[PATCH] D86044: [llvm-dwarfdump][test] Add a --statistics test for a DW_AT_artificial variable

[David Blaikie via Phabricator via llvm-commits]

dblaikie added inline comments.


================
Comment at: llvm/test/tools/llvm-dwarfdump/X86/statistics.ll:26-29
+; struct T {
+;   void *address();
+; };
+; void *T::address() { return this; }
----------------
MaskRay wrote:
> dblaikie wrote:
> > Does this function need a return value or would a void-returning member function suffice?
> `void T::address() { (void)this; }` works as well.
> 
> `void T::address() { this; }` (-Wunused-variable) does not generate `this`
> 
> It is also interesting that the !dbg metadata on `ret void, !dbg !17` is important. Dropping the metadata will cause the absence of `this` as a DW_TAG_formal_parameter (I think it has something to do with `DW_AT_location (DW_OP_fbreg -8)`)
Is the IR being produced with optimizations enabled? Because at least for a simple test it looks like a location is generated even for a function that never names/mentions "this" (& if adding "(void)this;" changes the DWARF generation I'd be surprised/concerned, really - I don't think it should change the DWARF or the code generated)
```
$ cat test.cpp
struct t1 { void f1(); }; void t1::f1() { }
$ clang++-tot test.cpp -g -c && llvm-dwarfdump-tot test.o
test.o: file format elf64-x86-64

.debug_info contents:
0x00000000: Compile Unit: length = 0x00000070, format = DWARF32, version = 0x0004, abbr_offset = 0x0000, addr_size = 0x08 (next unit at 0x00000074)

0x0000000b: DW_TAG_compile_unit
              DW_AT_producer    ("clang version 12.0.0 (git at github.com:llvm/llvm-project.git 545abf55a70686cd1335a95075bc2c6eba8b135d)")
              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      (0x0000000000000000)
              DW_AT_high_pc     (0x000000000000000a)

0x0000002a:   DW_TAG_structure_type
                DW_AT_calling_convention        (DW_CC_pass_by_value)
                DW_AT_name      ("t1")
                DW_AT_byte_size (0x01)
                DW_AT_decl_file ("/usr/local/google/home/blaikie/dev/scratch/test.cpp")
                DW_AT_decl_line (1)

0x00000033:     DW_TAG_subprogram
                  DW_AT_linkage_name    ("_ZN2t12f1Ev")
                  DW_AT_name    ("f1")
                  DW_AT_decl_file       ("/usr/local/google/home/blaikie/dev/scratch/test.cpp")
                  DW_AT_decl_line       (1)
                  DW_AT_declaration     (true)
                  DW_AT_external        (true)

0x0000003e:       DW_TAG_formal_parameter
                    DW_AT_type  (0x00000045 "t1*")
                    DW_AT_artificial    (true)

0x00000043:       NULL

0x00000044:     NULL

0x00000045:   DW_TAG_pointer_type
                DW_AT_type      (0x0000002a "t1")

0x0000004a:   DW_TAG_subprogram
                DW_AT_low_pc    (0x0000000000000000)
                DW_AT_high_pc   (0x000000000000000a)
                DW_AT_frame_base        (DW_OP_reg6 RBP)
                DW_AT_object_pointer    (0x00000061)
                DW_AT_specification     (0x00000033 "_ZN2t12f1Ev")

0x00000061:     DW_TAG_formal_parameter
                  DW_AT_location        (DW_OP_fbreg -8)
                  DW_AT_name    ("this")
                  DW_AT_type    (0x0000006e "t1*")
                  DW_AT_artificial      (true)

0x0000006d:     NULL
```


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D86044/new/

https://reviews.llvm.org/D86044