[lldb-dev] [RFC][PATCH] Keep un-canonicalized template types in the debug information
Robinson, Paul
Paul_Robinson at playstation.sony.com
Tue Sep 23 14:24:17 PDT 2014
> One of the issues is that there's only /so/ different we can be from
> GCC here before types/declarations/definitions won't match up in GDB.
I think this might get to the nub of it: I agree that GCC/GDB matters, I disagree that GCC/GDB is what matters. GCC/GDB compatibility may be an important use-case but it is not the Reference Implementation of DWARF, and in particular GCC/GDB compatibility is completely irrelevant to my environment. My environment is 100% Clang, and we care more about what the DWARF spec says than we do about whatever GCC/GDB might choose to do for one reason or another. So, if GCC/GDB compatibility means diverging so noticeably from what the spec says (i.e., that the name is as it is in the source program) maybe this is a point worth identifying as one where a divergence occurs, and make the choice target-dependent.
In a way it feels somewhat analogous to choices in supporting extensions/dialects of C++. For practical purposes it's very worthwhile to the community to support things that GCC supports, but that doesn't mean that GCC defines the standard. In the case at hand, Clang has strayed from the letter of the DWARF spec, and we'd really like to see a way back toward it.
We're entirely willing to do work toward getting things realigned (admittedly I personally have been mostly MIA for the past year, but I am seeing an occasional photon from down the far end of my current tunnel) given that the primary contributor and code owner are willing to go along with it.
Thanks,
--paulr
From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Monday, September 22, 2014 3:47 PM
To: Nick Lewycky
Cc: Robinson, Paul; lldb-dev at cs.uiuc.edu; llvm-commits at cs.uiuc.edu
Subject: Re: [lldb-dev] [RFC][PATCH] Keep un-canonicalized template types in the debug information
On Mon, Sep 22, 2014 at 3:40 PM, Nick Lewycky <nicholas at mxc.ca<mailto:nicholas at mxc.ca>> wrote:
Robinson, Paul wrote:
I think it comes down to how the information is planning to be used. A
consumer with the dwarf information today could, in fact, get to the
S<int> type from a user who types S<A> pretty easily right?
If the typedef actually appears in the DWARF, the consumer could figure
out what the user meant by typing S<A>, yes. In my experiments the
typedef is not always present, which leaves the user up a creek with no
paddle.
How the debugger presents the types of things is also a consideration,
however. This is more evident with a less trivial example, such as the
vector typedef I described previously. It is clearly a step backward in
the end-user debugging experience if people are used to seeing
S<int4>
which the debugger has been displaying all along, but suddenly they
start seeing instead
S<int __attribute__((ext_vector_type(4)))>
which is what has started happening. Especially if 'int4' no longer
appears as a typedef at all, this is Just Wrong.
In clang, ConvertTypeToDiagnosticString deals with vectors specially. The rationale, I think, is to prevent the compiler from showing the internal implementation detail of how float4 and friends are defined. I think that this is the wrong approach and would have preferred a second attribute. Does attribute nodebug on a typedef have any meaning yet? Could we repurpose it to mean that you shouldn't look through this typedef for compiler diagnostics nor debug info? Any any case, our behaviour on diagnostics and debug info should probably match here.
One of the issues is that there's only /so/ different we can be from GCC here before types/declarations/definitions won't match up in GDB. I believe GCC has some smarts to tolerate differences like S<0> versus S<0u> or S<'\0'> I think... at least some of those, but I don't know how it'll go with:
S<__attribute__((__vector_size__(4 * sizeof(int)))) int>
V
S<__vector(4) int>
(using a GCC-compatible syntax, vector_size(sizeof(int) * 4) rather than the ext_vector_type which isn't supported by GCC)
Huh... apparently GDB ignores the entire adornment and allows func(S<__vector(4) int>) to be called with a variable of type S<__attribute__((__vector_size__(5 * sizeof(int)))) int> even... not sure what to make of any of that.
Nick
Wolfgang did some bisection and traced this change to r205447, and the
intent of that change was centered on default template arguments. This
de-referencing of typedefs appears to have been an *unintended side
effect* of that patch.
