On Fri, Nov 18, 2022 at 9:26 AM Per Sundström XP
<per.xp.sundstrom@xxxxxxxxxxxx> wrote:
>
>
> > > ============ Vanilla ==========
> > > struct foo {
> > > struct {
> > > int aa;
> > > char ab;
> > > } a;
> > > long :64;
> > > int :4;
> > > char b;
> > > short c;
> > > };
> > > offsetof(struct foo, c)=18
> > >
> > > ============ Custom ==========
> > > struct foo {
> > > long: 8;
> > > long: 64;
> > > long: 64;
> > > char b;
> > > short c;
> > > };
> >
> > so I guess the issue is that the first 'long: 8' is padded to full
> > long: 64 ?
> >
> > looks like btf_dump_emit_bit_padding did not take into accout the gap
> > on the
> > begining of the struct
> >
> > on the other hand you generated that header file from 'min_core_btf'
> > btf data,
> > which takes away all the unused fields.. it might not beeen
> > considered as a
> > use case before
> >
> > jirka
> >
>
> > That could be the case, but I think the 'emit_bit_padding()' will not
> > really have a
> > lot to do for the non sparse headers ..
> > /Per
>
>
> Looks like something like this makes tings a lot better:
yep, this helps, though changes output with padding to more verbose
version, quite often unnecessarily. I need to thing a bit more on
this, but the way we currently calculate alignment of a type is not
always going to be correct. E.g., just because there is an int field,
doesn't mean that struct actually has 4-byte alignment.
We must take into account natural alignment, but also actual
alignment, which might be different due to __attribute__((packed)).
Either way, thanks for reporting!
>
> diff --git a/src/btf_dump.c b/src/btf_dump.c
> index 12f7039..a8bd52a 100644
> --- a/src/btf_dump.c
> +++ b/src/btf_dump.c
> @@ -881,13 +881,13 @@ static void btf_dump_emit_bit_padding(const
> struct btf_dump *d,
> const char *pad_type;
> int pad_bits;
>
> - if (ptr_bits > 32 && off_diff > 32) {
> + if (align > 4 && ptr_bits > 32 && off_diff > 32) {
> pad_type = "long";
> pad_bits = chip_away_bits(off_diff, ptr_bits);
> - } else if (off_diff > 16) {
> + } else if (align > 2 && off_diff > 16) {
> pad_type = "int";
> pad_bits = chip_away_bits(off_diff, 32);
> - } else if (off_diff > 8) {
> + } else if (align > 1 && off_diff > 8) {
> pad_type = "short";
> pad_bits = chip_away_bits(off_diff, 16);
> } else {
> /Per
