[PATCH] D80391: [Driver] Don't make -gsplit-dwarf imply -g2

[David Blaikie via Phabricator via cfe-commits]

dblaikie added inline comments.


================
Comment at: clang/docs/ReleaseNotes.rst:120
   `nonnull` attribute on `this` for configurations that need it to be nullable.
+- ``-gsplit-dwarf`` no longer implies ``-g2``.
 
----------------
MaskRay wrote:
> dblaikie wrote:
> > thakis wrote:
> > > This apparently bit us (chromium) in thinlto configs. Could this here be more explicit that `-gsplit-dwarf` now needs an explicit `-g2` in cflags, and if using thinlto, in ldflags as well?
> > Arguably I think maybe it should be fixed/made to work for (thin)lto with -g2 in cflags and -gsplit-dwarf in ldflags, without the need to specify -g2 in ldflags (if that's what you're saying is currently required). Since the -gN level would hopefully at best be ignored (because the IR carries the N in -gN with it, and it'd be unfortunate it enabling split dwarf meant you had to ignore all your per-file -gN levels, or that the -gN is ignored except for allowing -gsplit-dwarf to be respected).
> > 
> > @MaskRay ?
> LLD LTO does not have debug related options. `ld -g` is an ignored compatibility option. LLD somewhat inherited that from GNU ld. I don't think `-gsplit-dwarf` or `-g` can be separately controlled in ldflags.
> 
> @thakis Can you share the crbug.com discussion thread?
Hmm, I guess the thing that bit Chromium would've been the need to add -g2 to cflags where previously it might've been just -gsplit-dwarf for cflags and ldflags.

But it doesn't look like -g2 is needed in ldflags, only passing it to the compilation seems sufficient, then the -gsplit-dwarf passed to the link step is respected:
```
$ clang++-tot -flto=thin test.cpp -g -c && clang++-tot -flto=thin -fuse-ld=lld -gsplit-dwarf test.o && llvm-dwarfdump-tot a.out
a.out:  file format elf64-x86-64

.debug_info contents:
0x00000000: Compile Unit: length = 0x0000002c, format = DWARF32, version = 0x0004, abbr_offset = 0x0000, addr_size = 0x08 (next unit at 0x00000030)

0x0000000b: DW_TAG_compile_unit
              DW_AT_stmt_list   (0x00000000)
              DW_AT_comp_dir    ("/usr/local/google/home/blaikie/dev/scratch")
              DW_AT_GNU_dwo_name        ("a.out_dwo/1.dwo")
              DW_AT_GNU_dwo_id  (0xf29563d7c812deae)
              DW_AT_low_pc      (0x0000000000201730)
              DW_AT_high_pc     (0x0000000000201738)
              DW_AT_GNU_addr_base       (0x00000000)
$ clang++-tot -flto=thin test.cpp -g -c && clang++-tot -flto=thin -fuse-ld=lld  test.o && llvm-dwarfdump-tot a.out
a.out:  file format elf64-x86-64

.debug_info contents:
0x00000000: Compile Unit: length = 0x00000047, format = DWARF32, version = 0x0004, abbr_offset = 0x0000, addr_size = 0x08 (next unit at 0x0000004b)

0x0000000b: DW_TAG_compile_unit
              DW_AT_producer    ("clang version 12.0.0 (git at github.com:llvm/llvm-project.git 0d88d7d82bc44b211a8187650a06c6cd3492186a)")
              DW_AT_language    (DW_LANG_C_plus_plus_14)
              DW_AT_name        ("test.cpp")
              DW_AT_stmt_list   (0x00000000)
              DW_AT_comp_dir    ("/usr/local/google/home/blaikie/dev/scratch")
              DW_AT_low_pc      (0x0000000000201730)
              DW_AT_high_pc     (0x0000000000201738)

0x0000002a:   DW_TAG_subprogram
                DW_AT_low_pc    (0x0000000000201730)
                DW_AT_high_pc   (0x0000000000201738)
                DW_AT_frame_base        (DW_OP_reg6 RBP)
                DW_AT_name      ("main")
                DW_AT_decl_file ("/usr/local/google/home/blaikie/dev/scratch/test.cpp")
                DW_AT_decl_line (3)
                DW_AT_type      (0x00000043 "int")
                DW_AT_external  (true)

0x00000043:   DW_TAG_base_type
                DW_AT_name      ("int")
                DW_AT_encoding  (DW_ATE_signed)
                DW_AT_byte_size (0x04)

0x0000004a:   NULL
```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D80391/new/

https://reviews.llvm.org/D80391