r188739 - Revert "Revert "Revert "Revert "DebugInfo: Omit debug info for dynamic classes in TUs that do not have the vtable for that class""""

[Adrian Prantl]

On Dec 17, 2013, at 14:29, David Blaikie <dblaikie at gmail.com> wrote:

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> 
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> On Tue, Dec 17, 2013 at 12:23 PM, Greg Clayton <gclayton at apple.com> wrote:
> >
> > >
> > > We do need to have the option to turn this optimization off; preferably we would make off the default for Darwin. Other platforms that use LLDB as their primary debugger may want to do the same thing.
> > >
> > > Is there no way to fix LLDB so it actually loads in the other dsyms and finds the full definition of the type? It would seem unfortunate to have such a bloated debug info format (not only for this optimization, but for the existing -flimit-debug-info optimizations and anything else we might think of in the future where we can ensure that some debug info is already availabel in another file).
> >
> > We can fix LLDB but at what cost? If we don't know where a "foo" base class comes from, should we start download _all_ debug info for _all_ shared libraries until we find it? I don't really like that solution. I would like to see the full base class should always be there and would rather not have to use "-fno-limit-debug-info" which would make the debug info really really really bloated.
> >
> > Really really bloated?
> >
> > a) -flimit-debug-info is a more aggressive optimization than the vtable optimization - it will cause many pointer uses to emit only declarations and not definitions in the debug info. (it was even more aggressive before I fixed it 3 months ago - if you were using -flimit-debug-info prior to that then you're already far more tolerant to missing definitions than you realize)
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> I was saying that using "-fno-limit-debug-info" should not try to cull _any_ debug info and would result in all types being emitted with complete definitions, no?
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> No - what you're describing is GCC's -gfull, which Clang does not currently implement.
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> Here's the way things are:
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> Clang never emits types that are not referenced from some codegen'd costruct. That means an inline function with no callers has no debug info (& thus any types it uses (parameters, local variables, etc)aren't placed in the debug info), for example. Any type that /is/ referenced by a codegen'd construct will be emitted as a declaration if that's all that is available in this translation unit, or a definition if one is provided (even if the definition is provided after the use: struct foo; foo *f; struct foo { }; )
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> -flimit-debug-info (the default) goes a step further than that - it doesn't emit definitions, even if they're available in the source, if the type is not used in such a way that the definition is required (using Clang's "required to be complete" callback). So the above 'foo' example would not produce a definition of 'foo', but this example would: "struct foo; foo *f; struct foo { int i; }; void func() { i->i = 3; }"). The assumption being that if the type is never dereferenced in this translation unit the definition is not required and we can rely on the definition being provided in whatever other translation unit actually does the dereferencing. This feature was specifically requested/suggested by Chris Lattner, as far as I'm aware. When Eric measured the first implementation of this I think he recorded only a 2-3% savings. Since my improvements to this we haven't done any size comparisons, so I'm not sure what it's worth.
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> The vtable optimization, like the limit-debug-info approach, uses a similar (though stronger - because it relies on a language guarantee not just an informal heuristic) approach to guarantee that some other translation unit will contain the debug info for the type.
>  

I took the opportunity to document the effect of -flimit-debug-info in the man page. (CFE r197819).

-- adrian