Parse C++ class and inheritance from Debug Info

[ttreyer]

Extend the Dwarf parser to make class members and parents' members accessible.

This patch handles any shape of inheritance hierarchy, but does provide access to parents' shadowed members:

struct Parent { int abc, xyz; };
struct Child : Parent { int abc; };
...
child->abc          // Access the child's member
child->xyz          // Access the parent's member
child->Parent::abc  // Not implemented: access the parent's shadowed member

Multiple inheritance suffer from a similar problem: when a parent shadows the member of another parent.

struct GrandChild : Parent, Child {};
g_child->xyz // Access Parent's member
g_child->abc // Ambiguous! Will access Parent::abc, but it should reject and require a qualified name instead

Checklist

• Language changes are updated in man/adoc/bpftrace.adoc
• User-visible and non-trivial changes updated in CHANGELOG.md
• The new behaviour is covered by tests

[ajor]

I was able to cause an abort with this script when it's triggered:

uprobe:./tests/testprogs/uprobe_test_cxx:cpp:uprobeFunction1 { print(args.foo) }

This change could do with some semantic analyser unit tests as well, to confirm the AST transformation occurs as expected.

[ajor]

Suggested change

	EXPECT Child & c
	EXPECT Parent & p
	EXPECT uprobe:./testprogs/uprobe_test_cxx:cpp:"uprobeFunction3(Child&, Parent&)"
	Child & c
	Parent & p

The multi-line EXPECT will verify that the order of the arguments is printed correct too

[ajor]

Why not use the demangled name here?

[ajor]

It doesn't look like these fields or inheritance hierarchy are being used in the tests - are they needed? Best to keep things as simple as possible if not necessary

[ajor]

If we convert the wrapped reference here into a pointer, then I think that will simplify later passes by not having to care about reference types any more. I'm hoping we can get away with zero changes to codegen and field_analyser. With luck, it might help with the crash I found too.

Here's an example to explain what I mean a bit better.
Original AST:

. :: [reference (struct Foo)]
  builtin: args :: [struct uprobeFunction1_args]
  foo

Current transformation:

dereference :: [struct Foo]
  . :: [reference (struct Foo)]
    builtin: args :: [struct uprobeFunction1_args]
    foo

AST after converting reference into pointer:

dereference :: [struct Foo]
  . :: [pointer (struct Foo)]
    builtin: args :: [struct uprobeFunction1_args]
    foo

[ajor]

Btw, I like how much simpler your method is for generating a DWARF data source - maybe we could switch the C data source across to that way in the future?