[PATCH dwarves v2 4/4] btf_encoder: support btf_type_tag attribute

[Yonghong Song <yhs@fb.com>]

The following is an example.
  [$ ~] cat t.c
  #define __tag1 __attribute__((btf_type_tag("tag1")))
  #define __tag2 __attribute__((btf_type_tag("tag2")))
  int __tag1 * __tag1 __tag2 *g __attribute__((section(".data..percpu")));
  [$ ~] clang -O2 -g -c t.c
  [$ ~] pahole -JV t.o
  Found per-CPU symbol 'g' at address 0x0
  Found 1 per-CPU variables!
  File t.o:
  [1] TYPE_TAG tag1 type_id=5
  [2] TYPE_TAG tag2 type_id=1
  [3] PTR (anon) type_id=2
  [4] TYPE_TAG tag1 type_id=6
  [5] PTR (anon) type_id=4
  [6] INT int size=4 nr_bits=32 encoding=SIGNED
  search cu 't.c' for percpu global variables.
  Variable 'g' from CU 't.c' at address 0x0 encoded
  [7] VAR g type=3 linkage=1
  [8] DATASEC .data..percpu size=8 vlen=1
          type=7 offset=0 size=8
  [$ ~]

You can see for the source
  int __tag1 * __tag1 __tag2 *g __attribute__((section(".data..percpu")));
the following type chain is generated:
  var -> ptr -> tag2 -> tag1 -> ptr -> tag1 -> int

The following shows pahole option "--skip_encoding_btf_type_tag"
can prevent BTF_KIND_TYPE_TAG generation.
  [$ ~] pahole -JV t.o --skip_encoding_btf_type_tag
  Found per-CPU symbol 'g' at address 0x0
  Found 1 per-CPU variables!
  File t.o:
  [1] PTR (anon) type_id=2
  [2] PTR (anon) type_id=3
  [3] INT int size=4 nr_bits=32 encoding=SIGNED
  search cu 't.c' for percpu global variables.
  Variable 'g' from CU 't.c' at address 0x0 encoded
  [4] VAR g type=1 linkage=1
  [5] DATASEC .data..percpu size=8 vlen=1
          type=4 offset=0 size=8
  [$ ~]

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 btf_encoder.c | 7 +++++++
 dwarves.h     | 5 +++++
 2 files changed, 12 insertions(+)

diff --git a/btf_encoder.c b/btf_encoder.c
index 117656e..9d015f3 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -143,6 +143,7 @@ static const char * const btf_kind_str[NR_BTF_KINDS] = {
 	[BTF_KIND_DATASEC]      = "DATASEC",
 	[BTF_KIND_FLOAT]        = "FLOAT",
 	[BTF_KIND_DECL_TAG]     = "DECL_TAG",
+	[BTF_KIND_TYPE_TAG]     = "TYPE_TAG",
 };
 
 static const char *btf__printable_name(const struct btf *btf, uint32_t offset)
@@ -393,6 +394,9 @@ static int32_t btf_encoder__add_ref_type(struct btf_encoder *encoder, uint16_t k
 	case BTF_KIND_TYPEDEF:
 		id = btf__add_typedef(btf, name, type);
 		break;
+	case BTF_KIND_TYPE_TAG:
+		id = btf__add_type_tag(btf, name, type);
+		break;
 	case BTF_KIND_FWD:
 		id = btf__add_fwd(btf, name, kind_flag);
 		break;
@@ -862,6 +866,9 @@ static int btf_encoder__encode_tag(struct btf_encoder *encoder, struct tag *tag,
 	case DW_TAG_typedef:
 		name = namespace__name(tag__namespace(tag));
 		return btf_encoder__add_ref_type(encoder, BTF_KIND_TYPEDEF, ref_type_id, name, false);
+	case DW_TAG_LLVM_annotation:
+		name = tag__btf_type_tag(tag)->value;
+		return btf_encoder__add_ref_type(encoder, BTF_KIND_TYPE_TAG, ref_type_id, name, false);
 	case DW_TAG_structure_type:
 	case DW_TAG_union_type:
 	case DW_TAG_class_type:
diff --git a/dwarves.h b/dwarves.h
index 4425d3c..52d162d 100644
--- a/dwarves.h
+++ b/dwarves.h
@@ -637,6 +637,11 @@ static inline struct btf_type_tag_ptr_type *tag__btf_type_tag_ptr(struct tag *ta
 	return (struct btf_type_tag_ptr_type *)tag;
 }
 
+static inline struct btf_type_tag_type *tag__btf_type_tag(struct tag *tag)
+{
+	return (struct btf_type_tag_type *)tag;
+}
+
 /** struct namespace - base class for enums, structs, unions, typedefs, etc
  *
  * @tags - class_member, enumerators, etc
-- 
2.30.2