[6.0][llvm/Support/VirtualOutputBackends] Don't create unbuffered streams when the MirroringOutputBackend is used
#8927
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…when the `MirroringOutputBackend` is used `MirroringOutputBackend` was forcing the `raw_ostream`s to be unbuffered, causing significant slowdown due to I/O. When enabling Clang caching for building Clang or WebKit, the "from scratch" (all cache misses) build was slower than the regular non-caching build. For building debug Clang the overhead was ~22%, and for debug build of WebKit, it was ~15%. The overhead went away after removing the `SetUnbuffered()` calls. rdar://130514092 (cherry picked from commit d87edc2)
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Explanation: When compilation is performed with Clang caching enabled, there's a |
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@swift-ci Please test |
bnbarham
approved these changes
Jun 26, 2024
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MirroringOutputBackendwas forcing theraw_ostreams to be unbuffered, causing significant slowdown due to I/O. When enabling Clang caching for building Clang or WebKit, the "from scratch" (all cache misses) build was slower than the regular non-caching build. For building debug Clang the overhead was ~22%, and for debug build of WebKit, it was ~15%.The overhead went away after removing the
SetUnbuffered()calls.rdar://130514092
(cherry picked from commit d87edc2)