[llvm] [RFC] Emit dwarf data for signature-changed or new functions (PR #157349)

[David Faust via llvm-commits]

dafaust wrote:

Late to the discussion so it's quite possible I missed something, but have you looked at what GCC generates for cases like this? Why not use a similar format?

To use the example snippet in from the OP, compiled with `gcc -c -O2 -g` we get for "foo" the DWARF below.
First we have the original `DW_TAG_subprogram` reflecting the source, followed by a second `DW_TAG_subprogram` at the same level for the concrete instance referring via `AT_abstract_origin` to the original.  The formals of the second reflect optimizations: note how "b" has no location information and instead is `const_value: 1`.  The call site info for the call to `tar` is also now a child of the concrete subprogram.

```
 <1><e3>: Abbrev Number: 16 (DW_TAG_subprogram)
    <e4>   DW_AT_name        : foo
    <e8>   DW_AT_decl_file   : 1
    <e9>   DW_AT_decl_line   : 3
    <ea>   DW_AT_decl_column : 41
    <eb>   DW_AT_prototyped  : 1
    <eb>   DW_AT_type        : <0x6c>
    <ef>   DW_AT_inline      : 1	(inlined)
    <f0>   DW_AT_sibling     : <0x10d>
 <2><f4>: Abbrev Number: 2 (DW_TAG_formal_parameter)
    <f5>   DW_AT_name        : a
    <f7>   DW_AT_decl_file   : 1
    <f7>   DW_AT_decl_line   : 3
    <f7>   DW_AT_decl_column : 55
    <f8>   DW_AT_type        : <0x78>
 <2><fc>: Abbrev Number: 2 (DW_TAG_formal_parameter)
    <fd>   DW_AT_name        : d
    <ff>   DW_AT_decl_file   : 1
    <ff>   DW_AT_decl_line   : 3
    <ff>   DW_AT_decl_column : 68
    <100>   DW_AT_type        : <0x78>
 <2><104>: Abbrev Number: 2 (DW_TAG_formal_parameter)
    <105>   DW_AT_name        : b
    <107>   DW_AT_decl_file   : 1
    <107>   DW_AT_decl_line   : 3
    <107>   DW_AT_decl_column : 75
    <108>   DW_AT_type        : <0x4a>
 <2><10c>: Abbrev Number: 0
 <1><10d>: Abbrev Number: 17 (DW_TAG_subprogram)
    <10e>   DW_AT_abstract_origin: <0xe3>
    <112>   DW_AT_low_pc      : 0
    <11a>   DW_AT_high_pc     : 0x5
    <122>   DW_AT_frame_base  : 1 byte block: 9c 	(DW_OP_call_frame_cfa)
    <124>   DW_AT_call_all_calls: 1
 <2><124>: Abbrev Number: 7 (DW_TAG_formal_parameter)
    <125>   DW_AT_abstract_origin: <0xf4>
    <129>   DW_AT_location    : 0x34 (location list)
    <12d>   DW_AT_GNU_locviews: 0x30
 <2><131>: Abbrev Number: 7 (DW_TAG_formal_parameter)
    <132>   DW_AT_abstract_origin: <0xfc>
    <136>   DW_AT_location    : 0x46 (location list)
    <13a>   DW_AT_GNU_locviews: 0x42
 <2><13e>: Abbrev Number: 18 (DW_TAG_formal_parameter)
    <13f>   DW_AT_abstract_origin: <0x104>
    <143>   DW_AT_const_value : 1
 <2><144>: Abbrev Number: 6 (DW_TAG_call_site)
    <145>   DW_AT_call_return_pc: 0x5
    <14d>   DW_AT_call_tail_call: 1
    <14d>   DW_AT_call_origin : <0x51>
 <3><151>: Abbrev Number: 1 (DW_TAG_call_site_parameter)
    <152>   DW_AT_location    : 1 byte block: 55 	(DW_OP_reg5 (rdi))
    <154>   DW_AT_call_value  : 3 byte block: a3 1 55 	(DW_OP_entry_value: (DW_OP_reg5 (rdi)))
 <3><158>: Abbrev Number: 1 (DW_TAG_call_site_parameter)
    <159>   DW_AT_location    : 1 byte block: 54 	(DW_OP_reg4 (rsi))
    <15b>   DW_AT_call_value  : 3 byte block: a3 1 54 	(DW_OP_entry_value: (DW_OP_reg4 (rsi)))
```

https://github.com/llvm/llvm-project/pull/157349