[Issue 2005] New: dynamic va_arg looping/ align info in TypeInfo

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=2005

           Summary: dynamic va_arg looping/ align info in TypeInfo
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: spec
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla at digitalmars.com
        ReportedBy: fawzi at gmx.ch


Looping over va_arg that span a set of types that is unknown is error prone.

In dmd this seem to work because va_list is a void* and one can simply add the
size of the actual type to go to the next.
Actually such a method does not keep into account alignment issues, and no
alignment information is present in TypeInfo.

A clean way to solve this would be to add a function like

   void *dyn_va_arg(TypeInfo, inout va_list)

that returns a pointer to the actual argument and updates va_list.

If one wants to allow even more flexibility in the implementation of va_list
then the function should be

  void *dyn_va_arg_tmp(TypeInfo, inout va_list, out bool);

and the pointer returned by this functions would be invalided at the next call
of va_arg if bool says so.

Such functions could be readily implemented for dmd if typeinfo would give
access to the alignment information of its type (.talign?).

Also for gdc it should be possible to implement this function (at least the
second one), and it would solve or at least alleviate the much more serious
issues that gdc has in that department, something that I proposed in
http://d.puremagic.com/issues/show_bug.cgi?id=2004

Another possibility would be to change _args to a void*[] in D2, this would
also solve all the issues, but would add some overhead that the actual
implementation does not have.


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