[lldb-dev] Problem with ClangExpressionDeclMap
Zachary Turner
zturner at google.com
Tue Jan 13 11:12:18 PST 2015
Before I write out this email, I just want to state that my concern comes
mostly from the fact that the name lookup code seems to have many codepaths
that depend on not only the OS flavor, but also the mangling scheme. It
makes it very hard to maintain, improve, or even verify that things are
working correctly. For example, see this bug
<http://llvm.org/bugs/show_bug.cgi?id=21662>. Most of this could be
solved by just saying "clang, what is this thing?" BTW, I CC'ed David and
Chandler, because they know quite a lot about how name lookup works in
Clang and can probably provide quite a lot of edge cases that would break
name lookup in LLDB currently, similar to the aforementioned bug.
That being said, to respond to your point, since you have the forward
declaration, when you do "p *foo", the compiler should be able to tell you
"The type of this expression is <class Foo>". Then you look in the debug
info ignoring scope of any kind for the definition of <class Foo>. The
compiler only does the name lookup based on the current scope, but once it
knows the type it shouldn't have to worry about visibility anymore when
checking the symbol file. There may be cases where the debug info file
contains multiple definitions of <class Foo> in different source files, or
modules. In the case where it's not ambiguous, there's nothing to worry
about. In the case where it is, I don't think LLDB should try to be smart
about it anyway, because the user probably knows best.
So ambiguous types could be handled the same way as something that isn't
visible from the current function (another use case you said LLDB tries to
support) -- in particular there could be a syntax for specifying the
scope. Off the top of my head, something like this,
[<lib>,<file>,<function>]expression. Any of the 3 fields in the braces
could be unspecified. So, for example, suppose you have a file named
foo.cpp. This file is compiled into two separate libraries, foo1.so and
foo2.so, and looks like this:
// foo.cpp
int g_global = 0;
void func()
{
static int g_static = 1;
}
void func2()
{
int local = 2;
}
If the debugger is stopped in func() from foo1, you could do this:
(lldb) p g_global ; equivalent to "p [foo1, foo.cpp, ]g_global"
(lldb) p g_static ; equivalent to "p [foo1, foo.cpp, func]g_static"
(lldb) p local ; error, name not found
But you could also do this:
(lldb) p [foo2, , ]g_global ; No matter what module you're in, look for
all names in foo2.so that are called g_global, in any file. If you find
more than 1, it's ambiguous and needs to be refined by the user.
(lldb) p [foo2, , func]g_static ; No matter what module you're in, find
function local statics in module 'foo', in functions named 'func', declared
in any file. If you find more than 1 it's ambiguous.
But the default behavior,
(lldb) p g_global
Just searches based on the current stack frame and doesn't do anything
fancy, which is usually what you want.
(For the record, I know you guys have thought about this harder than I
have, so I'm just trying to understand. I still think delegating all the
name lookup to clang is more robust, but I still might be missing some
important use cases)
On Tue Jan 13 2015 at 10:39:21 AM <jingham at apple.com> wrote:
>
> > On Jan 13, 2015, at 9:58 AM, Zachary Turner <zturner at google.com> wrote:
> >
> > But then why does it work on Mac?
> >
> > And why is this workaround needed at all, when you can just have it
> check the basename in the first place (e.g. in FindExternalVisibleDecls)?
> >
> > I guess a higher level, more general question - Why is name lookup so
> complicated? Clang already knows everything, why does LLDB need to modify
> the default behavior of Clang's name lookup? I know LLDB can insert its
> own decls into the AST, but it seems like we can just get avoid returning
> those to the user, and still use clang for everything else.
> >
>
> I don't get what you mean by this comment. Name lookup in the debugger is
> pretty different from name lookup as it would be if we were sitting in the
> compiler trying to compile the expression as if it had been inserted in the
> current function at the PC. The compiler can be quite strict about what is
> and isn't visible, whereas the debugger has to be as lax as possible.
>
> For instance, suppose I am in a frame of a function that has a pointer to
> Foo, but that function is in a compile unit (and maybe even a shared
> library) that only contains a forward reference to Foo. If you had another
> shared library that contains a definition of Foo, you would certainly want
> to be able to print *Foo. Of course, from a strict compiler standpoint,
> this would be illegal, but telling debugger users that would not be a
> popular move.
