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

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added inline comments.


================
Comment at: llvm/test/tools/llvm-dwarfdump/X86/statistics.ll:26-29
+; struct T {
+;   void *address();
+; };
+; void *T::address() { return this; }
----------------
dblaikie wrote:
> 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
> ```
Reperformed my experiment. You are right: `this` does not need to appear in the C++ file. I'll drop `this`. 

My other comment about `!dbg metadata on ret void, !dbg !17` is correct.


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

https://reviews.llvm.org/D86044