[PATCH] D145891: [BPF] Generate BTF info using 'btf:type_tag' annotation

[Eduard Zingerman via Phabricator via llvm-commits]

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This is a follow-up for BPF mailing list discussion at [1].

Previous commit in a series updated DWARF generation for the following
example:

  int __attribute__((btf_type_tag("tag1"))) *g;

To generate DWARF that looks as follows:

  0x0000001e:   DW_TAG_variable
                  DW_AT_name      ("g")
                  DW_AT_type      (0x00000029 "int *")
  
  0x00000029:   DW_TAG_pointer_type
                  DW_AT_type      (0x00000032 "int")
  
  0x00000032:   DW_TAG_base_type
                  DW_AT_name      ("int")
  
  0x00000036:     DW_TAG_LLVM_annotation
                    DW_AT_name    ("btf:type_tag")
                    DW_AT_const_value     ("tag1")

The fresh part is attachment of `btf:type_tag` annotations to types
other than pointers. This commit changes BTF generation to rely on
`btf:type_tag` annotations to generate TYPE_TAG entries.
This necessitates the following changes:

- The logic for `BTFTypeTypeTag` chains creation is moved to `BTFDebug::addType()`;
- Special logic is added to avoid duplicate BTF entries for tagged and un-tagged type variants, e.g. in the following case:

  #define __tag1 __attribute__((btf_type_tag("tag1"))) #define __tag2 __attribute__((btf_type_tag("tag2")))

  struct foo {};

  struct bar { struct foo __tag1 aa; struct foo __tag2 bb; struct foo cc; };

  Debug information generated for this example contains three instances of `DICompositeType(name: "foo")` with different `annotations` fields, however single BTF definition for structure "foo" should be generated. Field `BTFDebug::DIDedupMap` and method `BTFDebug::lookupType()` are responsible for this logic;
- Care is taken to avoid references to type tags in relocation entries.

[1] https://lore.kernel.org/bpf/87r0w9jjoq.fsf@oracle.com/

Depends on D143967 <https://reviews.llvm.org/D143967>


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D145891

Files:
  llvm/lib/Target/BPF/BTFDebug.cpp
  llvm/lib/Target/BPF/BTFDebug.h
  llvm/test/CodeGen/BPF/BTF/type-tag-dedup-enum.ll
  llvm/test/CodeGen/BPF/BTF/type-tag-dedup-struct.ll
  llvm/test/CodeGen/BPF/BTF/type-tag-dedup-typedef.ll
  llvm/test/CodeGen/BPF/BTF/type-tag-dedup-union.ll
  llvm/test/CodeGen/BPF/BTF/type-tag-field-relo.ll
  llvm/test/CodeGen/BPF/BTF/type-tag-fixup-fwd.ll
  llvm/test/CodeGen/BPF/BTF/type-tag-fixup-resolved.ll
  llvm/test/CodeGen/BPF/BTF/type-tag-skip-var.ll
  llvm/test/CodeGen/BPF/BTF/type-tag-var.ll
  llvm/test/CodeGen/BPF/BTF/type-tag-void.ll

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