Em Wed, Aug 15, 2012 at 02:56:47PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Sat, Jul 21, 2012 at 08:22:33PM +0200, Nicolas escreveu:
> > Hello,
> >
> > I tried using pahole, but I get a brain fart alert with the file
> > attached (which is an extract from stdio.h). I'm running dwarves 1.10,
> > libdw1 0.152 and g++ 4.6.1, on i386.
> >
> > Here is what I get:
> >
> > $ g++ -Wall -g -c brainfart.cpp -o brainfart.o && pahole brainfart.o
> > class ios_base {
> > enum _Ios_Openmodeconst in; /* 0 4 */
> > typedef enum _Ios_Fmtflags fmtflags;
> >
> >
> > /* size: 1, cachelines: 1, members: 1 */
> > /* padding: 65533 */
> > /* last cacheline: 1 bytes */
> >
> > /* BRAIN FART ALERT! 1 != 4 + 0(holes), diff = -3 */
> >
> > };
> >
> > Moreover, on amd64, pahole segfaults if I do the same (with dwarves
> > 1.10, libdw1 0.152, g++ 4.6.3).
>
> that is strange, I just tried it on rhel6, x86_64, and just reproduced
> what you describe on i386:
>
> [acme@sandy pahole]$ uname -a
> Linux sandy.ghostprotocols.net 3.6.0-rc1+ #13 SMP Fri Aug 10 12:42:33 BRT 2012 x86_64 x86_64 x86_64 GNU/Linux
> [acme@sandy pahole]$ g++ -Wall -g -c brainfart.cpp -o brainfart.o && pahole brainfart.o
> class ios_base {
> enum _Ios_Openmodeconst in; /* 0 4 */
> typedef enum _Ios_Fmtflags fmtflags;
>
>
> /* size: 1, cachelines: 1, members: 1 */
> /* padding: 65533 */
> /* last cacheline: 1 bytes */
>
> /* BRAIN FART ALERT! 1 != 4 + 0(holes), diff = -3 */
>
> };
So the problem is that 'in' is (from the .cpp file):
static const _Ios_Openmode in = _S_in;
<2><82>: Abbrev Number: 10 (DW_TAG_member)
<83> DW_AT_name : in
<86> DW_AT_decl_file : 1
<87> DW_AT_decl_line : 15
<88> DW_AT_type : <0x9b>
<8c> DW_AT_external : 1
<8d> DW_AT_declaration : 1
<8e> DW_AT_const_value : 8
So 'in' type is...
<1><9b>: Abbrev Number: 12 (DW_TAG_const_type)
<9c> DW_AT_type : <0x49>
That links to:
<2><49>: Abbrev Number: 3 (DW_TAG_enumeration_type)
<4a> DW_AT_name : (indirect string, offset: 0xa9): _Ios_Openmode
<4e> DW_AT_byte_size : 4
<4f> DW_AT_decl_file : 1
<50> DW_AT_decl_line : 7
So it thinks that the size is 4 bytes:
65533 == (unsigned short)((short)(1 - 4))
I need to capture the fact that it is a static const, and investigate
the C++ structure encoding rules, why is that it takes 1 byte when it
has no per instance members?
- Arnaldo
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