[all-commits] [llvm/llvm-project] 843f2d: [Driver] Don't make -gsplit-dwarf imply -g2

[Fangrui Song via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 843f2dbf003f2a51d0d4ab8cf40647c99ded2e93
      https://github.com/llvm/llvm-project/commit/843f2dbf003f2a51d0d4ab8cf40647c99ded2e93
  Author: Fangrui Song <i at maskray.me>
  Date:   2020-12-08 (Tue, 08 Dec 2020)

  Changed paths:
    M clang/docs/ReleaseNotes.rst
    M clang/lib/Driver/ToolChains/Clang.cpp
    M clang/test/Driver/split-debug.c

  Log Message:
  -----------
  [Driver] Don't make -gsplit-dwarf imply -g2

RFC: http://lists.llvm.org/pipermail/cfe-dev/2020-May/065430.html
Agreement from GCC: https://sourceware.org/pipermail/gcc-patches/2020-May/545688.html

g_flags_Group options generally don't affect the amount of debugging
information. -gsplit-dwarf is an exception. Its order dependency with
other gN_Group options make it inconvenient in a build system:

* -g0 -gsplit-dwarf -> level 2
   -gsplit-dwarf "upgrades" the amount of debugging information despite
   the previous intention (-g0) to drop debugging information
* -g1 -gsplit-dwarf -> level 2
  -gsplit-dwarf "upgrades" the amount of debugging information.
* If we have a higher-level -gN, -gN -gsplit-dwarf will supposedly decrease the
  amount of debugging information. This happens with GCC -g3.

The non-orthogonality has confused many users. GCC 11 will change the semantics
(-gsplit-dwarf no longer implies -g2) despite the backwards compatibility break.
This patch matches its behavior.

New semantics:

* If there is a g_Group, allow split DWARF if useful
  (none of: -g0, -gline-directives-only, -g1 -fno-split-dwarf-inlining)
* Otherwise, no-op.

To restore the original behavior, replace -gsplit-dwarf with -gsplit-dwarf -g.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D80391