I want my typedef'd template parameters back please…
--paulr
*From:*Eric Christopher [mailto:echristo at gmail.com<mailto:echristo at gmail.com>]
*Sent:* Thursday, September 18, 2014 10:07 PM
*To:* Robinson, Paul
*Cc:* David Blaikie; lldb-dev at cs.uiuc.edu<mailto:lldb-dev at cs.uiuc.edu>; llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu>;
Frédéric Riss
*Subject:* Re: [lldb-dev] [RFC][PATCH] Keep un-canonicalized template
types in the debug information
On Thu, Sep 18, 2014 at 5:57 PM, Robinson, Paul
<Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>
<mailto:Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>>> wrote:
The (limited) feedback I've had from the committee is along these lines.
If the program uses the type name "S<A>" for something, the DWARF
should fully describe the type named "S<A>" because that's the name
as-in-the-source-program. If you use both S<A> and S<int> in the
program in different places, then you need to describe both in the
DWARF. There is sadly no standard way to associate the two as
aliases. Yes in C++ they are the same; in standard DWARF they are not.
Yeah, I'm not sure I agree with this. I've seen the thread and I'm not
sure I like the logic.
The typedef S<A> => S<int> hack might work [if the debugger can
tolerate that]. It is obviously not a real typedef. You could mark
it artificial as an indication that something funny is going on
(artificial typedefs being highly atypical).
The DW_AT_specification hack is just wrong, because neither S<A> nor
S<int> is completing the other.
I need to step back from the typedef hack. I believe our debugger
throws away the <brackets> on the theory that it can reconstruct
them from template-parameter children; that is, the <bracket> part
of the name is redundant. The typedef hack does not provide those
children, and the <brackets> are not redundant, so this is likely to
be a problem for us. Feh. I'd forgotten about that detail when I
started liking the typedef hack. Yes, this means I don't have a
suggestion, apart from emitting things redundantly as needed to
preserve as-in-the-source-program.
Here's a bizarre data point. Going back to at least 3.2, Clang has
emitted S<int> instead of S<A>. But with my vector example, it used
to use the typedef name up through 3.4. That changed in 3.5, where
the type name 'int4' has entirely disappeared from the DWARF.
Clearly that's a bug; the type name needs to be in there somewhere.
One more thing:
it'd be good to figure out how to deal with all possible names for
the type, even the ones the user hasn't written (eg: typedef int A;
typedef int B; and make sure that the debugger can handle S<int>,
S<A> and S<B> in their code, even though the user only wrote one of
those in the source).
The answer to this "how to deal" question is with debugger smarts,
not more complicated DWARF. DWARF is about the program as-written
and as-compiled, not about
anything-the-user-might-conceivably-try-to-write-in-the-debugger.
Handling this in DWARF is a combinatorial nightmare, for completely
speculative purposes. Not gonna happen.
I think it comes down to how the information is planning to be used. A
consumer with the dwarf information today could, in fact, get to the
S<int> type from a user who types S<A> pretty easily right? Now if you'd
like a way to print out the textual representation of every type as it
was used in the program that's likely to be less possible without some
serious duplication of dwarf. You could use an unnamed type for the base
and then use DW_AT_specification with just a bare DW_AT_name to avoid
some of the unpleasantness of the specification hack, but then you come
to the problem of template arguments etc. It's fairly crazy to consider,
but a user could quite easily write:
new std::vector<int, allocator>()
with some allocator that was never used in the program with vector and
expect the code to be generated at run time and the rest of the type to
be found.
Anyhow, I think the best bet is for the most general type to be left in
the debug information and then the typedefs etc to be their own DIEs.
Unless we have some use that we're not talking about here?
