[Bug fortran/26910] [fortran] Printing dynamic string with DW_AT_string_length of class loclistpr fails

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26910

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
The parameter s in function f:
...
 <2><1ed>: Abbrev Number: 13 (DW_TAG_formal_parameter)
    <1ee>   DW_AT_name        : s
    <1f2>   DW_AT_type        : <0x25a>
...
has type:
...
 <1><25a>: Abbrev Number: 17 (DW_TAG_string_type)
    <25b>   DW_AT_string_length: 0xbf (location list)
    <25f>   DW_AT_byte_size   : 4
    <260>   DW_AT_sibling     : <0x26e>
...
matching with location list:
...
    000000bf 0000000000400730 0000000000400764 (DW_OP_reg4 (rsi))
    000000d2 0000000000400764 0000000000400783 (DW_OP_reg6 (rbp))
    000000e5 0000000000400783 0000000000400784 (DW_OP_GNU_entry_value:
(DW_OP_reg4 (rsi)); DW_OP_stack_value)
...

Doing a print of pc at the breakpoint:
...
4         print *, s
(gdb) info registers pc
pc             0x400734            0x400734 <f+4>
...
tells us that the string length can be found in register rsi, which contains:
...
(gdb) info registers rsi
rsi            0x3                 3
...

So, the information seems to be there.  Why doesn't gdb use it?

We find the answer by stepping through read_tag_string_type.  We hit:
...
(gdb) 
17837         if (!attr_to_dynamic_prop (attr, die, cu, &prop, prop_type))
(gdb) 
17838           length = 1;
...
and that's the reason why the length is one.

So, why don't we manage to convert the attribute to a dynamic prop?

Stepping through attr_to_dynamic_prop shows why:
...
(gdb) 
18590     if (attr->form_is_block ())
(gdb) 
18613     else if (attr->form_is_ref ())
(gdb) 
18676     else if (attr->form_is_constant ())
(gdb) 
18680         dwarf2_invalid_attrib_class_complaint (dwarf_form_name
(attr->form),
(gdb) 
18682         return 0;
...

The actual form is:
...
(gdb) p attr.form
$2 = DW_FORM_sec_offset
...
and that's not handled in attr_to_dynamic_prop.

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