[PATCH] D153898: [DebugInfo] Enable debug info emission for extern variables in C++

[Yuze Chi via Phabricator via cfe-commits]

chiyuze added a comment.

Regarding cost, only BPF target triggers this debug info generation for extern variables. There is no impact on x86 builds of clang:

Without patch:

  bloaty build/RelWithDebInfo/83d47ba15a1229a21aaca8a8d6a33e0e90aabfd4/bin/clang
      FILE SIZE        VM SIZE    
   --------------  -------------- 
    74.7%  2.98Gi   0.0%       0    .debug_info
     9.2%   377Mi   0.0%       0    .debug_loclists
     5.2%   210Mi   0.0%       0    .debug_str
     4.3%   177Mi   0.0%       0    .debug_line
     2.0%  81.5Mi  53.7%  81.5Mi    .text
     1.7%  68.0Mi   0.0%       0    .debug_rnglists
     1.1%  43.2Mi  28.5%  43.2Mi    .rodata
     0.5%  21.4Mi   0.0%       0    .debug_abbrev
     0.5%  20.3Mi   0.0%       0    .strtab
     0.2%  8.79Mi   5.8%  8.79Mi    .eh_frame
     0.2%  6.73Mi   4.4%  6.73Mi    .rela.dyn
     0.1%  4.86Mi   0.0%       0    .symtab
     0.1%  4.09Mi   2.7%  4.09Mi    .dynstr
     0.1%  3.60Mi   2.4%  3.60Mi    .data.rel.ro
     0.1%  3.30Mi   0.0%       0    .debug_aranges
     0.0%  1.19Mi   0.8%  1.19Mi    .dynsym
     0.0%  1.09Mi   0.7%  1.09Mi    .eh_frame_hdr
     0.0%       0   0.4%   611Ki    .bss
     0.0%   449Ki   0.1%   127Ki    [25 Others]
     0.0%   393Ki   0.3%   393Ki    .gnu.hash
     0.0%   332Ki   0.2%   332Ki    .data
   100.0%  3.99Gi 100.0%   151Mi    TOTAL

With patch:

  bloaty build/RelWithDebInfo/23bc4427ab49716ce2c24c81529d9c90953b3c54/bin/clang
      FILE SIZE        VM SIZE    
   --------------  -------------- 
    74.7%  2.98Gi   0.0%       0    .debug_info
     9.2%   377Mi   0.0%       0    .debug_loclists
     5.2%   210Mi   0.0%       0    .debug_str
     4.3%   177Mi   0.0%       0    .debug_line
     2.0%  81.5Mi  53.7%  81.5Mi    .text
     1.7%  68.0Mi   0.0%       0    .debug_rnglists
     1.1%  43.2Mi  28.5%  43.2Mi    .rodata
     0.5%  21.4Mi   0.0%       0    .debug_abbrev
     0.5%  20.3Mi   0.0%       0    .strtab
     0.2%  8.79Mi   5.8%  8.79Mi    .eh_frame
     0.2%  6.73Mi   4.4%  6.73Mi    .rela.dyn
     0.1%  4.86Mi   0.0%       0    .symtab
     0.1%  4.09Mi   2.7%  4.09Mi    .dynstr
     0.1%  3.60Mi   2.4%  3.60Mi    .data.rel.ro
     0.1%  3.30Mi   0.0%       0    .debug_aranges
     0.0%  1.19Mi   0.8%  1.19Mi    .dynsym
     0.0%  1.09Mi   0.7%  1.09Mi    .eh_frame_hdr
     0.0%       0   0.4%   611Ki    .bss
     0.0%   449Ki   0.1%   127Ki    [25 Others]
     0.0%   393Ki   0.3%   393Ki    .gnu.hash
     0.0%   332Ki   0.2%   332Ki    .data
   100.0%  3.99Gi 100.0%   151Mi    TOTAL

For BPF, this debug info is needed to generate proper BTF and use kconfig <https://nakryiko.com/posts/bpf-core-reference-guide/#kconfig-extern-variables>. This patch just enables it for C++, which makes it possible for us to leverage templates to deduplicate almost identical BPF code for IPv4 and IPv6.

https://reviews.llvm.org/D70696 has more details regarding why it is needed for BPF in general and https://lore.kernel.org/bpf/CAKH8qBt4xqBUpXefqPk5AyU1Rr0-h-vCJzS_0Bu-987gL4wi4A@mail.gmail.com/ has more details regarding why we are trying to compile BPF from C++.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D153898/new/

https://reviews.llvm.org/D153898