-eric
--paulr
*From:*David Blaikie [mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>
<mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>>]
*Sent:* Thursday, September 18, 2014 4:03 PM
*To:* Robinson, Paul
*Cc:* llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu> <mailto:llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu>>;
Greg Clayton; Frédéric Riss; lldb-dev at cs.uiuc.edu<mailto:lldb-dev at cs.uiuc.edu>
<mailto:lldb-dev at cs.uiuc.edu<mailto:lldb-dev at cs.uiuc.edu>>; jingham at apple.com<mailto:jingham at apple.com>
<mailto:jingham at apple.com<mailto:jingham at apple.com>>
*Subject:* Re: [lldb-dev] [RFC][PATCH] Keep un-canonicalized
template types in the debug information
On Thu, Sep 18, 2014 at 2:40 PM, Robinson, Paul
<Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>
<mailto:Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>>> wrote:
David,
Sorry, thought you were protesting the typedef idea as interfering
with deduplication or type-unit commonality.
So to recap, if we have source like this:
typedef int A;
template<typename T> struct S { T member; };
S<A> s_a;
then we'll get
DW_TAG_typedef
DW_AT_name "A"
DW_AT_type -> int
DW_TAG_structure_type
DW_AT_name "S<A>"
DW_TAG_member
DW_AT_name "member"
DW_AT_type -> int // or the typedef for "A" ?
DW_TAG_template_type_parameter
DW_AT_name "T"
DW_AT_type -> (the typedef for "A")
Are you suggesting putting the rest of S<int> here too? Or how would
S<A> refer to S<int> for the rest of the implementation?
DW_TAG_variable
DW_AT_name "s_a"
DW_AT_type -> (the above structure_type)
Ah, no - just a typedef of the template:
1: DW_TAG_structure_type // the debug info we already produce today
(S<int>)
...
2: DW_TAG_typedef
DW_AT_name "S<A>"
DW_AT_type (1)
And honestly, the variable would still be of type (1).
Duplicating the entire type for each way of naming the same type is,
I'm fairly sure, not going to work for debuggers today. If someone
wants to propose a way of encoding this that will need new
code/support from debuggers, etc, then I feel the right venue to
discuss that is the DWARF committee - because you'll need buy-in
from producers and consumers. Without having that discussion, I
believe just providing a typedef of the template specialization is
probably a benefit to users.
If we want to talk about a 'right' representation of this for DWARF
that would necessitate more substantial changes to both DWARF
producers and consumers... I think it'll be a bit more involved than
even what you're proposing. If we're going to deal with that, it'd
be good to figure out how to deal with all possible names for the
type, even the ones the user hasn't written (eg: typedef int A;
typedef int B; and make sure that the debugger can handle S<int>,
S<A> and S<B> in their code, even though the user only wrote one of
those in the source).
Yes?
--paulr
*From:*David Blaikie [mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>
<mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>>]
*Sent:* Thursday, September 18, 2014 1:09 PM
*To:* Robinson, Paul
*Cc:* llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu>
<mailto:llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu>>; Greg Clayton; Frédéric Riss;
lldb-dev at cs.uiuc.edu<mailto:lldb-dev at cs.uiuc.edu> <mailto:lldb-dev at cs.uiuc.edu<mailto:lldb-dev at cs.uiuc.edu>>;
jingham at apple.com<mailto:jingham at apple.com> <mailto:jingham at apple.com<mailto:jingham at apple.com>>
*Subject:* Re: [lldb-dev] [RFC][PATCH] Keep un-canonicalized
template types in the debug information
On Thu, Sep 18, 2014 at 1:05 PM, David Blaikie
<dblaikie at gmail.com<mailto:dblaikie at gmail.com> <mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>>> wrote:
On Thu, Sep 18, 2014 at 1:00 PM, Robinson, Paul
<Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>
<mailto:Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>>> wrote:
>From: David Blaikie [mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>
<mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>>]
>On Wed, Sep 17, 2014 at 11:54 AM, Robinson, Paul
<Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>
<mailto:Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>>> wrote:
>> From: David Blaikie [mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>
<mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>>]
>> > On Wed, Sep 17, 2014 at 7:45 AM, Robinson, Paul
<Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>
<mailto:Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>>> wrote:
>> > > From: David Blaikie [mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>
<mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>>]
>> > > On Sat, Sep 13, 2014 at 6:54 PM, Robinson, Paul
<Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>
<mailto:Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>>> wrote:
>> > > > > > On 09 Sep 2014, at 00:01, jingham at apple.com<mailto:jingham at apple.com>
<mailto:jingham at apple.com<mailto:jingham at apple.com>> wrote:
>> > > > > > >
>> > > > > > > From the debugger's standpoint, the functional
concern is that if you do
>> > > > > > something more real, like:
>> > > > > > >
>> > > > > > > typedef int A;
>> > > > > > > template <typename T>
>> > > > > > > struct S
>> > > > > > > {
>> > > > > > > T my_t;
>> > > > > > > };
>> > > > > > >
>> > > > > > > I want to make sure that the type of my_t is
given as "A" not as "int".
