Why is gcc 4.9 (trunk) so slow?

Question

I have built an up-to-date vanilla GCC from svn with these flags:

../configure \  
    --enable-languages=c,c++ \  
    --disable-nls \  
    --enable-multilib \  
    --prefix=/opt/other/gcc-svn \  
    --program-suffix=-svn \  
    --with-system-zlib

First with clang 3.4, then I thought it may be clang's fault (with a grain of salt) and rebuilt GCC once
more with GCC 4.8.1, which led to the exact same result.

The resulting GCC is about 17 seconds slower than GCC 4.8.1 when I try to compile a C++ project with approx. 150k lines of code.

These are the build times I get (-O3):

• g++ 4.9: 48 seconds
• g++ 4.8: 31 seconds
• clang 3.4: 13 seconds

Did I miss a configure flag or is GCC 4.9 really that much slower?!

Answer 1

If the compiler is slower than expected, passing the -ftime-report flag can help figuring out what's going on.

Luckily, it also helped in this case: With -ftime-report GCC printed

'Extra diagnostic checks enabled; compiler may run slowly. Configure with --enable-checking=release to disable checks.'

Thomas has rebuilt the compiler from source accordingly and the problem is gone!