>
> Similarly, folks want to be able to use global variables (and eventually
> function statics when we work out a syntax for that) that aren't visible in
> the current function.
>
> So the debugger has to get into the business of broadening name lookup to
> meet these needs.
>
> Jim
>
>
> > On Tue Jan 13 2015 at 2:54:15 AM Abid, Hafiz <Hafiz_Abid at mentor.com>
> wrote:
> > In SymbolFileDWARF::FindFunctions (), there is code that runs only for
> Linux and BSD. That finds the function in global namespace.
> >
> >
> >
> > Regards,
> >
> > Abid
> >
> >
> >
> > From: lldb-dev-bounces at cs.uiuc.edu [mailto:lldb-dev-bounces at cs.uiuc.edu]
> On Behalf Of Zachary Turner
> > Sent: 12 January 2015 19:51
> > To: lldb-dev at cs.uiuc.edu; Jim Ingham
> > Subject: [lldb-dev] Problem with ClangExpressionDeclMap
> >
> >
> >
> > Was looking into this bug on Windows, and I traced it down to an issue
> in ClangExpressionDeclMap. ClangExpressionDeclMap::FindExternalVisibleDecls
> has a piece of code that looks like this:
> >
> >
> >
> > if (namespace_decl && module_sp)
> >
> > {
> >
> > const bool include_symbols = false;
> >
> >
> >
> > module_sp->FindFunctions(name,
> >
> > &namespace_decl,
> >
> > eFunctionNameTypeBase,
> >
> > include_symbols,
> >
> > include_inlines,
> >
> > append,
> >
> > sc_list);
> >
> > }
> >
> > else if (target && !namespace_decl)
> >
> > {
> >
> > const bool include_symbols = true;
> >
> >
> >
> > // TODO Fix FindFunctions so that it doesn't return
> >
> > // instance methods for eFunctionNameTypeBase.
> >
> >
> >
> > target->GetImages().FindFunctions(name,
> >
> > eFunctionNameTypeFull,
> >
> > include_symbols,
> >
> > include_inlines,
> >
> > append,
> >
> > sc_list);
> >
> > }
> >
> >
> >
> > So it's only searching for the base name if the function is in a
> namespace, and if it's at global scope it's searching for the full name.
> >
> >
> >
> > I'm not sure why this works on other platforms, but on Windows it
> doesn't work because if I have this code:
> >
> >
> >
> > int foo(int x)
> >
> > {
> >
> > }
> >
> >
> >
> > then the full name of this function is ?foo@@YAHH at Z
> >
> >
> >
> > If I change eFunctionNameTypeFull to eFunctionNameTypeBase then
> everything works, and "p foo" finds the function. So I've got a couple of
> questions:
> >
> >
> >
> > 1) Why all this complicated logic? I would expect that if I type "p
> foo" then it would just print everything whose base name is foo and is
> visible from within the current scope? Am I underthinking this? There's
> special cases for namespaces, global scope, instance methods, variables,
> functions, and it looks for things in certain orders, etc. Is there any
> reason why it can't just find everything with a basename of foo visible
> within the current stack frame?
> >
> >
> >
> > 2) I'm not sure what this comment means:
> >
> > // TODO Fix FindFunctions so that it doesn't return
> >
> > // instance methods for eFunctionNameTypeBase.
> >
> > I changed it to eFunctionNameTypeBase and it fixes the original bug on
> Windows. I inserted a class with an instance method named foo() and also a
> global method named foo(), and running "p foo" doesn't find the instance
> method, only the global method. Is it possible this was somehow fixed, and
> it was forgotten to change this back to eFunctionNameTypeBase?
> >
> >
> >
> > 3) Why does this work on other platforms? full names are mangled on
> other platforms too, so I don't know how "p foo" doesn't run into this same
> issue on other platforms. Any ideas?
> >
> >
> >
> >
> >
> > Currently the best fix I have for this bug on Windows is to change
> eFunctionNameTypeFull to eFunctionNameTypeBase. but I want to get some
> thoughts on that worrisome comment before I go forward with that patch.
> >
>
>
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