>> > > > > > The reason for that is that it is not uncommon to
have data formatters
>> > > > > > that trigger off the typedef name. This happens
when you use some common
>> > > > > > underlying type like "int" but the value has some
special meaning when it
>> > > > > > is formally an "A", and you want to use the data
formatters to give it an
>> > > > > > appropriate presentation. Since the data
formatters work by matching type
>> > > > > > name, starting from the most specific on down, it
is important that the
>> > > > > > typedef name be preserved.
>> > > > > > >
>> > > > > > > However, it would be really odd to see:
>> > > > > > >
>> > > > > > > (lldb) expr -T -- my_s
>> > > > > > > (S<int>) $1 = {
>> > > > > > > (A) my_t = 5
>> > > > > > > }
>> > > > > > >
>> > > > > > > instead of:
>> > > > > > >
>> > > > > > > (lldb) expr -T -- my_s
>> > > > > > > (S<A>) $1 = {
>> > > > > > > (A) my_t = 5
>> > > > > > > }
>> > > > > > >
>> > > > > > > so I am in favor of presenting the template
parameter type with the most
>> > > > > > specific name it was given in the overall template
type name.
>> > > > > >
>> > > > > > OK, we get this wrong today. I’ll try to look into it.
>> > > > > >
>> > > > > > What’s your take on the debug info representation
for the templated class
>> > > > > > type? The tentative patch introduces a typedef
that declares S<A> as a
>> > > > > > typedef for S<int>. The typedef doesn’t exist in
the code, thus I find it
>> > > > > > a bit of a lie to the debugger. I was more in
favour of something like :
>> > > > > >
>> > > > > > DW_TAG_variable
>> > > > > > DW_AT_type: -> DW_TAG_structure_type
>> > > > > > DW_AT_name: S<A>
>> > > > > > DW_AT_specification: -> DW_TAG_structure_type
>> > > > > > DW_AT_name: S<int>
>> > > > > >
>> > > > > > This way the canonical type is kept in the debug
information, and the
>> > > > > > declaration type is a real class type aliasing the
canonical type. But I’m
>> > > > > > not sure debuggers can digest this kind of aliasing.
>> > > > > >
>> > > > > > Fred
>> > > > >
>> > > > > Why introduce the extra typedef? S<A> should have a
template parameter
>> > > > > entry pointing to A which points to int. The info
should all be there
>> > > > > without any extra stuff. Or if you think something
is missing, please
>> > > > > provide a more complete example.
>> > > > My immediate concern here would be either loss of
information or bloat
>> > > > when using that with type units (either bloat because
each instantiation
>> > > > with differently spelled (but identical) parameters is
treated as a separate
>> > > > type - or loss when the types are considered the same
and all but one are
>> > > > dropped at link time)
>> > > You'll need to unpack that more because I'm not
following the concern.
>> > > If the typedefs are spelled differently, don't they
count as different types?
>> > > DWARF wants to describe the program as-written, and
there's no S<int> written
>> > > in the program.
>> > >
>> > > Maybe not in this TU, but possibly in another TU? Or by
the user.
>> > >
>> > > void func(S<int>);
>> > > ...
>> > > typedef int A;
>> > > S<A> s;
>> > > func(s); // calls the same function
>> > >
>> > > The user probably wants to be able to call void func
with S<int> or S<A>
>> > Sure.
