[PATCH] D67481: [ELF] Add -z separate-segments to complement separate-code and noseparate-code

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: grimar, jakehehrlich, mcgrathr, peter.smith, phosek, ruiu.
Herald added subscribers: llvm-commits, arichardson, emaste.
Herald added a reviewer: espindola.
Herald added a project: LLVM.

D64906 <https://reviews.llvm.org/D64906> allows PT_LOAD to have overlapping p_offset ranges. In the
default R RX RW RW layout + -z noseparate-code case, we do not tail pad
segments when transiting to another segment. This can save at most
3*maxPageSize bytes.

a) Before D64906 <https://reviews.llvm.org/D64906>, we tail pad R, RX and the first RW.
b) With -z separate-code, we tail pad R and RX, but not the first RW (RELRO).

In some cases, b) saves one file page. In some cases, b) wastes one
virtual memory page. The waste is a concern on Fuchsia. Because it uses
compressed binaries, it doesn't benefit from the saved file page.

This patch adds -z separate-segments to restore the behavior before
D64906 <https://reviews.llvm.org/D64906>. It can affect section addresses and can thus be used as a
debugging mechanism (see PR43214 and ld.so partition bug in
crbug.com/998712).


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D67481

Files:
  ELF/Config.h
  ELF/Driver.cpp
  ELF/Writer.cpp
  test/ELF/fill-trap.s
  test/ELF/separate-segments.s

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