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Clang compile for v8_base on host is very slow. |
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Issue descriptionFor Chromecast cross-compiles, compiling v8_base for the host with clang hangs for about 10 minutes. We have done some research in an internal bug (b/28027960), and suspect a clang corner-case bug. Here are the key observations: 1) This is not seen when using gcc. 2) Changing the optimization used results in a massive speedup (no other differences). Here are the compile times for counters.cc: -O2: 8m41.966s (default) -O0: 3.535s 3) The heavy use of FOR_EACH_INTRINSIC seems to be a likely suspect. Register allocation may be tripping up clang when optimization is on. Changing the char* to a std::string here causes the performance issues to disappear: https://code.google.com/p/chromium/codesearch#chromium/src/v8/src/counters.h&l=486 Next steps are to find a way to reproduce this on a public Chromium build and assign to v8 for triage.
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Apr 7 2016
I created a minimal test case that exhibits the clang slowness issue. time g++ -O2 -m32 -std=c++11 -c clang_test.cc -o clang_test.o real 0m5.948s user 0m5.901s sys 0m0.043s time ../../third_party/llvm-build/Release+Asserts/bin/clang++ -O2 -m32 -std=c++11 -c clang_test.cc -o clang_test.o real 1m13.431s user 1m12.337s sys 0m0.976s
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Apr 7 2016
dupe of https://bugs.chromium.org/p/v8/issues/detail?id=4856 maybe
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Apr 7 2016
Nice repro though!
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Apr 7 2016
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Comment 1 by dpranke@chromium.org
, Apr 7 2016