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;
}
This question aside, I think the changes fine.
[...]
