[PATCH v2 0/3] perf annotate: Data type profiling support for C++ classes and virtual calls

[Yanbo Zhao]

Hello,

Data type profiling currently only understands C struct/union types.
For C++ workloads, member accesses through base class subobjects
cannot be resolved, and virtual function calls (indirect calls through
the vtable) lose the return type of the callee: the register holding
the returned value becomes unknown, which matters when it's used
directly to access memory like 'p->next()->val' where no DWARF
variable describes the temporary.

This series extends data type profiling to C++:

Patch 1 introduces die_is_compound_type() covering DW_TAG_class_type
as well and accepts DW_TAG_inheritance in the offset-based member
lookup so that it descends into base class subobjects.  It handles
empty base classes, members placed in the tail padding of a base and
virtual base classes.

Patch 2 adds the DWARF helpers for virtual calls: the vtable slot index
of a virtual function, the virtual function at a slot of a class
(following the primary base class chain), and the class of the vtable
pointer at an offset of a type.

Patch 3 tracks the vtable pointer and the virtual function pointer
loaded from it in the x86 instruction tracking, and resolves the
return type of 'call *N(%reg)' and 'call *%reg' through them.

Tested on x86-64 with GCC 15 (-O2 -g) using small programs covering
single/multiple inheritance, empty base optimization, tail padding
reuse, virtual inheritance, direct calls through the vtable and the
speculatively devirtualized form.  In all cases the access to the
returned pointer after a virtual call is annotated with the right type
and member, e.g.:

  movq    (%rbp), %rax           # data-type: struct Node +0 (_vptr.Node)
  movq    (%rax), %rax
  cmpq    %r14, %rax
  je      0x1240
  movq    %rbp, %rdi
  callq   *%rax
  addq    8(%rax), %r12          # data-type: struct Node +0x8 (val)

The existing results for C code are unchanged and the added cost on
the common path (a pointer dereference) is a tag check on the resolved
member type.

Changes in v2:

Patch 1:
- Explain DW_TAG_inheritance with an example DWARF in the commit
  message (Namhyung).
- Skip virtual base classes whose location is a runtime expression
  instead of falling back to offset 0 (Sashiko).
- Match a base class in the offset lookup only if it actually has a
  member at the offset, to handle empty base optimization and tail
  padding reuse where a member of the derived class shares the offset
  with the base (Sashiko).
- Keep looking at the next sibling in fill_member_name() when an
  anonymous child (base class) has nothing at the offset.

Patch 2:
- Drop the non-existent DW_AT_vtable_elem_index and the DW_LANG_*
  fallback macros (Namhyung).
- Drop cu_get_language(), cu_is_cplusplus(), die_get_base_class(),
  die_get_parent() and die_find_member_by_offset() which are not
  needed anymore (Namhyung).
- Document that die_get_vtable_index() returns the vtable slot index
  and that GCC and Clang both emit the index as DW_OP_constu
  (Namhyung, Sashiko).
- Fix die_find_virtual_func() to return the function DIE instead of
  the DW_TAG_inheritance DIE when found in a base class (Sashiko).
- Follow only the primary base class chain in die_find_virtual_func()
  since non-primary bases have their own secondary vtables, and skip
  an empty base at offset 0 which is not the primary base.
- Add die_get_vptr_class() to find the class of the vtable pointer
  through base class subobjects, and die_is_vtbl_ptr_type() to
  identify the vtable pointer by its type ('__vtbl_ptr_type').

Patch 3:
- Remove the receiver ('this' pointer) register update after the call
  which was dead code and not needed, and the arg0_reg field (Sashiko,
  Namhyung).  The 'this' pointer lives in a callee-saved register or
  on the stack across the call and DWARF location lists cover it.
- Handle 'call *%reg' by tracking the function pointer loaded from the
  vtable as TSR_KIND_VFUNC_PTR with its return type (Sashiko).  This
  form is common due to speculative devirtualization by GCC.
- Strip the leading '*' of an indirect call operand in call__parse()
  instead of extract_reg_offset() so that both forms are parsed.
- Ignore void virtual functions instead of aborting (Namhyung).
- Keep the existing pointer dereference branch and its fall-through
  intact; the vtable pointer is detected from the resolved member
  type there instead of a separate lookup before it.
- Treat the new register kinds as pointers when saved to the stack.

v1: https://lore.kernel.org/r/20260821050207.4517-1-yzhao62@xxxxxxxx

Thanks,
Yanbo

Yanbo Zhao (3):
  perf dwarf-aux: Add die_is_compound_type() to handle C++ class types
  perf dwarf-aux: Add C++ vtable helpers
  perf annotate: Resolve C++ virtual function calls in x86 insn tracking

 tools/perf/util/annotate-arch/annotate-x86.c |  76 +++++-
 tools/perf/util/annotate-data.c              |  61 +++--
 tools/perf/util/annotate-data.h              |   4 +
 tools/perf/util/disasm.c                     |   5 +
 tools/perf/util/dwarf-aux.c                  | 265 ++++++++++++++++++-
 tools/perf/util/dwarf-aux.h                  |  21 ++
 6 files changed, 406 insertions(+), 26 deletions(-)

base-commit: 45d15e89a783a0a279b7a54f9018c230127380d7
-- 
2.53.0