Re: Compilation of object creation in C++

[Andrew Pinski <pinskia at gmail dot com>]

On Wed, Aug 19, 2015 at 7:16 PM, Uday P. Khedker <uday@cse.iitb.ac.in> wrote:
>
> We are working on an analysis for identifying the class of an object flow
> sensitively for
> flow sensitive de-virtualization (i.e. replacing a virtual function call by
> a call to the
> function of a known class in the hierarchy). This is a regular ipa pass. It
> find outs the
> class of an object at point of creation and then propagates it.

Most of this is already in GCC 5 and above.  Including the IPA pass.
Have you looked into that pass yet?


>
> In the examples we have seen so far, given a statement
>
>   B beta ("str");
>
> It is is transformed by gcc-4.7.2 as shown below :
>
>      __comp_ctor  (&beta, "str");
>
> We use this information to identify the class.
>
> However, for case in the source of GNU package gperf-3.0.4, a statement
>
>       Output_Enum style ("  ");
>
> is transformed by gcc 4.7.2 as
>
>       # DEBUG this => &style
>       # DEBUG indent => ""
>       # DEBUG D#62 => &style.D.4064
>       # DEBUG this => D#62
>       MEM[(struct Output_Constants *)&style]._vptr.Output_Constants =
> &MEM[(void *)&_ZTV16Output_Constants + 16B];
>       style.D.4064._vptr.Output_Constants = &MEM[(void *)&_ZTV11Output_Enum
> + 16B];
>       style._indentation = "";
>
> Why is this different? Why is __comp_ctor not invoked in each case?

This looks like the function has been inlined as it is short.

Thanks,
Andrew Pinski

>
> The class hierarchy in gperf is as given below :
>
> struct Output_Constants
> {
>   virtual void          output_start () = 0;
>   virtual void          output_item (const char *name, int value) = 0;
>   virtual void          output_end () = 0;
>                         Output_Constants () {}
>   virtual               ~Output_Constants () {}
> };
>
> struct Output_Enum : public Output_Constants
> {
>   virtual void          output_start ();
>   virtual void          output_item (const char *name, int value);
>   virtual void          output_end ();
>                         Output_Enum (const char *indent)
>                           : _indentation (indent) {}
>   virtual               ~Output_Enum () {}
> private:
>   const char *_indentation;
>   bool _pending_comma;
> };
>
> Thanks and regards,
>
> Uday Khedker.
>
>