[all-commits] [llvm/llvm-project] 0db402: [lld] Buffer writes when composing a single diagno...

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 0db402c5b4ef60ca8d20dda8bb59c7288a534e2a
      https://github.com/llvm/llvm-project/commit/0db402c5b4ef60ca8d20dda8bb59c7288a534e2a
  Author: Fangrui Song <i at maskray.me>
  Date:   2021-09-09 (Thu, 09 Sep 2021)

  Changed paths:
    M lld/Common/ErrorHandler.cpp
    M lld/include/lld/Common/ErrorHandler.h
    M llvm/include/llvm/Support/raw_ostream.h

  Log Message:
  -----------
  [lld] Buffer writes when composing a single diagnostic

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
strace/truss).

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). This can speed up
`relocation R_X86_64_PC32 out of range` diagnostic printing as well
with `--noinhibit-exec --no-fatal-warnings`.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D87272