[PATCH] D80765: [ELF] Handle bitcode comdat groups separately to deduplicate thinlto comdat sections

Wed Jun 17 23:11:35 PDT 2020

MaskRay added a subscriber: pcc.
MaskRay added a comment.

Apologies for my slowness getting to this patch.

`symtab->ltoOutputComdatGroups` does work:

  % readelf -g a3.o.lto.o a3.o1.lto.o 

  File: a3.o.lto.o

  COMDAT group section [    4] `.group' [4068369915778327548] contains 2 sections:
     [Index]    Name
     [    5]   .debug_types
     [    6]   .rela.debug_types

  File: a3.o1.lto.o

  COMDAT group section [    4] `.group' [4068369915778327548] contains 2 sections:
     [Index]    Name
     [    5]   .debug_types
     [    6]   .rela.debug_types

The  .debug_types from a3.o1.lto.o can be discarded by the new COMDAT logic. However, I am concerned that making it general as this patch does can miss some codegen bugs. See @pcc's argument in 
in https://reviews.llvm.org/D56015#1339411 . Since .debug_types (notably, a non-SHF_ALLOC section) is the only COMDAT rule this patch will discard, how about special casing .debug_types (i.e. if isLTOOutput && the group is related to .debug_types)?

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D80765/new/

https://reviews.llvm.org/D80765