Cool, thanks! I think I understand now.
On Tue, 22 Jun 2021 at 23:58, Martin Jambor <mjam...@suse.cz> wrote: > > Hi, > > On Fri, Jun 18 2021, Erick Ochoa via Gcc wrote: > > Hi, > > > > I am having some trouble understanding the semantics of OBJ_TYPE_REF. > > I understand that it is made of three operands: > > > > 1. OBJ_TYPE_REF_EXPR: An expression that evaluates the value to use. > > 2. OBJ_TYPE_REF_OBJECT: Is the object on whose behalf the lookup is > > being performed > > 3. OBJ_TYPE_REF_TOKEN: An integer index to the virtual method table. > > > > The language here is very general and makes it hard for me to understand. Is > > > > 1. OBJ_TYPE_REF_EXPR: The virtual function > > well, it is never the actual function as in a FUNCTION_DECL, it is > always a pointer to an unknown function. > > > 2. OBJ_TYPE_REF_OBJECT: The "this" object > > > > ? > > > > Why is the index needed if it looks like the virtual function is > > already pointed to by the first operand? > > As I wrote above, the first operand is just a pointer. Sometimes other > optimizations can figure out where it leads but often not (or not early > enough for inlining). > > In some situations we can get at the actual DECL (and subsequently have > a direct call) using type based devirtualization and for that we use the > other two arguments of OBJ_TYPE_REF. See the big leading comment in > ipa-devirt.c. > > > > I am reading Hubicka's post > > on devirtualization and Part II > > (https://hubicka.blogspot.com/2014/01/devirtualization-in-c-part-2-low-level.html) > > seems to agree with me on the example below. There is also no mention > > of OBJ_TYPE_REF on part II, but there is on part III. > > I believe the gimple quoted there has been simplified to make it easier > for the reader, since it does not describe the type-based > devirtualization but how other optimizations can together deduce the > call target, and so OBJ_TYPE_REF has been omitted not to confuse anyone. > > If you dump the IL for th example yourself, you will see OBJ_TYPE_REF in > the earliest dumps and you will see that it disappears as soon as the > call becomes direct. > > Hope this helps, > > Martin
