[llvm-bugs] [Bug 32037] New: clang is slower than gcc when compiling some codes in chrome

[via llvm-bugs]

http://bugs.llvm.org/show_bug.cgi?id=32037

            Bug ID: 32037
           Summary: clang is slower than gcc when compiling some codes in
                    chrome
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: laszio at google.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 18016
  --> http://bugs.llvm.org/attachment.cgi?id=18016&action=edit
row_common.cc, preprocessed

This is about compile time, not code quality.

When building chrome, there are some objects that take noticeably longer than
gcc. Here are the top 5:

diff / clang-4.0 time / gcc-4.9.2 time / object
17.752848, 58.020705, 40.267857,
obj/third_party/sqlite/chromium_sqlite3/sqlite3.o
10.927715, 15.727189, 4.799474, obj/third_party/libyuv/libyuv/row_common.o
8.964672, 19.499460, 10.534788, obj/v8/v8_base/wasm-interpreter.o
7.865181, 11.146169, 3.280988, obj/third_party/libvpx/libvpx/variance.o
5.407040, 15.328771, 9.921731,
obj/components/policy/cloud_policy_proto_generated_compile_proto/cloud_policy.pb.o

Although this is comparing apples to oranges, it would be nice to make clang
run faster if there are some low hanging fruits. One possibility might be
row_common.o, which is compiled with -march=corei7. Removing the option make it
4x faster.

$ time clang -target x86_64-cros-linux-gnu -O2 -c row_common.cc -march=corei7
real    0m6.920s
user    0m6.900s
sys     0m0.022s

$ time clang -target x86_64-cros-linux-gnu -O2 -c row_common.cc
real    0m1.565s
user    0m1.526s
sys     0m0.037s


Top 10 time-consuming functions:

# Overhead  Command           Shared Object     Symbol                          
# ........  ................  ................ 
.................................
#
    12.08%  clang-4.0         clang-4.0         [.]
combineX86ShufflesRecursively                               
     8.81%  clang-4.0         libc-2.23.so      [.] malloc                      
     8.05%  clang-4.0         clang-4.0         [.] llvm::extractConstantMask   
     6.86%  clang-4.0         clang-4.0         [.] llvm::APInt::shlSlowCase    
     4.84%  clang-4.0         clang-4.0         [.] llvm::APInt::lshr           
     2.13%  clang-4.0         libstdc++.so.6.0  [.] operator new                
     1.92%  clang-4.0         clang-4.0         [.] peekThroughBitcasts         
     1.63%  clang-4.0         clang-4.0         [.] llvm::DecodePSHUFBMask      
     1.61%  clang-4.0         clang-4.0         [.] llvm::APInt::operator|=     
     1.39%  clang-4.0         clang-4.0         [.] llvm::APInt::initSlowCase

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