds for GCC.
One of the things I really like is that most of GCC's command-line options are supported. so I used -ftime-report to see where all that time is spent, but all the detailed stats are for the code generation stages, which is the only bit that isn't slow!
| ---User Time--- | --System Time-- | --User+System-- | ---Wall Time--- | --- Name --- |
| 137.5871 ( 50.6%) | 0.2380 ( 67.8%) | 137.8250 ( 50.6%) | 138.0570 ( 50.6%) | Clang front-end timer |
| 131.2420 ( 48.2%) | 0.0610 ( 17.4%) | 131.3030 ( 48.2%) | 131.3707 ( 48.2%) | LLVM IR Generation Time |
| 3.1755 ( 1.2%) | 0.0520 ( 14.8%) | 3.2275 ( 1.2%) | 3.2308 ( 1.2%) | Code Generation Time |
| 272.0046 (100.0%) | 0.3509 (100.0%) | 272.3556 (100.0%) | 272.6585 (100.0%) | Total |
That was with a build from subversion a few weeks ago (trunk 121966) so I updated and got sensible times, although the optimised build is still slower than I expected:
$ time make CXX=clang++ objs/Order.o ... real 0m7.380s user 0m6.674s sys 0m0.471s $ time make CXX=clang++ CXXFLAGS="-O2 -DNDEBUG" objs/Order.o ... real 0m17.667s user 0m17.131s sys 0m0.309s
I don't remember the earlier build being so slow a few weeks ago, so maybe some gremlins came and moved the files to a really slow bit of the filesystem over Christmas.
Using Clang on a large codebase did find a few bugs which GCC and Sun CC didn't object to (one notable feature is that clang++ found errors in templates which were never instantiated, which even EDG didn't find) and I also found a bug in clang, but one of the main reasons I wanted to try it was the static analyzer. Unfortunately it only works for C and Obj-C and trying to analyze C++ fails miserably. I guess I'll wait and try it again at a later date.