Parallel builds of libstdc++ on APFS filesystem (with 1 ns granularity) on macOS 10.13 often fail (failure rate for “make -j2” to “make -j8” is about 60% from my own builds and results reported by others): https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81797 This is reproducible with several versions of GNU make.
Changing libstdc++’s makefile to mark install-headers with .NOTPARALLEL fixes the issue. We've carried that patch in Homebrew (https://brew.sh) for a few months now, and have had no report of build issues since then. Bootstrapped and regtested on x86_64-apple-darwin17 (as well as other platforms). OK to commit? FX
parallel.ChangeLog
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parallel.diff
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