gdb: don't treat empty enums as flag enums

  In C++ it is possible to use an empty enum as a strong typedef.  For
example, a user could write:

  enum class my_type : unsigned char {};

Now my_type can be used like 'unsigned char' except the compiler will
not allow implicit conversion too and from the native 'unsigned char'
type.

This is used in the standard library for things like std::byte.

Currently, when GDB prints a value of type my_type, it looks like
this:

  (gdb) print my_var
  $1 = (unknown: 0x4)

Which isn't great.  This gets worse when we consider something like:

  std::vector<my_type> vec;

When using a pretty-printer, this could look like this:

  std::vector of length 2, capacity 2 = {(unknown: 0x2), (unknown: 0x4)}

Clearly not great.  This is described in PR gdb/30148.

The problem here is in dwarf2/read.c, we assume all enums are flag
enums unless we find an enumerator with a non-flag like value.
Clearly an empty enum contains no non-flag values, so we assume the
enum is a flag enum.

I propose adding an extra check here; that is, an empty enum should
never be a flag enum.

With this the above cases look more like:

  (gdb) print my_var
  $1 = (unknown: 0x4)

and:

  std::vector of length 2, capacity 2 = {2, 4}

Which look much better.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30148
---
 gdb/dwarf2/read.c                   |  2 ++
 gdb/testsuite/gdb.cp/empty-enum.cc  | 31 +++++++++++++++++++
 gdb/testsuite/gdb.cp/empty-enum.exp | 48 +++++++++++++++++++++++++++++
 3 files changed, 81 insertions(+)
 create mode 100644 gdb/testsuite/gdb.cp/empty-enum.cc
 create mode 100644 gdb/testsuite/gdb.cp/empty-enum.exp


base-commit: 733da2ced8f948b299bdd10b7ff40146095f43f9
  

On Feb 20 2023, Andrew Burgess via Gdb-patches wrote:

> In C++ it is possible to use an empty enum as a strong typedef.  For
> example, a user could write:
>
>   enum class my_type : unsigned char {};
>
> Now my_type can be used like 'unsigned char' except the compiler will
> not allow implicit conversion too and from the native 'unsigned char'
> type.
>
> This is used in the standard library for things like std::byte.
>
> Currently, when GDB prints a value of type my_type, it looks like
> this:
>
>   (gdb) print my_var
>   $1 = (unknown: 0x4)
>
> Which isn't great.  This gets worse when we consider something like:
>
>   std::vector<my_type> vec;
>
> When using a pretty-printer, this could look like this:
>
>   std::vector of length 2, capacity 2 = {(unknown: 0x2), (unknown: 0x4)}
>
> Clearly not great.  This is described in PR gdb/30148.
>
> The problem here is in dwarf2/read.c, we assume all enums are flag
> enums unless we find an enumerator with a non-flag like value.
> Clearly an empty enum contains no non-flag values, so we assume the
> enum is a flag enum.
>
> I propose adding an extra check here; that is, an empty enum should
> never be a flag enum.
>
> With this the above cases look more like:
>
>   (gdb) print my_var
>   $1 = (unknown: 0x4)

I guess this should look different from the output above?
  

Andreas Schwab <schwab@suse.de> writes:

> On Feb 20 2023, Andrew Burgess via Gdb-patches wrote:
>
>> In C++ it is possible to use an empty enum as a strong typedef.  For
>> example, a user could write:
>>
>>   enum class my_type : unsigned char {};
>>
>> Now my_type can be used like 'unsigned char' except the compiler will
>> not allow implicit conversion too and from the native 'unsigned char'
>> type.
>>
>> This is used in the standard library for things like std::byte.
>>
>> Currently, when GDB prints a value of type my_type, it looks like
>> this:
>>
>>   (gdb) print my_var
>>   $1 = (unknown: 0x4)
>>
>> Which isn't great.  This gets worse when we consider something like:
>>
>>   std::vector<my_type> vec;
>>
>> When using a pretty-printer, this could look like this:
>>
>>   std::vector of length 2, capacity 2 = {(unknown: 0x2), (unknown: 0x4)}
>>
>> Clearly not great.  This is described in PR gdb/30148.
>>
>> The problem here is in dwarf2/read.c, we assume all enums are flag
>> enums unless we find an enumerator with a non-flag like value.
>> Clearly an empty enum contains no non-flag values, so we assume the
>> enum is a flag enum.
>>
>> I propose adding an extra check here; that is, an empty enum should
>> never be a flag enum.
>>
>> With this the above cases look more like:
>>
>>   (gdb) print my_var
>>   $1 = (unknown: 0x4)
>
> I guess this should look different from the output above?

Oops.  Yeah, I updated the std::vector example below, but this one
should be:

  (gdb) print my_var
  $1 = 0x4

I've updated the commit message locally.

Thanks,
Andrew
  

>>>>> "Andrew" == Andrew Burgess via Gdb-patches <gdb-patches@sourceware.org> writes:

Andrew> In C++ it is possible to use an empty enum as a strong typedef.  For
Andrew> example, a user could write:

Andrew>   enum class my_type : unsigned char {};

Andrew> Now my_type can be used like 'unsigned char' except the compiler will
Andrew> not allow implicit conversion too and from the native 'unsigned char'
Andrew> type.

...
Andrew> I propose adding an extra check here; that is, an empty enum should
Andrew> never be a flag enum.

Looks good to me, thank you.

Reviewed-By: Tom Tromey <tom@tromey.com>

Tom
  

Tom Tromey <tom@tromey.com> writes:

>>>>>> "Andrew" == Andrew Burgess via Gdb-patches <gdb-patches@sourceware.org> writes:
>
> Andrew> In C++ it is possible to use an empty enum as a strong typedef.  For
> Andrew> example, a user could write:
>
> Andrew>   enum class my_type : unsigned char {};
>
> Andrew> Now my_type can be used like 'unsigned char' except the compiler will
> Andrew> not allow implicit conversion too and from the native 'unsigned char'
> Andrew> type.
>
> ...
> Andrew> I propose adding an extra check here; that is, an empty enum should
> Andrew> never be a flag enum.
>
> Looks good to me, thank you.
>
> Reviewed-By: Tom Tromey <tom@tromey.com>

Pushed.

Thanks,
Andrew