>> >
>> > > (and, actually, in theory, with S<B> where B is another
typedef of int, but
>> > > that'll /really/ require DWARF consumer support and/or
new DWARF wording).
>> >
>> > Not DWARF wording. DWARF doesn't say when you can and
can't call something;
>> > that's a debugger feature and therefore a debugger decision.
>> >
>> What I mean is we'd need some new DWARF to help explain
which types are
>> equivalent (or the debugger would have to do a lot of
spelunking to try
>> to find structurally equivalent types - "S<B>" and "S<A>",
go look through
>> their DW_TAG_template_type_params, see if they are typedefs
to the same
>> underlying type, etc... )
>> >
>> >
>> > > We can't emit these as completely independent types - it
would be verbose
>> > > (every instantiation with different typedefs would be a
whole separate type
>> > > in the DWARF, not deduplicated by type units, etc) and wrong
>> >
>> > Yes, "typedef int A;" creates a synonym/alias not a new
type, so S<A> and S<int>
>> > describe the same type from the C++ perspective, so you
don't want two complete
>> > descriptions with different names, because that really
would be describing them
>> > as separate types. What wrinkles my brow is having S<int>
be the "real"
>> > description even though it isn't instantiated that way in
the program. I wonder
>> > if it should be marked artificial... but if you do
instantiate S<int> in another
>> > TU then you don't want that. Huh. It also seems weird to
have this:
>> > DW_TAG_typedef
>> > DW_AT_name "S<A>"
>> > DW_AT_type -> S<int>
>> > but I seem to be coming around to thinking that's the most
viable way to have
>> > a single actual instantiated type, and still have the
correct names of things
>*mostly* correct; this still loses "A" as the type of the data
member.
>
>For the DW_TAG_template_type_parameter, you mean? No, it wouldn't.
>
> (as a side note, if you do actually have a data member (or
any other mention) of
>the template parameter type, neither Clang nor GCC really get
that 'right' -
>"template<typename T> struct foo { T t; }; foo<int> f;" - in
both Clang and GCC,
>the type of the 't' member of foo<int> is a direct reference
to the "int" DIE, not
>to the DW_TAG_template_type_parameter for "T" -> int)
Huh. And DWARF doesn't say you should point to the
template_type_parameter...
I thought it did, but no. Okay, so nothing is lost, but it feels
desirable
to me, that uses of the template parameter should cite it in the
DWARF as well.
But I guess we can leave that part of the debate for another time.
>
>Crud.
>But I haven't come up with a way to get that back without
basically instantiating
>S<A> and S<int> separately.
>
>> >
>> Yep - it's the only way I can think of giving this
information in a way that's
>> likely to work with existing consumers. It would probably be
harmless to add
>> DW_AT_artificial to the DW_TAG_typedef, if that's any help
to any debug info
>> consumer.
>
>Hmmm no, S<A> is not the artificial name;
>
>It's not the artificial name, but it is an artificial typedef.
If the source only says S<A>, then the entire S<int> description
is artificial,
because *that's not what the user wrote*. So both the typedef
and the class type
are artificial. Gah. Let's forget artificial here.
>
>some debuggers treat DW_AT_artificial
>as meaning "don't show this to the user."
>
>In some sense that's what I want - we never wrote the typedef
in the source
>so I wouldn't want to see it rendered in the "list of
typedefs" (or even
>probably in the list of types, maybe).
>
>But S<A> is the name we *do* want to
>show to the user.
>
>Maybe. Sometimes. But there could be many such aliases for the
type. (& many
>more that were never written in the source code, but are still
valid in the
>source language (every other typedef of int, every other way
to name the int
>type (decltype, etc)))
But you *lose* cases where the typedef is the *same*
*everywhere*. And in
many cases that typedef is a valuable thing, not the trivial
rename we've
been bandying about. This is a more real example:
typedef int int4 __attribute__((ext_vector_type(4)));
template<typename T> struct TypeTraits {};
template<>
struct TypeTraits<int4> {
static unsigned MysteryNumber;
};
unsigned TypeTraits<int4>::MysteryNumber = 3U;
Displaying "TypeTraits<int __attribute__((ext_vector_type(4)))>"
is much
worse than "TypeTraits<int4>" (and not just because it's shorter).
