[Bug debug/87440] New: GCC creates debug that confuses gdb

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87440

            Bug ID: 87440
           Summary: GCC creates debug that confuses gdb
           Product: gcc
           Version: 8.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

static int foo (int i)
{
  volatile int j = i + 3;
  return j - 2;
}
int main()
{
  volatile int z = foo (-1);
  return z;
}

> gcc t.c -O -g
> gdb ./a,out
(gdb) start
(gdb) s
foo (i=-1) at t.c:7
7       {
(gdb) info locals
j = 0
j = <optimized out>

we get 'j' two times (this is with gdb 8.2) because we emit a stray lexical
block for the DW_TAG_inlined_subroutine:

 <2><128>: Abbrev Number: 4 (DW_TAG_inlined_subroutine)
    <129>   DW_AT_abstract_origin: <0x170>
    <12d>   DW_AT_low_pc      : 0x4004d7
    <135>   DW_AT_high_pc     : 0xf
    <13d>   DW_AT_call_file   : 1
    <13e>   DW_AT_call_line   : 8
 <3><13f>: Abbrev Number: 5 (DW_TAG_formal_parameter)
    <140>   DW_AT_abstract_origin: <0x17c>
    <144>   DW_AT_location    : 0x0 (location list)
 <3><148>: Abbrev Number: 6 (DW_TAG_lexical_block)
    <149>   DW_AT_low_pc      : 0x4004d7
    <151>   DW_AT_high_pc     : 0xf
 <4><159>: Abbrev Number: 7 (DW_TAG_variable)
    <15a>   DW_AT_abstract_origin: <0x185>
    <15e>   DW_AT_location    : 2 byte block: 91 70     (DW_OP_fbreg: -16)

vs. the abstract origin which has:

 <1><170>: Abbrev Number: 10 (DW_TAG_subprogram)
    <171>   DW_AT_name        : foo
    <175>   DW_AT_decl_file   : 1
    <176>   DW_AT_decl_line   : 1
    <177>   DW_AT_prototyped  : 1
    <177>   DW_AT_type        : <0x164>
    <17b>   DW_AT_inline      : 1       (inlined)
 <2><17c>: Abbrev Number: 11 (DW_TAG_formal_parameter)
    <17d>   DW_AT_name        : i
    <17f>   DW_AT_decl_file   : 1
    <180>   DW_AT_decl_line   : 1
    <181>   DW_AT_type        : <0x164>
 <2><185>: Abbrev Number: 12 (DW_TAG_variable)
    <186>   DW_AT_name        : j
    <188>   DW_AT_decl_file   : 1
    <189>   DW_AT_decl_line   : 3
    <18a>   DW_AT_type        : <0x16b>

note how there's no outermost lexical block there.

This is because gen_subprogram_die calls decls_for_scope on DECL_INITIAL
rather than gen_lexical_block_die.  gen_inlined_subroutine_die tries to do
the same but fails to merge the "fake" block the inliner generates for
the parameter replacements with the actual outer scope of the function.