[PATCH] Output debug information for structure members only referenced by a pointer or typedef

[Keith Walker]

> On Wed, Jun 19, 2013 at 5:34 AM, Keith Walker <keith.walker at arm.com>
> wrote:
> > This patch addresses an issue introduced by r183296 that means that
> the
> > members of a structure are no longer output in the DWARF debugging
> > information if the structure is only referenced by a pointer or
> typedef
> > (reported in PR16214).
>
> Does r184252 address your issue?

Unfortunately no.

> What's the particular regression you're trying to fix related to
> pointers without typedefs? I'm pretty sure even pre-r183296 a simple

Actually it appears I simplified my example program I was using to investigate the problem too much.

The problem is actually to do with a typdef'ed pointer to a structure as mentioned in PR16214.

This program when compiled with "-g -O0" demonstrates the problem that the member i of struct s in not output (unless you use -fno-limit-debug-info).

-----------------------------------
#include <stdlib.h>

struct s {
    int i;
};

typedef struct s *sptr;

int main()
{
    sptr sp = (sptr)malloc(sizeof(struct s));
    sp->i = 1;
    return sp->i;
}
-----------------------------------

Unfortunately during my investigation I had taken one step further and simplified it to ....

-----------------------------------
#include <stdlib.h>

struct s {
    int i;
};


int main()
{
    struct s *sp = (struct s*)malloc(sizeof(struct s));
    sp->i = 1;
}
-----------------------------------

Which generated the same debugging information so I mistakenly thought the problem was not due to the typedef.

However, upon further investigation adding back the return statement so there is a read of the member sp->i and hey-presto the expected member information is generated.

So actually the problem I was attempting to fix is that which is mentioned it PR16214.

> piece of code involving a struct definition and the declaration of a
> variable that is a pointer to that type would've only emitted the
> declaration of that type. Did you observe different behavior?

Previously to r183296 the member information was generated .... but I guess that is actually because the -flimit-debug-info was not limiting the generated information in all cases.

