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Date: Fri, 10 Jan 2020 13:47:29 -0300
From: Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To: Stanislav Fomichev <sdf@...ichev.me>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, daniel@...earbox.net,
ast@...com, andriin@...com, morbo@...gle.com
Subject: Re: pahole and LTO
Em Fri, Jan 10, 2020 at 08:44:10AM -0800, Stanislav Fomichev escreveu:
> tl;dr - building the kernel with clang and lto breaks BTF generation because
> pahole doesn't seem to understand cross-cu references.
tl;dr response:
Yeah, so it may be the time to fix that, elfutils has interfaces for it,
and the tools that come with it handle cross-cu references, so we need
to study that and make pahole understand it.
- Arnaldo
> Can be reproduced with the following:
> $ cat a.c
> struct s;
>
> void f1() {}
>
> __attribute__((always_inline)) void f2(struct s *p)
> {
> if (p)
> f1();
> }
> $ cat b.c
> struct s {
> int x;
> };
>
> void f2(struct s *p);
>
> int main()
> {
> struct s s = { 10 };
> f2(&s);
> }
> $ clang -fuse-ld=lld -flto {a,b}.c -g
>
> $ pahole a.out
> tag__recode_dwarf_type: couldn't find 0x3f type for 0x99 (inlined_subroutine)!
> lexblock__recode_dwarf_types: couldn't find 0x3f type for 0x99 (inlined_subroutine)!
> struct s {
> int x; /* 0 4 */
>
> /* size: 4, cachelines: 1, members: 1 */
> /* last cacheline: 4 bytes */
> };
>
> From what I can tell, pahole internally loops over each cu and resolves only
> local references, while the dwarf spec (table 2.3) states the following
> about 'reference':
> "Refers to one of the debugging information entries that describe the program.
> There are four types of reference. The first is an offset relative to the
> beginning of the compilation unit in which the reference occurs and must
> refer to an entry within that same compilation unit. The second type of
> reference is the offset of a debugging information entry in any compilation
> unit, including one different from the unit containing the reference. The
> third type of reference is an indirect reference to a type definition using
> an 8-byte signature for that type. The fourth type of reference is a reference
> from within the .debug_info section of the executable or shared object file to
> a debugging information entry in the .debug_info section of a supplementary
> object file."
>
> In particular: "The second type of reference is the offset of a debugging
> information entry in any compilation unit, including one different from the
> unit containing the reference."
>
>
> So the question is: is it a (known) issue? Is it something that's ommitted
> on purpose? Or it's not implemented because lto is not (yet) widely used?
>
>
> Here is the dwarf:
>
> $ readelf --debug-dump=info a.out
> Contents of the .debug_info section:
>
> Compilation Unit @ offset 0x0:
> Length: 0x44 (32-bit)
> Version: 4
> Abbrev Offset: 0x0
> Pointer Size: 8
> <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
> <c> DW_AT_producer : (indirect string, offset: 0x11): clang version 10.0.0 (https://github.com/llvm/llvm-project.git 5fe4679cc9cfb4941b766db07bf3cd928075d204)
> <10> DW_AT_language : 12 (ANSI C99)
> <12> DW_AT_name : (indirect string, offset: 0x0): a.c
> <16> DW_AT_stmt_list : 0x0
> <1a> DW_AT_comp_dir : (indirect string, offset: 0x7a): /usr/local/google/home/sdf/tmp/lto
> <1e> DW_AT_low_pc : 0x201730
> <26> DW_AT_high_pc : 0x6
> <1><2a>: Abbrev Number: 2 (DW_TAG_subprogram)
> <2b> DW_AT_low_pc : 0x201730
> <33> DW_AT_high_pc : 0x6
> <37> DW_AT_frame_base : 1 byte block: 56 (DW_OP_reg6 (rbp))
> <39> DW_AT_name : (indirect string, offset: 0xa4): f1
> <3d> DW_AT_decl_file : 1
> <3e> DW_AT_decl_line : 3
> <3f> DW_AT_external : 1
> <1><3f>: Abbrev Number: 3 (DW_TAG_subprogram)
> <40> DW_AT_name : (indirect string, offset: 0x4): f2
> <44> DW_AT_decl_file : 1
> <45> DW_AT_decl_line : 5
> <46> DW_AT_prototyped : 1
> <46> DW_AT_external : 1
> <46> DW_AT_inline : 1 (inlined)
> <1><47>: Abbrev Number: 0
> Compilation Unit @ offset 0x48:
> Length: 0x7f (32-bit)
> Version: 4
> Abbrev Offset: 0x0
> Pointer Size: 8
> <0><53>: Abbrev Number: 1 (DW_TAG_compile_unit)
> <54> DW_AT_producer : (indirect string, offset: 0x11): clang version 10.0.0 (https://github.com/llvm/llvm-project.git 5fe4679cc9cfb4941b766db07bf3cd928075d204)
> <58> DW_AT_language : 12 (ANSI C99)
> <5a> DW_AT_name : (indirect string, offset: 0x7): b.c
> <5e> DW_AT_stmt_list : 0x3a
> <62> DW_AT_comp_dir : (indirect string, offset: 0x7a): /usr/local/google/home/sdf/tmp/lto
> <66> DW_AT_low_pc : 0x201740
> <6e> DW_AT_high_pc : 0x1f
> <1><72>: Abbrev Number: 4 (DW_TAG_subprogram)
> <73> DW_AT_low_pc : 0x201740
> <7b> DW_AT_high_pc : 0x1f
> <7f> DW_AT_frame_base : 1 byte block: 56 (DW_OP_reg6 (rbp))
> <81> DW_AT_name : (indirect string, offset: 0x9d): main
> <85> DW_AT_decl_file : 1
> <86> DW_AT_decl_line : 7
> <87> DW_AT_type : <0xae>
> <8b> DW_AT_external : 1
> <2><8b>: Abbrev Number: 5 (DW_TAG_variable)
> <8c> DW_AT_location : 2 byte block: 91 78 (DW_OP_fbreg: -8)
> <8f> DW_AT_name : (indirect string, offset: 0xb): s
> <93> DW_AT_decl_file : 1
> <94> DW_AT_decl_line : 9
> <95> DW_AT_type : <0xb5>
> <2><99>: Abbrev Number: 6 (DW_TAG_inlined_subroutine)
> <9a> DW_AT_abstract_origin: <0x3f>
> <9e> DW_AT_low_pc : 0x201752
> <a6> DW_AT_high_pc : 0x5
> <aa> DW_AT_call_file : 1
> <ab> DW_AT_call_line : 10
> <ac> DW_AT_call_column :
> <2><ad>: Abbrev Number: 0
> <1><ae>: Abbrev Number: 7 (DW_TAG_base_type)
> <af> DW_AT_name : (indirect string, offset: 0xd): int
> <b3> DW_AT_encoding : 5 (signed)
> <b4> DW_AT_byte_size : 4
> <1><b5>: Abbrev Number: 8 (DW_TAG_structure_type)
> <b6> DW_AT_name : (indirect string, offset: 0xb): s
> <ba> DW_AT_byte_size : 4
> <bb> DW_AT_decl_file : 1
> <bc> DW_AT_decl_line : 1
> <2><bd>: Abbrev Number: 9 (DW_TAG_member)
> <be> DW_AT_name : (indirect string, offset: 0xa2): x
> <c2> DW_AT_type : <0xae>
> <c6> DW_AT_decl_file : 1
> <c7> DW_AT_decl_line : 2
> <c8> DW_AT_data_member_location: 0
> <2><c9>: Abbrev Number: 0
> <1><ca>: Abbrev Number: 0
--
- Arnaldo
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