On Thu, Nov 14, 2024 at 12:04:24PM -0800, Yonghong Song wrote:
>
>
>
> On 11/14/24 10:21 AM, Eduard Zingerman wrote:
> > On Thu, 2024-11-14 at 08:51 -0800, Yonghong Song wrote:
> >
> > [...]
> >
> > > > + /* match DW_OP_entry_value(DW_OP_regXX) at any location */
> > > > + case DW_OP_entry_value:
> > > > + case DW_OP_GNU_entry_value:
> > > > + if (dwarf_getlocation_attr(attr, expr, &entry_attr) == 0 &&
> > > > + dwarf_getlocation(&entry_attr, &entry_ops, &entry_len) == 0 &&
> > > > + entry_len == 1) {
> > > > + ret = entry_ops->atom;
> > > Could we have more than one DW_OP_entry_value? What if the second one
> > > matches execpted_reg? From dwarf5 documentation, there is no say about
> > > whether we could have more than one DW_OP_entry_value or not.
> > >
> > > If we have evidence that only one DW_OP_entry_value will appear in parameter
> > > locations, a comment will be needed in the above.
> > >
> > > Otherwise, let us not do 'goto out' here. Rather, let us compare
> > > entry_ops->atom with expected_reg. Do 'ret = entry_ops->atom' and
> > > 'goto out' only if entry_ops->atom == expected_reg. Otherwise,
> > > the original 'ret' value is preserved.
> > Basing on this description in lldb source:
> > https://github.com/llvm/llvm-project/blob/1cd981a5f3c89058edd61cdeb1efa3232b1f71e6/lldb/source/Expression/DWARFExpression.cpp#L538
> > It would be surprising if DW_OP_entry_value records had different expressions.
> > However, there are 50 instances of such behaviour in my clang 18.1.8 built kernel., e.g.:
> >
> > 0x01f75d14: DW_TAG_subprogram
> > DW_AT_low_pc (0xffffffff818c43a0)
> > DW_AT_high_pc (0xffffffff818c43c9)
> > DW_AT_frame_base (DW_OP_reg7 RSP)
> > DW_AT_call_all_calls (true)
> > DW_AT_name ("hwcache_align_show")
> > DW_AT_decl_file ("/home/eddy/work/bpf-next/mm/slub.c")
> > DW_AT_decl_line (6621)
> > DW_AT_prototyped (true)
> > DW_AT_type (0x01f51a9b "ssize_t")
> >
> > 0x01f75d26: DW_TAG_formal_parameter
> > DW_AT_location (indexed (0xa0f) loclist = 0x0062c64f:
> > [0xffffffff818c43a9, 0xffffffff818c43b5): DW_OP_reg5 RDI
> > [0xffffffff818c43b5, 0xffffffff818c43c1): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
> > [0xffffffff818c43c1, 0xffffffff818c43c9): DW_OP_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value)
> > DW_AT_name ("s")
> > DW_AT_decl_file ("/home/eddy/work/bpf-next/mm/slub.c")
> > DW_AT_decl_line (6621)
> > DW_AT_type (0x01f4f449 "kmem_cache *")
> >
> > The following change seem not to affect pahole execution time:
> >
> > @@ -1234,7 +1234,8 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
> > dwarf_getlocation(&entry_attr, &entry_ops, &entry_len) == 0 &&
> > entry_len == 1) {
> > ret = entry_ops->atom;
> > - goto out;
> > + if (expr->atom == expected_reg)
> > + goto out;
> > }
> > break;
> > }
>
> Should we do
> ...
> dwarf_getlocation(&entry_attr, &entry_ops, &entry_len) == 0 &&
> entry_len == 1 && expr->atom == expected_reg) {
> ret = entry_ops->atom;
> goto out;
> }
> ...
> ?
+1
jirka
>
> >
> > This question aside, I think the changes fine.
> >
> > [...]
> >
>
