Pretty print of C / C++ enumerations

[Jonathan Wakely]

This is not the right list. Bug reports should go to bugzilla, and
discussion of changes/fixes to GCC code should go to the gcc mailing
list.
On Wed, 19 Sep 2018 at 13:49, will wray <wjwray@gmail.com> wrote:
>
> Also, the docs need a fix.
> The GCC Internals doc is inconsistent on enumerator value.
>
> GCC Internals https://gcc.gnu.org/onlinedocs/gccint.pdf
> Chapter 11 GENERIC 11.3 Types (p 158 of current pdf):
>
>   ENUMERAL_TYPE
>     Used to represent an enumeration type
>     ... the TREE_VALUE will be an INTEGER_CST giving the value
>      assigned to that constant...
>
>  *This is incorrect - TREE_VALUE is not an INTEGER_CST -
>   this is the bug in gcc enumerator pretty print.*
>
> Chapter 11 GENERIC  11.4 Declarations (p 161)
>
>   CONST_DECL
>     These nodes are used to represent enumeration constants.
>     The value of the constant is given by DECL_INITIAL which
>     will be an INTEGER_CST with the same type as the TREE_TYPE
>     of the CONST_DECL, i.e., an ENUMERAL_TYPE.
>
>  *This is correct - DECL_INITIAL is needed to get the value*
>
> My guess is that DECL_INITIAL was introduced at some point
> but not all the code and docs were updated for the change.
>
>
> On Tue, Sep 18, 2018 at 3:58 PM will wray <wjwray@gmail.com> wrote:
>
> > There's a bug in gcc pretty printing of C / C++ enumerators:
> >
> >   GCC should print the id of an enumerated value
> >   but prints a C-style cast (the fallback for non-enumerated values)
> >
> > The bug shows up in __PRETTY_FUNCTION__ output, for instance.
> > You can see it in the error output of this Compiler Explorer example
> > https://godbolt.org/z/1df0Sk
> >
> >   enum e { a, b, c=0 };
> >   template <auto> struct wauto; // deliberately incomplete to trigger..
> >   wauto<a> v; // error: aggregate 'wauto<(e)0> v' has incomplete type..
> >
> > The error output should print 'a' in place of '(e)0'
> >
> > (Clang gets it right with 'wauto<a>',
> >  MSVC wrongly prints 'wauto<0>' so also loses type information.)
> >
> > This mail shows a one-line hack to fix the bug... almost...
> > I'd like advice or assistance towards a proper fix.
> >
> > The function is ~30 lines of code so I paste it after an intro.
> >
> > The intent of the code is to print, for a given integral constant:
> >
> >    - The enum id of the first enumerator with that value, or
> >    - a C-style cast if no enumerator is found with that value.
> >
> > In  c-pretty-print.c
> >     c_pretty_printer::constant calls pp_c_enumeration_constant
> >
> > /******************** pp_c_enumeration_constant ****************/
> >
> > /* Attempt to print out an ENUMERATOR. Return true on success.
> >    Else return false; that means the value was obtained by a cast,
> >    in which case print out the type-id part of the cast-expression
> >    -- the casted value is then printed by pp_c_integer_literal. */
> >
> > static bool
> > pp_c_enumeration_constant (c_pretty_printer *pp, tree e)
> > {
> >   bool value_is_named = true;
> >   tree type = TREE_TYPE (e);
> >   tree value;
> >
> >   /* Find the name of this constant. */
> >   for (value = TYPE_VALUES (type);
> >        value != NULL_TREE && !tree_int_cst_equal (TREE_VALUE (value), e);
> >        value = TREE_CHAIN (value))
> >     ;
> >
> >   if (value != NULL_TREE)
> >     pp->id_expression (TREE_PURPOSE (value));
> >   else
> >   {
> >     /* Value must have been cast. */
> >     pp_c_type_cast (pp, type);
> >     value_is_named = false;
> >   }
> >
> >   return value_is_named;
> > }
> > /***************************************************************/
> > The code iterates over the enumerators, comparing with the given value,
> > prints the id if found else falls back to print the C-style cast.
> > However, the comparison always fails so it always prints the cast
> > (after iterating over all the enumerators) and never the id.
> >
> > The comparison in the enumeration loop can be fixed by adding DECL_INITIAL
> > to 'unwrap' the enumerator's value so it compares correctly as INTEGER_CST,
> > terminating the loop and pretty-printing the id instead of the C-style
> > cast.
> >
> >   /* Find the name of this constant. */
> >   for (value = TYPE_VALUES (type);
> >        value != NULL_TREE && !tree_int_cst_equal (DECL_INITIAL( TREE_VALUE
> > (value)), e);
> >                                                   ^^^^^^^^^^^^^
> >        ^
> >        value = TREE_CHAIN (value))
> >      ;
> >
> > With this patch, the loop exits early when it finds an enumerator of the
> > given value and prints the found-enumerator's id via:
> >
> >   if (value != NULL_TREE)
> >     pp->id_expression (TREE_PURPOSE (value));
> >
> > However...
> > This code is located in c-pretty-print.c
> > and has not been updated to deal with C++11 scoped enums.
> >
> > I can share a more complete fix that duplicates c-pretty-print.c
> > code to cxx-pretty-print.c and modifies it for scoped enums.
> > I'm learning as I hack but would like guidance on a proper fix
> > from anyone more familiar with gcc pretty print and/or grammar -
> > the guideline comment, at the top of the file, states:
> >
> > /* The pretty-printer code is primarily designed to closely follow
> >    (GNU) C and C++ grammars... */
> >
> > Another thought.
> > The pretty-printing code will need an overhaul for C++20 with
> > the proposed changes for generalised non-type template args.
> >
> > Thanks for any pointers.
> >
> >