[all-commits] [llvm/llvm-project] 890e8c: [Support] Add MemoryBuffer::dontNeedIfMmap

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 890e8c8f7e9e448a870224068d6b4181d96a293d
      https://github.com/llvm/llvm-project/commit/890e8c8f7e9e448a870224068d6b4181d96a293d
  Author: Fangrui Song <i at maskray.me>
  Date:   2021-12-30 (Thu, 30 Dec 2021)

  Changed paths:
    M llvm/include/llvm/Support/FileSystem.h
    M llvm/include/llvm/Support/MemoryBuffer.h
    M llvm/lib/Support/MemoryBuffer.cpp
    M llvm/lib/Support/Unix/Path.inc
    M llvm/lib/Support/Windows/Path.inc

  Log Message:
  -----------
  [Support] Add MemoryBuffer::dontNeedIfMmap

On *NIX systems, this API calls madvise(MADV_DONTNEED) on read-only file mappings.
It should not be used on a writable buffer.
The API is used to implement ld.lld LTO memory saving trick (D116367).

Note: on read-only file mappings, Linux's MADV_DONTNEED semantics match POSIX
POSIX_MADV_DONTNEED and BSD systems' MADV_DONTNEED.

On Windows, VirtualAllocEx MEM_COMMIT/MEM_RESET have similar semantics
but are unfortunately not drop-in replacements. dontNeedIfMmap is currently a no-op.

Reviewed By: aganea

Differential Revision: https://reviews.llvm.org/D116366