More to the point, having the debugger *complain* when the user says
something like "ptype TypeTraits<int4>" is a problem.
Reducing debug-info size is a worthy goal, but don't degrade the
debugging
experience to get there.
I'm not sure which part of what I've said seemed like a
suggestion to degrade the debugging experience to minimize debug
info size (the proposition that we should use a typedef or other
alias on top of the canonical type? It wouldn't cause "ptype
TypeTraits<int4>" to complain - indeed for GDB ptyping a typedef
gives /exactly/ the same output as if you ptype the underlying
type - it doesn't even mention that there's a typedef involved:
typedef fooA foo<int>;
(keyboard shortcuts are hard - accidentally sent before I finished)
(gdb) ptype fooA
type = struct foo<int> [with T = int] {
<no data fields>
}
But in any case, I think what I'm saying boils down to:
Short of changing debug info consumers, I think the only thing
we can do is DW_TAG_typedef. That'll work for existing consumers.
Anything else will need possibly new DWARF wording, or at least
an agreement between a variety of debug info consumers and
producers that some new cliche/use of existing DWARF be used to
describe these situations.
I could be wrong - if someone wants to try prototyping the
DW_TAG_structure_type proposal Fred had and see if existing
debuggers work with that, sure.
I'm not opposed to someone coming up with a standardizable more
descriptive form than DW_TAG_typedef, but that conversation
probably needs to happen with the DWARF Committee more than the
LLVM community.
- David
--paulr
>
>
>> That said, I'm not opposed to proposing something to
DWARF to define some more
>> 'proper' way to describe this.
>
>Yah. I've been thinking about the DW_AT_specification
idea too, which would be
>something like this:
> DW_TAG_class_type
> DW_AT_name "S<A>"
> DW_AT_specification -> S<int>
>
> DW_TAG_template_type_parameter
> DW_AT_name "T"
> DW_AT_type -> A
>
>The problem with this is you don't know where T is
used in the template, so
>you *still* don't know when to use A as the type of
"field". Also it's kind
>of an abuse of DW_AT_specification. If we can't get A
as the type of "field"
>then the typedef is more straightforward and
understandable.
>
>It's still a lot of DWARF to emit for every way the
user has named the template
>& I'm not sure how much value it provides - are there
use cases you have in mind
>that would benefit from the increased fidelity of
knowing which template argument
>corresponds to the way the user wrote the type.
>
> (& what would we emit if the user named the type in
some other more exotic way:
>int func(); template<typename T> struct S { }; ...
S<decltype(func())> s; )
>
>
>Maybe I'll pop a note to the DWARF committee for a
broader set of opinions.
>
>>
>> One other open question is then, when, if ever, to
reference the DW_TAG_typedef
>> rather than the underlying type? Do we just
reference it whenever the user
>> writes using that name?
>>
>> void f(S<A>);
>> ...
>> void f(S<B>) { ... }
>>
>> etc... (this would be just as possible/we could
maybe treat it the same as if
>> the user wrote "void f(A); ... void f(B) { ... }")
>
>That's what I would do, and I think is more conformant
to the DWARF spec.
>--paulr
>
>>
>> > (because DWARF is all about the name "as it
appears in the source program.")
>> >
>> > > (the debugger wouldn't know these are actually
the same type so wouldn't
>> > > allow function calls, etc).
>> > >
>> > > - David
>> > >
>> > > >
>> > > >
>> > > > > Jim
>> > > > >
>> > > >> On Sep 8, 2014, at 12:38 PM, Frédéric Riss
<friss at apple.com<mailto:friss at apple.com> <mailto:friss at apple.com<mailto:friss at apple.com>>> wrote:
>> > > >>
>> > > >>
>> > > >>> On 08 Sep 2014, at 19:31, Greg Clayton
<gclayton at apple.com<mailto:gclayton at apple.com> <mailto:gclayton at apple.com<mailto:gclayton at apple.com>>> wrote:
>> > > >>>
>> > > >>> This means you will see "S<A>" as the type
for your variables in the
>> > > debugger when you view variables or children of
structs/unions/classes. I
>> > > think this is not what the user would want to
see. I would rather see
>> > > "S<int>" as the type for my variable than see
"S<A>”.
