https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94469
--- Comment #11 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #9)
> (In reply to Tom de Vries from comment #8)
> > (In reply to Richard Biener from comment #7)
> > > (In reply to Richard Biener from comment #6)
> > > > Btw, I still wonder how it works with abstract functions, inline and
> > > > concrete instances. Works in gdb, that is - will dig into it a bit
> > > > after
> > > > lunch.
> > >
> > > So here I see us (without LTO) putting DW_AT_location attributes on the
> > > abstract instance copy. That kind-of makes it not abstract anymore, no?
> > > But that way we avoid emitting multiple DIEs for local statics. With
> > > -flto the same problem appears there because we cannot annotate the
> > > early CUs DIE with a location so the concrete instance copy
> > > gets [generated and] the location.
> > >
> > > So while I intended to have the early CUs behave like fully abstract
> > > the actual DWARF is different. I understand that if I emit the early CU
> > > as
> > > partial unit it becomes abstract?
> >
> > Well, that's my theory.
> >
> > I've created a minimal dwarf assembler variant corresponding to the C
> > test-case (with only var aaa), and I could reproduce the problem, however
> > after changing the tag to DW_TAG_partial_unit still a symbol table for that
> > partial unit was created. It seems that the inter-cu ref handling code is
> > responsible for that. I'll try to fix this.
> >
> > > Note we do emit DW_AT_const_value
> > > to early optimized out decls - would those still be found when the early
> > > CU
> > > is partial [and not imported anywhere]?
> >
> > I think so, but I could check with the dwarf assembler test-case.
>
> OK, so that doesn't work anymore.
>
> static const int i = 1;
>
> int main()
> {
> return i;
> }
>
> with -O2 -flto -g and DW_TAG_compile_unit I see
>
> (gdb) start
> Temporary breakpoint 1 at 0x4003a0: file t.c, line 5.
> Starting program: /home/abuild/rguenther/obj-gcc-g/gcc/a.out
>
> Temporary breakpoint 1, main () at t.c:5
> 5 return i;
> (gdb) p i
> 1
>
> while when using DW_TAG_partial_unit:
>
> (gdb) p i
> No symbol "i" in current context.
>
> Compilation Unit @ offset 0xc7:
> Length: 0x3d (32-bit)
> Version: 4
> Abbrev Offset: 0x64
> Pointer Size: 8
> <0><d2>: Abbrev Number: 1 (DW_TAG_compile_unit)
> <d3> DW_AT_producer : (indirect string, offset: 0x1d0): GNU GIMPLE
> 10.0
> .1 20200406 (experimental) -mtune=generic -march=x86-64 -g -g -O2 -O2
> -fno-openm
> p -fno-openacc -fno-pie -fltrans
> <d7> DW_AT_language : 12 (ANSI C99)
> <d8> DW_AT_name : (indirect string, offset: 0x250): <artificial>
> <dc> DW_AT_comp_dir : (indirect string, offset: 0x25d):
> /abuild/rguenth
> er/obj-gcc-g/gcc
> <e0> DW_AT_ranges : 0x40
> <e4> DW_AT_low_pc : 0x0
> <ec> DW_AT_stmt_list : 0xe9
> <1><f0>: Abbrev Number: 2 (DW_TAG_subprogram)
> <f1> DW_AT_abstract_origin: <0x13c>
> <f5> DW_AT_low_pc : 0x4003a0
> <fd> DW_AT_high_pc : 0x6
> <105> DW_AT_frame_base : 1 byte block: 9c
> (DW_OP_call_frame_cfa)
> <107> DW_AT_GNU_all_call_sites: 1
> <1><107>: Abbrev Number: 0
> Compilation Unit @ offset 0x108:
> Length: 0x3d (32-bit)
> Version: 4
> Abbrev Offset: 0x88
> Pointer Size: 8
> <0><113>: Abbrev Number: 1 (DW_TAG_partial_unit)
> <114> DW_AT_producer : (indirect string, offset: 0x27d): GNU C17
> 10.0.1
> 20200406 (experimental) -mtune=generic -march=x86-64 -g -O2 -flto
> <118> DW_AT_language : 12 (ANSI C99)
> <119> DW_AT_name : t.c
> <11d> DW_AT_comp_dir : (indirect string, offset: 0x25d):
> /abuild/rguent
> her/obj-gcc-g/gcc
> <121> DW_AT_stmt_list : 0x127
> <1><125>: Abbrev Number: 2 (DW_TAG_variable)
> <126> DW_AT_name : i
> <128> DW_AT_decl_file : 1
> <129> DW_AT_decl_line : 1
> <12a> DW_AT_decl_column : 18
> <12b> DW_AT_type : <0x137>
> <12f> DW_AT_const_value : 1
> <1><130>: Abbrev Number: 3 (DW_TAG_base_type)
> <131> DW_AT_byte_size : 4
> <132> DW_AT_encoding : 5 (signed)
> <133> DW_AT_name : int
> <1><137>: Abbrev Number: 4 (DW_TAG_const_type)
> <138> DW_AT_type : <0x130>
> <1><13c>: Abbrev Number: 5 (DW_TAG_subprogram)
> <13d> DW_AT_external : 1
> <13d> DW_AT_name : (indirect string, offset: 0x2ce): main
> <141> DW_AT_decl_file : 1
> <142> DW_AT_decl_line : 3
> <143> DW_AT_decl_column : 5
> <144> DW_AT_type : <0x130>
> <1><148>: Abbrev Number: 0
>
Ack, I've managed to reproduce this using a dwarf assembly test-case (PR
gdb/25796 - "Symbol with inherited DW_AT_const_value not found" @
https://sourceware.org/bugzilla/show_bug.cgi?id=25796), and submitted a gdb
patch for this.
Note that the problem is specific to gdb's partial symbol tables feature, so
that problem doesn't occur when using -readnow.
> I understand that if I would again add imports this would likely be resolved
> but at the expense of re-creating the original issue (but just with two
> instances rather than three)?
Agreed.
