Hi,
On Wed, 2019-04-03 at 10:19 -0300, Arnaldo Carvalho de Melo wrote:
> Most are false positives, as DWARF (let alone BTF) has explicit ways
> to tell that these are all __attribute__((aligned(something)) and
> pahole still don't infer those by realizing that its not a natural
> alignment, a heuristic that would catch most of these bigger holes,
> such as:
DWARF does have DW_AT_alignment and GCC8 does output it.
DW_AT_alignment Non-default alignment of type, subprogram or
variable
2.24 Alignment
A debugging information entry may have a DW_AT_alignment attribute
whose value of class constant is a positive, non-zero, integer
describing the alignment of the entity.
For example, an alignment attribute whose value is 8 indicates that
the entity to which it applies occurs at an address that is a
multiple of eight (not a multiple of 2^8 or 256).
$ cat align.c
struct tt {
char *ptr;
int __attribute__((__aligned__(64))) i;
int n;
} t;
$ gcc -g -c align.c
$ eu-readelf --debug-dump=info align.o
DWARF section [ 4] '.debug_info' at offset 0x40:
[Offset]
Compilation unit at offset 0:
Version: 4, Abbreviation section offset: 0, Address size: 8, Offset
size: 4
[ b] compile_unit abbrev: 1
producer (strp) "GNU C17 8.2.1 20180905 (Red Hat
8.2.1-3) -mtune=generic -march=x86-64 -g"
language (data1) C99 (12)
name (strp) "align.c"
comp_dir (strp) "/tmp"
stmt_list (sec_offset) 0
[ 1d] structure_type abbrev: 2
name (string) "tt"
byte_size (data1) 128
alignment (data1) 64
decl_file (data1) align.c (1)
decl_line (data1) 1
decl_column (data1) 8
sibling (ref4) [ 4f]
[ 2a] member abbrev: 3
name (string) "ptr"
decl_file (data1) align.c (1)
decl_line (data1) 2
decl_column (data1) 9
type (ref4) [ 4f]
data_member_location (data1) 0
[ 37] member abbrev: 4
name (string) "i"
decl_file (data1) align.c (1)
decl_line (data1) 3
decl_column (data1) 40
type (ref4) [ 5c]
HERE => alignment (data1) 64
data_member_location (data1) 40
[ 43] member abbrev: 3
name (string) "n"
decl_file (data1) align.c (1)
decl_line (data1) 4
decl_column (data1) 7
type (ref4) [ 5c]
data_member_location (data1) 44
[ 4f] pointer_type abbrev: 5
byte_size (data1) 8
type (ref4) [ 55]
[ 55] base_type abbrev: 6
byte_size (data1) 1
encoding (data1) signed_char (6)
name (strp) "char"
[ 5c] base_type abbrev: 7
byte_size (data1) 4
encoding (data1) signed (5)
name (string) "int"
[ 63] variable abbrev: 8
name (string) "t"
decl_file (data1) align.c (1)
decl_line (data1) 5
decl_column (data1) 3
type (ref4) [ 1d]
external (flag_present) yes
location (exprloc)
[ 0] addr .bss+0
