[PATCH] D96035: [dsymutil][DWARFlinker] implement separate multi-thread processing for compile units.

Tue Jan 4 16:54:28 PST 2022

clayborg added a comment.

Looking at a few normal CUs after this I see that there are a lot of imported declarations we can probably get rid of if no one references them:

  0x04517e06:   DW_TAG_namespace
                  DW_AT_name	("std")

  0x04517e0b:     DW_TAG_namespace
                    DW_AT_name	("__1")
                    DW_AT_export_symbols	(true)

  0x04517e10:       DW_TAG_namespace
                      DW_AT_name	("chrono")

  0x04517e15:         DW_TAG_imported_module
                        DW_AT_decl_file	("/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/usr/include/c++/v1/chrono")
                        DW_AT_decl_line	(2923)
                        DW_AT_import	(0x0000000001b05f36)

  0x04517e1d:         NULL

  0x04517e1e:       DW_TAG_imported_declaration
                      DW_AT_decl_file	("/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/usr/include/c++/v1/cstddef")
                      DW_AT_decl_line	(49)
                      DW_AT_import	(0x0000000003fe0299)

  0x04517e25:       DW_TAG_imported_declaration
                      DW_AT_decl_file	("/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/usr/include/c++/v1/cstddef")
                      DW_AT_decl_line	(50)
                      DW_AT_import	(0x0000000003fe0375)

  0x04517e2c:       DW_TAG_imported_declaration
                      DW_AT_decl_file	("/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/usr/include/c++/v1/cstddef")
                      DW_AT_decl_line	(53)
                      DW_AT_import	(0x0000000003fe019b)

  0x04517e33:       DW_TAG_imported_declaration
                      DW_AT_decl_file	("/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/usr/include/c++/v1/cstdlib")
                      DW_AT_decl_line	(99)
                      DW_AT_import	(0x0000000003fe0375)

  ... <many many more>

These imported declarations amounted to 3% of overall CU size for a single compile unit that I tested. I know that the stock dsymutil whitelists all of these which causes these and all the types the reference to be kept, so if we can get rid of any DW_TAG_imported_declaration DIEs that are not referenced, we might also be able to get rid of the type that they reference as well as all types from a C++ header file that have corresponding C header files like "cstdlib" -> "stdlib.h", they will have this issue and waste space.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D96035/new/

https://reviews.llvm.org/D96035