llvm::errs() is unbuffered. On a POSIX platform, composing a diagnostic
string may invoke the ::write syscall multiple times, which can be slow.
Buffer writes to a temporary SmallString when composing a single diagnostic to
reduce the number of ::write syscalls to one (also easier to read under
For an invocation of ld.lld with 62000+ lines of
ld.lld: warning: symbol ordering file: no such symbol: warnings (D87121),
the buffering decreases the write time from 1s to 0.4s (for /dev/tty) and
from 0.4s to 0.1s (for a tmpfs file).