[fpc-devel] Wrong debug info when using clang backend

[Martin Frb]

Not suer if the issue is within Fpc or clang...

Should this be reported against Fpc?

- Fedora 33
- Fpc 3.3.1  from Sept 26th
- make all  OPT=" " OPTNEW=" -Clv11.0 " FPCMAKEOPT=" -Clv11.0 " LLVM=1
- clang --version
   clang version 11.0.0 (Fedora 11.0.0-3.fc33)
   Target: x86_64-unknown-linux-gnu
   Thread model: posix
   InstalledDir: /usr/bin


Program at the end of this mail was compiled with -O-

Assembler for nested procedure Bar
/home/m/laz/projects/clang1/clang_nested_proc_1.lpr:8 begin
00000000004012D0 897424F8                 mov [rsp-$08],esi
00000000004012D4 895424F0                 mov [rsp-$10],edx
/home/m/laz/projects/clang1/clang_nested_proc_1.lpr:9  a := 1;
00000000004012D8 C74424F801000000         mov [rsp-$08],$00000001
/home/m/laz/projects/clang1/clang_nested_proc_1.lpr:10  b := 1;
00000000004012E0 C74424F001000000         mov [rsp-$10],$00000001
/home/m/laz/projects/clang1/clang_nested_proc_1.lpr:11  c:=2;
00000000004012E8 C74424E402000000         mov [rsp-$1C],$00000002
/home/m/laz/projects/clang1/clang_nested_proc_1.lpr:12  d:=2;
00000000004012F0 C74424E002000000         mov [rsp-$20],$00000002
/home/m/laz/projects/clang1/clang_nested_proc_1.lpr:13  d:=2;
00000000004012F8 C74424E002000000         mov [rsp-$20],$00000002
/home/m/laz/projects/clang1/clang_nested_proc_1.lpr:14  end;
0000000000401300 C3                       ret

Variables d and c are encoded using a location list
  <2><9c>: Abbrev Number: 3 (DW_TAG_variable)
     <9d>   DW_AT_location    : 0x24 (location list)
     <a1>   DW_AT_name        : (indirect string, offset: 0xab): D
     <a5>   DW_AT_decl_file   : 1
     <a6>   DW_AT_decl_line   : 7
     <a7>   DW_AT_type        : <0xdb>
  <2><ab>: Abbrev Number: 3 (DW_TAG_variable)
     <ac>   DW_AT_location    : 0x48 (location list)
     <b0>   DW_AT_name        : (indirect string, offset: 0xad): C
     <b4>   DW_AT_decl_file   : 1
     <b5>   DW_AT_decl_line   : 7
     <b6>   DW_AT_type        : <0xdb>

Contents of the .debug_loc section:
     Offset   Begin            End              Expression
     00000000 00000000004012c3 00000000004012c5 (DW_OP_breg7 (rsp): 0)
     00000014 <End of list>
     00000024 0000000000401300 0000000000401301 (DW_OP_breg7 (rsp): -32)
     00000038 <End of list>
     00000048 0000000000401300 0000000000401301 (DW_OP_breg7 (rsp): -28)
     0000005c <End of list>

Both location lists specify the variable is only available from 
0000000000401300 0000000000401301
- That covers exactly the asm "ret" statement.
- Yet the variables are stored on the frame ([rsp-$1C] / [rsp-$20]) => 
so they are available before.



program clang_nested_proc_1;
{$mode objfpc}

procedure Foo;
   procedure Bar(a, b: integer);
   var
     c,d: integer;
   begin
     a := 1;
     b := 1;
     c:=2;
     d:=2;
     d:=2;
   end;
begin
   Bar(1,2);
end;

begin
   Foo;
end.



line info confirms the addresses from the asm.

Contents of the .debug_line section:
CU: clang_nested_proc_1.lpr:
File name                            Line number    Starting address    
View    Stmt
clang_nested_proc_1.lpr                       15 0x4012b0               x
clang_nested_proc_1.lpr                       16 0x4012b4               x
clang_nested_proc_1.lpr                       17 0x4012c3               x
clang_nested_proc_1.lpr                        8 0x4012d0               x
clang_nested_proc_1.lpr                        8 0x4012d0       1       x
clang_nested_proc_1.lpr                        9 0x4012d8               x
clang_nested_proc_1.lpr                       10 0x4012e0               x
clang_nested_proc_1.lpr                       11 0x4012e8               x
clang_nested_proc_1.lpr                       12 0x4012f0               x
clang_nested_proc_1.lpr                       13 0x4012f8               x
clang_nested_proc_1.lpr                       14 0x401300               x
clang_nested_proc_1.lpr                       19 0x401310               x
clang_nested_proc_1.lpr                       19 0x401311               x
clang_nested_proc_1.lpr                       20 0x401316               x
clang_nested_proc_1.lpr                       21 0x40131b               x
clang_nested_proc_1.lpr                       21 0x401322               x