>> > > >>
>> > > >> I find it more accurate for the debugger to
report what has actually
>> > > been put in the code. Moreover when a typedef is
used, it’s usually to
>> > > make things more readable not to hide
information, thus I guess it would
>> > > usually be as informative while being more
compact. The debugger needs to
>> > > have a way to describe the real type behind the
abbreviated name though,
>> > > we must not have less information compared to
what we have today.
>> > > >>
>> > > >> Another point: this allows the debugger to
know what S<A> actually is.
>> > > Without it, the debugger only knows the
canonical type. This means that
>> > > currently you can’t copy/paste a piece of code
that references that kind
>> > > of template names and have it parse correctly. I
/think/ that having this
>> > > information in the debug info will allow more of
this to work.
>> > > >>
>> > > >> But we can agree to disagree :-) It would be
great to have more people
>> > > chime and give their opinion.
>> > > >>
>> > > >> Fred
>> > > >>
>> > > >>>> On Sep 5, 2014, at 4:00 PM, Adrian Prantl
<aprantl at apple.com<mailto:aprantl at apple.com> <mailto:aprantl at apple.com<mailto:aprantl at apple.com>>> wrote:
>> > > >>>>
>> > > >>>>
>> > > >>>>> On Sep 5, 2014, at 3:49 PM, Eric
Christopher <echristo at gmail.com<mailto:echristo at gmail.com> <mailto:echristo at gmail.com<mailto:echristo at gmail.com>>>
>> > > wrote:
>> > > >>>>>
>> > > >>>>>
>> > > >>>>>
>> > > >>>>>
>> > > >>>>> On Fri, Sep 5, 2014 at 3:43 PM, Duncan P.
N. Exon Smith
>> > > <dexonsmith at apple.com<mailto:dexonsmith at apple.com>
<mailto:dexonsmith at apple.com<mailto:dexonsmith at apple.com>>> wrote:
>> > > >>>>>
>> > > >>>>>> On 2014 Sep 5, at 16:01, Frédéric Riss
<friss at apple.com<mailto:friss at apple.com> <mailto:friss at apple.com<mailto:friss at apple.com>>> wrote:
>> > > >>>>>>
>> > > >>>>>> I couldn’t even find a subject expressing
exactly what this patch
>> > > is about… First of all, it’s meant to start a
discussion, and I’m not
>> > > proposing it for inclusion right now.
>> > > >>>>>>
>> > > >>>>>> The issue I’m trying to address is that
template types are always
>> > > canonicalized when emitted in the debug
information (this is the desugar()
>> > > call in UnwrapTypeForDebugInformation).
>> > > >>>>>>
>> > > >>>>>> This means that if the developer writes:
>> > > >>>>>>
>> > > >>>>>> typedef int A;
>> > > >>>>>> template <typename T>
>> > > >>>>>> struct S {};
>> > > >>>>>>
>> > > >>>>>> S<A> var;
>> > > >>>>>>
>> > > >>>>>> The variable var will have type S<int>
and not S<A>. In this simple
>> > > example, it’s not that much of an issue, but for
heavily templated code,
>> > > the full expansion might be really different
from the original
>> > > declaration.
