[Bug c++/58258] New: cp_tree_equal and unknown symbol take up 70% of (astronomical) build time

["geoffers+gccbug at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58258

            Bug ID: 58258
           Summary: cp_tree_equal and unknown symbol take up 70% of
                    (astronomical) build time
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: geoffers+gccbug at gmail dot com

Created attachment 30709
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30709&action=edit
Slow compiling file

With the attached file (sorry about the size â I'm going away on holiday and
have run out of time to reduce it and would likely forget about it â though it
is an improvement on the 2.3MB preprocessed source file it started off as!),
g++ takes ~90s to compile, whereas clang++ takes ~3s. (On the original,
non-reduced testcase, g++ took ~2h, clang++ took ~20s â a 400x difference.)
This, somewhat obviously, isn't acceptable to me as an end-user.

Notably expensive, from `perf record g++ -c -O0 -Wfatal-errors -std=c++11
-fpreprocessed -w -S -o /dev/null testcase.i`:

 44.89%  cc1plus  cc1plus            [.] 0x000000000065a167
 14.73%  cc1plus  cc1plus            [.] cp_tree_equal(tree_node*, tree_node*)
  8.05%  cc1plus  cc1plus            [.] dependent_name(tree_node*)
  5.29%  cc1plus  cc1plus            [.] operand_equal_p(tree_node const*,
tree_node const*, unsigned int)
  5.05%  cc1plus  cc1plus            [.] ggc_internal_alloc_stat(unsigned long)

To contrast with a limited sample of the original (I'm not recording the
two-hour compilation, the perf.data file would be huge!):

+  54.76%  cc1plus  cc1plus            [.] 0x00000000006482d9                   
+  20.42%  cc1plus  cc1plus            [.] cp_tree_equal(tree_node*,
tree_node*)                                                                    
+  13.97%  cc1plus  cc1plus            [.] dependent_name(tree_node*)

I'm using the ArchLinux packaged GCC/Clang, i.e.,

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/4.8.1/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /build/gcc/src/gcc-4.8-20130725/configure --prefix=/usr
--libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch
--enable-gnu-unique-object --enable-linker-build-id --enable-cloog-backend=isl
--disable-cloog-version-check --enable-lto --enable-gold --enable-ld=default
--enable-plugin --with-plugin-ld=ld.gold --with-linker-hash-style=gnu
--disable-install-libiberty --disable-multilib --disable-libssp
--disable-werror --enable-checking=release
Thread model: posix
gcc version 4.8.1 20130725 (prerelease) (GCC)