Fix:
Kill one of the const qualifiers.

Targeting to 3.4 to get on the radar...
Could someone who understands 'const' in C and/or DWARF better than me confirm that this behavior is, in fact, a bug?

I can confirm that the behavior is the same in mainline (will be 3.4).

Confirmed.

really confirmed, ICC 6.0 does not produce the extra DW_TAG_const_type and also this 
is not a regression.

One more thing, a type def is not needed:
int const ii[20][20];
will produce the extra DW_TAG_const_type.

Patch :
http://gcc.gnu.org/ml/gcc-patches/2004-02/msg00030.html

See
    http://gcc.gnu.org/ml/gcc-patches/2004-02/msg01911.html
for patch review.

 http://gcc.gnu.org/ml/gcc-patches/2004-02/msg01911.html

seems to be the wrong reference (it's about loop peeling).

Here is the corrected link
http://gcc.gnu.org/ml/gcc-patches/2004-02/msg01995.html
or follow the link to the patch and click on followups.

Created attachment 12702 [details]
PR/8354 patch discussion thread

As reported in Bug #20588, the referenced links to the gcc-patch mailing list archives are broken.  The referenced mail messages are included in this attachment.

I looked into fixing GCC's DWARF 2
generation for types that are qualified (for example, "const"
and "volatile", and in the case of UPC, "shared", "strict",
and "relaxed") in order to ensure that GDB has an accurate
understanding of when a particular object's type is
qualified:
http://sourceware.org/ml/gdb/2006-11/msg00185.html
http://sourceware.org/ml/gdb/2006-12/msg00133.html
http://sourceware.org/ml/gdb/2006-12/msg00177.html

In the latter e-mail reply, copied below, I sketched
out three possible ways to fix the DWARF 2 information
that GCC presently generates for qualified types.

It would be great if someone familiar with the GCC
front-end reviews my analysis, and either corrects it,
or improves upon the suggested approaches.

Additionally, I'm a bit unsure on what
the "correct" DWARF 2 information should look like in
all cases.  For example should the members/fields of
a struct/union that is C-V qualified also be individually
qualified?  One commercial compiler that I tried did not
do this.

Alternatively, would all the possible DWARF 2
type qualifier representation issues best be handled
within GDB?  This would move the problem from GCC back
to GDB, and at the same time improve upon
inter-operability.

--- copy of http://sourceware.org/ml/gdb/2006-12/msg00177.html ---
Jim Blandy wrote: 
>
> "Gary Funck" <gary@intrepid.com> writes:
> > The main difficulty is that GCC doesn't create new qualified
> > types for declarations.  Rather, it sets TREE_READONLY()
> > and TREE_THIS_VOLATILE() in the DECL node for declarations
> > such as:
> >
> >    volatile int A[10];
> 
> Ugh.  That's a shame.  Can't dwarf2out.c fix things up as it builds
> the tree of struct die_structs from the GCC 'tree' type tree?

In theory, it should be possible.  I worked on this a bit, but I think
to do it right, this fix will require contribution/direction from someone
more conversant in the GCC front-end, and more knowledgeable about how
the other language front-ends use both the DWARF 2 generation
routines, and the extent to which they depend upon the type information
remaining in its current form.

Three approaches to fixing the front end to create
appropriate DWARF 2 information come to mind:

(1) Change the GCC front-end, so that when it creates
type information and associates the type information
with a declaration that it fully qualifies the type definition,
in a way that preserves language semantics, yet also ensures
that the correct DWARF 2 information is generated for qualified
types.

(2) Create the fully qualified type definition in dwarf2out.c
(probably in routines that handle DECL items such as
gen_formal_parameter_die(), gen_variable_die() and gen_field_die()).
There are two likely sub-approaches: (i) keep this fully
qualified type definition on the side, parallel to the existing
type definition structure, or (ii) smash the new fully qualified
type into the DECL node's TREE_TYPE() value.  Keeping a
parallel definition may be difficult because various parts
of dwarfout.c may need to refer back to the DECL node's type
value, and all places that do this will have to be found and fixed.
Cross-type references and typedef's create another set of problems.
Rewriting the declaration's type value is the most straightforward,
but runs the risk of violating various assumptions made by the
language front-ends, and will require a rather elaborate "deep copy"
mechanism to make sure the fix up is done correctly.