>> > > >>>>>>
>> > > >>>>>> The attached patch makes us emit an
intermediate typedef for the
>> > > variable’s type:
>> > > >>>>>>
>> > > >>>>>> 0x0000002a: DW_TAG_variable [2]
>> > > >>>>>> DW_AT_name [DW_FORM_strp] (
>> > > .debug_str[0x00000032] = “var")
>> > > >>>>>> DW_AT_type [DW_FORM_ref4] (cu + 0x0040 =>
>> > > {0x00000040})
>> > > >>>>>> DW_AT_external [DW_FORM_flag] (0x01)
>> > > >>>>>> DW_AT_decl_file [DW_FORM_data1] (0x01)
>> > > >>>>>> DW_AT_decl_line [DW_FORM_data1] (8)
>> > > >>>>>> DW_AT_location [DW_FORM_block1] (<0x09>
03 70 6c 00 00
>> > > 00 00 00 00 )
>> > > >>>>>>
>> > > >>>>>> 0x00000040: DW_TAG_typedef [3]
>> > > >>>>>> DW_AT_type [DW_FORM_ref4] (cu + 0x004b =>
>> > > {0x0000004b})
>> > > >>>>>> DW_AT_name [DW_FORM_strp] (
>> > >.debug_str[0x00000035] = “S<A>")
>> > > >>>>>> DW_AT_decl_file [DW_FORM_data1] (0x01)
>> > > >>>>>> DW_AT_decl_line [DW_FORM_data1] (6)
>> > > >>>>>>
>> > > >>>>>> 0x0000004b: DW_TAG_structure_type [4] *
>> > > >>>>>> DW_AT_name [DW_FORM_strp] (
>> > >.debug_str[0x0000003e] = “S<int>")
>> > > >>>>>> DW_AT_byte_size [DW_FORM_data1] (0x01)
>> > > >>>>>> DW_AT_decl_file [DW_FORM_data1] (0x01)
>> > > >>>>>> DW_AT_decl_line [DW_FORM_data1] (6)
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>> Which basically is what I want, although
I don’t like that it
>> > > introduces a typedef where there is none in the
code. I’d prefer that to
>> > > be:
>> > > >>>>>>
>> > > >>>>>> DW_TAG_variable
>> > > >>>>>> DW_AT_type: -> DW_TAG_structure_type
>> > > >>>>>> DW_AT_name: S<A>
>> > > >>>>>> DW_AT_specification: -> DW_TAG_structure_type
>> > > >>>>>> DW_AT_name: S<int>
>> > > >>>>>> …
>> > > >>>>>>
>> > > >>>>>> The patch also has the nice property of
omitting the defaulted
>> > > template arguments in the first level typedef.
For example you get
>> > > vector<A> instead of vector<int,
std::__1::allocator<int> >.
>> > > >>>>>
>> > > >>>>> If you specify `vector<int>` in C++ do you
get that instead of
>> > > >>>>> `vector<int, std::__1::allocator<int>>`?
>> > > >>>>>
>> > > >>>>> Yeah, I mentioned this as possibly causing
problems with debuggers
>> > > or other consumers, but I don't have any proof
past "ooooo scary!”.
>> > > >>>>
>> > > >>>> Well, [+lldb-dev], could this confuse
debuggers? :-)
>> > > >>>>
>> > > >>>> -- adrian
>> > > >>>>>
>> > > >>>>> That said, I like the idea personally :)
>> > > >>>>>
>> > > >>>>> -eric
>> > > >>>>>
>> > > >>>>>
>> > > >>>>>> Now there is one thing I really don’t
like about the patch. In
>> > > order not to emit typedefs for types that don’t
need it, I use string
>> > > comparison between the desugared and the
original type. I haven’t
>> > > quantified anything, but doing the construction
of the type name for every
>> > > template type and then comparing it to decide to
use it or not seems like
>> > > a big waste.
>> > > >>>>>
>> > > >>>>> Maybe someone on cfe-dev knows a better way.
>> > > >>>>>
>> > > >>>>>>
>> > > >>>>>> Thoughts?
>> > > >>>>>>
>> > > >>>>>> <template-arg-typedefs.diff>
>> > > >>>>>>
>> > > >>>>>> Fred
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>>
_______________________________________________
>> > > >>>>>> llvm-commits mailing list
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>> > > >>>>>>
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>> > > >>>>>
>> > > >>>>>
>> > > >>>>
>> > > >>>> _______________________________________________
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>> > > >>>>
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>> > > >>>
>> > > >>
>> > > >>
>> > > >> _______________________________________________
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>> > > >>
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>> > >
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