[Bug debug/91536] New: gcc generates invalid DW_OP_GNU_parameter_ref

[robert at ocallahan dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91536

            Bug ID: 91536
           Summary: gcc generates invalid DW_OP_GNU_parameter_ref
           Product: gcc
           Version: 9.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: robert at ocallahan dot org
  Target Milestone: ---

Compiling the following test program with `gcc -g -O2 -o ~/tmp/test
~/tmp/test.cpp`, `gcc (GCC) 9.1.1 20190503 (Red Hat 9.1.1-1)`

volatile char volatile_store;
struct X {
  int field;
  X() : field(111) {}
  __attribute__((noinline))
  void method() {
    volatile_store = 2;
  }
};
int main(void) {
  X x;
  x.method();
  return 0;
}

gdb can't print `this` in `method`:

(gdb) break method
Breakpoint 1 at 0x401120: file /home/roc/tmp/test.cpp, line 7.
(gdb) run
Starting program: /home/roc/tmp/test 
Breakpoint 1, X::method (this=<optimized out>) at /home/roc/tmp/test.cpp:7
7           volatile_store = 2;

gcc has generated a location list for `this` that uses
`DW_OP_GNU_parameter_ref`:

< 2><0x00000177>      DW_TAG_formal_parameter
                        DW_AT_abstract_origin       <0x00000127>
                        DW_AT_location              <loclist at offset
0x00000055 with 1 entries follows>
                        [ 0]<offset pair low-off: 0x00401120 addr 0x00401120
high-off: 0x00401128 addr 0x00401128>DW_OP_GNU_parameter_ref 0x00000127
DW_OP_stack_value

`method`'s return address is 0x401025 in this case, which corresponds to this
`DW_TAG_GNU_call_site`:

< 2><0x0000010b>      DW_TAG_GNU_call_site
                        DW_AT_low_pc                0x00401025
                        DW_AT_abstract_origin       <0x00000160>

Unfortunately this `DW_TAG_GNU_call_site` is completely useless because it
doesn't have any variables with `DW_AT_GNU_call_site_value`, so no wonder gdb
can't find the value of `this`. The subprgoram at 0x160 is not helpful, that's
just the subprogram containing the `DW_TAG_formal_parameter` at 0x177.

Aside: the DWARF5 spec and the original proposal for `DW_TAG_(GNU_)call_site`
(http://www.dwarfstd.org/ShowIssue.php?issue=100909.2 I think) don't list
`DW_AT_low_pc`  and `DW_AT_abstract_origin` as valid attributes, and thus don't
document what they mean here.