(3) Run a final pass over the internal DWARF tree built within
dwarfout.c to fix up the type qualifiers, basically propagating
the const_ and volatile_ qualifiers down to lower levels as
required.  This is probably doable, but will slow down DWARF
generation for all compilations, even if most compilations
seldom use "const" and "volatile" (and other qualifiers, such
as UPC's "shared", "strict", and "relaxed").  In this case, it
seems that dwarf2out.c is fixing representation
issues that more correctly should be solved in the front-end.

Given the tradeoffs, choice (1) above, where the type description
is fully qualified at the time the type is bound to the DECL item,
seems more correct, but may impact the correct operation of
the various language front-ends and therefore will require more
care and more study.  Choice (2) is perhaps a bad compromise,
and choice (3) is likely workable, but kludgy.

Jim Wilson posted thi follow up to the GDB list:
http://sourceware.org/ml/gdb/2007-01/msg00007.html

 From: Jim Wilson <wilson at specifix dot com> 
 Date: Mon, 01 Jan 2007 14:15:47 -0800 
 Subject: RE: how to support C type qualifiers applied to arrays? 

On Thu, 2006-12-14 at 12:22 -0800, Gary Funck wrote:
> The main difficulty is that GCC doesn't create new qualified
> types for declarations.  Rather, it sets TREE_READONLY()
> and TREE_THIS_VOLATILE() in the DECL node for declarations
> such as:
>    volatile int A[10];

If you look at the types created by the C front end, they are OK.
c_build_qualified_type knows how to handle an array correctly.

The problem arises in the DWARF2 output code.  gen_type_die calls
type_main_variant for all types other than vector types, which strips
off the const and volatile type modifiers.  Then it clumsily tries to
put them back later in gen_variable_die, except that for array types, it
puts them back in the wrong place.

This seems to answer the question I asked long ago.  Why are we trying
to put back qualifiers from the decl?  Because gen_type_die stripped
them off.  This seems wrong.

If we fix gen_type_die to stop calling type_main_variant, and if we fix
gen_variable_die to stop adding back the type qualifiers, then I get the
right result.  So I think I was on the right track before, we just need
another little change to gen_type_die in addition to what I already
described.

I haven't investigated this in detail yet.  There may be other parts of
the code that expect to see a type main variant here, so we might need
other cascading fixes.  This still seems fixable to me though.

http://sourceware.org/ml/gdb/2007-01/msg00147.html

 From: "Gary Funck" <gary at intrepid dot com> 
 To: <gdb at sourceware dot org> 
 Date: Tue, 9 Jan 2007 09:01:38 -0800 
 Subject: RE: how to support C type qualifiers applied to arrays? 


Jim Wilson wrote (in part):
> This seems to answer the question I asked long ago.  Why are we trying
> to put back qualifiers from the decl?  Because gen_type_die stripped
> them off.  This seems wrong.
>

It seems that the use of type_main_variant()
dates back to the original port of the DWARF 1 generator
to the DWARF 2 implementation.  I didn't try understanding the
DWARF 1 generator in detail, but it seemed that the use of the
type's main variant helped limit the number of unique type dies
that were generated.  In any event, it doesn't appear that there
is a compelling reason to continue to use the type_main_variant()
for DWARF 2 generatiion, especially in light of the fact that
in some cases the DWARF 2 information is incorrect in the current
implmentation.

There are of course the risks that you mentioned
in your reply that there may now be implicit assumptions made
by the various clients (C, C++, Fortran, Ada, for example)
regarding dependencies upon the type_main_variant(), but we
may have to deal with those issues as they arise.
 
> If we fix gen_type_die to stop calling type_main_variant, and 
> if we fix
> gen_variable_die to stop adding back the type qualifiers, 
> then I get the
> right result.  So I think I was on the right track before, we 
> just need
> another little change to gen_type_die in addition to what I already
> described.

I'll give this a try, and follow up.