[LLVMbugs] [Bug 13075] New: slow compilation

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13075

             Bug #: 13075
           Summary: slow compilation
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: fischman at chromium.org
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified


clang version 3.2 (trunk 155835)

The attached 2.6MB .E file is a pre-processed (-E) file from the chromium build
of WebKit, and clang takes ~3s to compile it down to a 3.4M .o file (on a beefy
linux machine).

$ /usr/bin/time clang -w -g   -x c++  -c
webcore_rendering.RenderMediaControlsChromium.E -o /dev/null
2.99user 0.08system 0:03.08elapsed 99%CPU (0avgtext+0avgdata
705440maxresident)k
0inputs+0outputs (0major+56787minor)pagefaults 0swaps
Dropping the -g drops the compile time to 2.2s-2.3s, although this still seems
excessively long.

When I do the same pre-processing for a g++-based ARM cross-compile of this
file, the -E output is still 2.6M, but g++ 4.6.3 takes only ~70ms (!) to
compile it down to a 1.4M .o file (once caches are warmed up).

So for some reason clang is 42x slower than gcc in producing output that is
less than 3x the size.

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