>
> > Files:
> >   test/CodeGenCXX/debug-info-namespace.cpp
> >   test/CodeGenCXX/debug-info.cpp
> >   lib/CodeGen/CGDebugInfo.cpp
> >
> > Index: test/CodeGenCXX/debug-info-namespace.cpp
> > ===================================================================
> > --- test/CodeGenCXX/debug-info-namespace.cpp    (revision 184291)
> > +++ test/CodeGenCXX/debug-info-namespace.cpp    (working copy)
> > @@ -57,7 +57,7 @@
> >  // CHECK: [[M6]] = metadata !{i32 {{[0-9]*}}, metadata [[FUNC]],
> metadata
> > [[FOO:![0-9]*]], i32 21} ; [ DW_TAG_imported_declaration ]
> >  // CHECK: [[FOO]] {{.*}} ; [ DW_TAG_structure_type ] [foo] [line 5,
> size 0,
> > align 0, offset 0] [fwd] [from ]
> >  // CHECK: [[M7]] = metadata !{i32 {{[0-9]*}}, metadata [[FUNC]],
> metadata
> > [[BAR:![0-9]*]], i32 22} ; [ DW_TAG_imported_declaration ]
> > -// CHECK: [[BAR]] {{.*}} ; [ DW_TAG_structure_type ] [bar] [line 6,
> {{.*}}]
> > [fwd] [from ]
> > +// CHECK: [[BAR]] {{.*}} ; [ DW_TAG_structure_type ] [bar] [line 6,
> {{.*}}]
> > [from ]
> >  // CHECK: [[M8]] = metadata !{i32 {{[0-9]*}}, metadata [[FUNC]],
> metadata
> > [[F1]], i32 23} ; [ DW_TAG_imported_declaration ]
> >  // CHECK: [[M9]] = metadata !{i32 {{[0-9]*}}, metadata [[FUNC]],
> metadata
> > [[I]], i32 24} ; [ DW_TAG_imported_declaration ]
> >  // CHECK: [[M10]] = metadata !{i32 {{[0-9]*}}, metadata [[FUNC]],
> metadata
> > [[BAZ:![0-9]*]], i32 25} ; [ DW_TAG_imported_declaration ]
> > Index: test/CodeGenCXX/debug-info.cpp
> > ===================================================================
> > --- test/CodeGenCXX/debug-info.cpp      (revision 184291)
> > +++ test/CodeGenCXX/debug-info.cpp      (working copy)
> > @@ -102,18 +102,26 @@
> >  typedef a at;
> >
> >  struct b {
> > +  int j;
> >  };
> >
> > +struct c;
> > +
> >  typedef b bt;
> > +typedef c ct;
> >
> >  void func() {
> >    at a_inst;
> >    bt *b_ptr_inst;
> >    const bt *b_cnst_ptr_inst;
> > +  ct *c_ptr_inst;
> >  }
> >
> >  // CHECK: metadata [[A_MEM:![0-9]*]], i32 0, null, null} ; [
> > DW_TAG_structure_type ] [a]
> >  // CHECK: [[A_MEM]] = metadata !{metadata [[A_I:![0-9]*]], metadata
> > !{{[0-9]*}}}
> >  // CHECK: [[A_I]] = {{.*}} ; [ DW_TAG_member ] [i] {{.*}} [from int]
> > -// CHECK: ; [ DW_TAG_structure_type ] [b] {{.*}}[fwd]
> > +// CHECK: metadata [[B_MEM:![0-9]*]], i32 0, null, null} ; [
> > DW_TAG_structure_type ] [b]
> > +// CHECK: [[B_MEM]] = metadata !{metadata [[B_J:![0-9]*]]}
> > +// CHECK: [[B_J]] = {{.*}} ; [ DW_TAG_member ] [j] {{.*}} [from int]
> > +// CHECK: ; [ DW_TAG_structure_type ] [c] {{.*}}[fwd]
> >  }
> > Index: lib/CodeGen/CGDebugInfo.cpp
> > ===================================================================
> > --- lib/CodeGen/CGDebugInfo.cpp (revision 184291)
> > +++ lib/CodeGen/CGDebugInfo.cpp (working copy)
> > @@ -615,7 +615,8 @@
> >                                                       llvm::DIFile
> Unit) {
> >    if (DebugKind > CodeGenOptions::LimitedDebugInfo)
> >      return getOrCreateType(PointeeTy, Unit);
> > -  return getOrCreateType(PointeeTy, Unit, true);
> > +  bool Declaration = isa<RecordType>(PointeeTy);
> > +  return getOrCreateType(PointeeTy, Unit, Declaration);
>
> This is not the right fix - we don't want to always emit definitions
> for types referenced via pointers or typedefs in all cases.
>
> Limited debug info (r184252 probably addresses your issue when
> building with -fno-limit-debug-info) is intended to reduce debug info
> by taking steps including emitting only the declaration of types that
> are only used in ways that a definition would not be needed.
>
> In your test case there's no use of the concrete type 'b' - only some
> pointers to 'b' that are never dereferenced. (the test here is "if a
> struct definition were replaced with a declaration, would the code
> still compile" - if that condition is met, then the intent is that
> Clang emit only a declaration for that type when building with
> -flimit-debug-info (which is the default))

I hadn't realised -flimit-debug-info was the default ...silly me!

> Does that make sense?

Yes.

> As for how to fix it - we'd have to look at how to plumb in hooks for
> the debug info to be invoked (to request the full definition of a
> type) when we visit expressions that require the full definition (eg
> in Adrian's r184252 the commented line in the test case that
> dereferences the pointer).

Yes .... this look exactly like the problem that I was wanting to fix .... unfortunately it looks like my attempted fix was a sledge hammer approach. :-)

>
> >  }
> >
> >  llvm::DIType CGDebugInfo::CreatePointerLikeType(unsigned Tag,


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