On 12/19/22 9:27 AM, Jose E. Marchesi wrote:
Hi Yonghong.
On 12/15/22 10:43 AM, Jose E. Marchesi wrote:
Of the two problems discussed:
1. DW_TAG_LLVM_annotation not being able to denote annotations to
non-pointed based types. clang currently ignores these instances.
We discussed two possible options to deal with this:
1.1 To continue ignoring these cases in the front-end, keep the dwarf
expressiveness limitation, and document it.
1.2 To change DW_TAG_LLVM_annotation so it behaves like a qualifier
DIE (like const, volatile, etc.) so it can apply to any type.
Thanks for the detailed update. Yes, we do want to __tag behaving like
a qualifier.
Today clang only support 'base_type <type_tag> *' style of code.
But we are open to support non-pointer style of tagging like
'base_type <type_tag> global_var'. Because of this, the following
dwarf output should be adopted:
C: int __tag1 * __tag2 * p;
dwarf: ptr -> __tag2 --> ptr -> __tag1 -> int
or
C: int __tag1 g;
dwarf: var_g -> __tag1 --> int
The above format *might* require particular dwarf tools to add support
for __tag attribute. But I think it is a good thing in the long run
esp. if we might add support to non-pointer types. In current
implementation, dwarf tools can simply ignore the children of ptr
which they may already do it.
I wonder, since these annotations are atomic, is there a reason for not
using an attribute instead of a DIE tag? Something like DW_AT_annotation.
Yes, we can. My suggestion is to facilitate gcc implementation.
Currently clang uses an attribute instead of a DIE tag. I am totally
fine if gcc uses the same dwarf representation mechanism as clang.
The attribute could then be used by any DIE (declaration, type, ...) and
existing DWARF consumers that don't support the new attribute would
happily just ignore it.
clang already use attributes to represent btf_type_tag and btf_decl_tag.
One of early considerations to use attribute in clang indeed is to avoid
existing tool changes as much as possible.
