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<base href="http://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - clang is slower than gcc when compiling some codes in chrome"
href="http://bugs.llvm.org/show_bug.cgi?id=32037">32037</a>
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<th>Summary</th>
<td>clang is slower than gcc when compiling some codes in chrome
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>-New Bugs
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>laszio@google.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<pre>Created <span class=""><a href="attachment.cgi?id=18016" name="attach_18016" title="row_common.cc, preprocessed">attachment 18016</a> <a href="attachment.cgi?id=18016&action=edit" title="row_common.cc, preprocessed">[details]</a></span>
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</pre>
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