Em Wed, Oct 16, 2019 at 10:11:12AM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Tue, Oct 15, 2019 at 09:38:28PM -0700, Charles Machalow escreveu:
> > Hello all,
> >
> > I've built Pahole 1.15 from source on WSL Ubuntu. Though I have is a
> > very simple example of a C struct that doesn't seem to work correctly
> > if compiled as C++ via g++, though works fine when compiled as C via
> > gcc.
> >
> > Is Pahole intended to work with code compiled as C++ (as opposed to
> > C)? If so, take a look at the simple test.c file (can be compiled via
>
> It should work with C++ as well, albeit the support that is there was
> mostly in response to reports from users, notably from the guys at
> CERN's ATLAS project, long ago.
>
> > gcc or g++). Also take a look at the output from running pahole
> > against the created executable. via both g++/gcc builds It seems like
> > the nested struct is duplicated if compiled on C++. Does this seem
> > like a bug?
>
> I'll check, thanks for the report.
Humm, this is subtle, when building with C we end up with a reordering
of how the structs are stored in the DWARF info:
[acme@quaco charles_machalow]$ grep DW_TAG_structure_type -A1 a.out.from.C.readelf
<1><9c>: Abbrev Number: 5 (DW_TAG_structure_type)
<9d> DW_AT_name : (indirect string, offset: 0xc7): _INNER
--
<1><c3>: Abbrev Number: 7 (DW_TAG_structure_type)
<c4> DW_AT_byte_size : 32
--
<1><12b>: Abbrev Number: 5 (DW_TAG_structure_type)
<12c> DW_AT_name : (indirect string, offset: 0x0): _MY_STRUCT
[acme@quaco charles_machalow]$
[acme@quaco charles_machalow]$ grep DW_TAG_structure_type -A1 a.out.from.C++.readelf
<1><9c>: Abbrev Number: 5 (DW_TAG_structure_type)
<9d> DW_AT_name : (indirect string, offset: 0x7e): _INNER
--
<1><c3>: Abbrev Number: 5 (DW_TAG_structure_type)
<c4> DW_AT_name : (indirect string, offset: 0x0): _MY_STRUCT
--
<2><d0>: Abbrev Number: 7 (DW_TAG_structure_type)
<d1> DW_AT_byte_size : 32
[acme@quaco charles_machalow]$
See that there is one DW_TAG_structure_type that doesn't have a name, as
expected, and it is inside the typedef definition, so in C++ it gets
that <2> right before the Abbrev, meaning it is defined, in C++, only
inside that typedef scope, while in C it ends up having a <1>, which
means it is "generally available".
I'll continue this later to try to figure out why pahole is printing
this thing twice for the C++ case, having that <2> versus <1> as the
clue.
BTW, I've reproduced this with:
[acme@quaco charles_machalow]$ readelf -wi a.out.from.C | grep DW_TAG_compile_unit -A4
<0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
<c> DW_AT_producer : (indirect string, offset: 0x61): GNU C17 9.2.1 20190827 (Red Hat 9.2.1-1) -mtune=generic -march=x86-64 -g
<10> DW_AT_language : 12 (ANSI C99)
<11> DW_AT_name : (indirect string, offset: 0x51): test.c
<15> DW_AT_comp_dir : (indirect string, offset: 0xd8): /home/acme/git/pahole/examples/charles_machalow
[acme@quaco charles_machalow]$ readelf -wi a.out.from.C++ | grep DW_TAG_compile_unit -A4
<0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
<c> DW_AT_producer : (indirect string, offset: 0x85): GNU C++14 9.2.1 20190827 (Red Hat 9.2.1-1) -mtune=generic -march=x86-64 -g
<10> DW_AT_language : 4 (C++)
<11> DW_AT_name : (indirect string, offset: 0x51): test.c
<15> DW_AT_comp_dir : (indirect string, offset: 0xda): /home/acme/git/pahole/examples/charles_machalow
[acme@quaco charles_machalow]$
- Arnaldo
>
> > In both cases, GCC is version 7.4.0.
> >
> > test.c:
> > #include <stdint.h>
> >
> > typedef struct _INNER {
> > long myLong;
> > } INNER;
> >
> > typedef struct _MY_STRUCT {
> > struct
> > {
> > char a;
> > int b;
> > uint64_t ccc;
> > uint8_t _8_1 : 1;
> > uint8_t _8_4 : 4;
> > uint8_t _8_3 : 3;
> > INNER i;
> > };
> >
> > } MY_STRUCT;
> >
> > int main()
> > {
> > MY_STRUCT m;
> > return 0;
> > }
> >
> > pahole_g++:
> > csm10495@csm10495-desk:~$ g++ -g test.c && pahole a.out -q
> > struct _INNER {
> > long int myLong;
> > };
> > struct _MY_STRUCT {
> > struct {
> > char a;
> > int b;
> > uint64_t ccc;
> > uint8_t _8_1:1;
> > uint8_t _8_4:4;
> > uint8_t _8_3:3;
> > INNER i;
> > };
> >
> > struct {
> > char a;
> > int b;
> > uint64_t ccc;
> > uint8_t _8_1:1;
> > uint8_t _8_4:4;
> > uint8_t _8_3:3;
> > INNER i;
> > };
> > };
> >
> > pahole_gcc:
> > csm10495@csm10495-desk:~$ gcc -g test.c && pahole a.out -q
> > struct _INNER {
> > long int myLong;
> > };
> > struct _MY_STRUCT {
> > struct {
> > char a;
> > int b;
> > uint64_t ccc;
> > uint8_t _8_1:1;
> > uint8_t _8_4:4;
> > uint8_t _8_3:3;
> > INNER i;
> > };
> > };
> >
> >
> > Thanks all.
> >
> > - Charlie Scott Machalow
>
> --
>
> - Arnaldo
--
- Arnaldo
