Re: pahole and LTO

[Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>]

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