Notes on Doug's Method Lookup Patch

Jun 27, 2000 by Simon Cozens

Methods and subroutines
What entersub knows about methods
When to convert methods
How to convert methods
Finding the stash
Extract the GV entry
Rewrite entersub
Conclusion
Exercises

Speeding up method lookups

Last week, Doug MacEachern provided an interesting and extremely clever patch to speed up method calls. His benchmarks showed that it completely reduced the overhead of using methods, and appeared to suggest that method calls could actually be faster than subroutine calls. Let’s have a look at the patch in detail, to see how it’s done.

Here’s the original musing from Sarathy, which started it all off:

    If @ISA isn't modified at run time, Foo::->bar() could be resolved
    at compile time to a subroutine call, as can this:

    my Foo $obj = shift;
    $obj->bar();

As all the tutorials on Perl’s object-oriented programming rightly state, when you call a method, you’re calling a subroutine. The difference, however, is that a method call has to perform a lookup to find out which package its subroutine lives in.

In the normal case without inheritance, Foo->bar() will be subroutine bar in package Foo. However, because of inheritance, you can’t tell just by looking at Foo->bar() where bar is going to be - Foo may inherit bar from Frob, from example.

To resolve this, when you call a method, Perl checks for a subroutine bar in package Foo. If it doesn’t find one, it checks Foo’s parents: all the packages listed in Foo’s @ISA array. It then checks the grandparents, and so on. This lookup has to be done at run-time, instead of compile time, because @ISA is an ordinary variable - someone may change the inheritance tree from under your feet.

Sarathy’s thought was that if you know for sure what’s going to be in @ISA at the time you perform the compilation, you change an expensive method call into a slightly less expensive subroutine call. And that’s what Doug’s patch does.

We can divide the patch up into two parts: the function method_to_entersub performs the conversion and tells us how it’s done, and the part of the code which calls method_to_entersub, telling us when it’s done. We’ll look first at the when.

Before we do that, though, a little detour about how subroutine calling works internally.

Perl constructs the entersub op in a similar way for a method call and an ordinary subroutine call. The difference is that for a subroutine call, the entersub op is preceded by an op that fetches the GV that holds the subroutine code, and for a method call, the entersub op is preceded by an op that looks up the method name in the inheritance tree. You can see this in the following B::Terse output.

        % perl -MO=Terse,exec -e 'bar()'
        ...
        SVOP (0xa043b00) gv  GV (0xa04ecf8) *bar              <- cvop
        UNOP (0xa0b6a60) entersub [1]
        ...

For a method call, though, Perl also attaches an op representing the class (let’s call it o2, since that’s as good a name as any) and svop, instead of being a GV, becomes a special op to signify that a method should be called in that class:

    % perl -MO=Terse,exec -e 'Foo->bar()'
    ...
    SVOP (0xa043b00) const  PV (0xa0411f0) "Foo"          <- o2
    SVOP (0xa043b40) method_named  PVIV (0xa04ecf8) "bar" <- cvop
    UNOP (0xa0d0500) entersub [1]
    ...

For class methods, o is a constant, while for object methods, o2 fetches the object, allowing us to work out the class at run time:

    % perl -MO=Terse,exec -e '$foo->bar()'
    ...
    SVOP (0xa043b00) gvsv  GV (0xa04ecf8) *foo            <- o2
    SVOP (0xa0d5720) method_named  PVIV (0xa0411f0) "bar" <- cvop
    UNOP (0xa0d04d0) entersub [1]
    ...

method_named is the op representing, unsurprisingly, a named method. You can also call methods on the fly, like this:

                Foo->$methname()

and in this case, the method op is preceded by an op that fetches the method name from $methname. In this example, it’s a simple variable lookup:

    % perl -MO=Terse,exec -e 'Foo->$x()'
    ...
    SVOP (0xa043b00) const  PV (0xa0411f0) "Foo"          <- o2
    SVOP (0xa043b60) gvsv  GV (0xa04ed70) *x              <- child of cvop
    UNOP (0xa043b40) method                               <- cvop
    UNOP (0xa0d04d0) entersub [1]
    ...

What we want to do is trap both these method-style cvops, method_named and method, and see if we can perform the method lookup at compile time. Once we know what subroutine will actually be called, we can change the op tree so that it calls the subroutine directly, so that at run time when the method is actually called, Perl won’t have to search the inheritance tree any more.

 --- ./op.c.orig Fri Jun  2 15:49:32 2000
 +++ ./op.c  Thu Jun 15 22:45:33 2000
 @@ -6243,6 +6309,12 @@
          } 
      }
      else if (   cvop->op_type == OP_METHOD 
               || cvop->op_type == OP_METHOD_NAMED) {
 +        if (o2->op_type == OP_CONST || o2->op_type == OP_PADSV) {
 +            OP *nop;
 +            if ((nop = method_to_entersub(aTHX_ o2, cvop))) {
 +                return nop;
 +            }
 +        }
          if (o2->op_type == OP_CONST) {
              o2->op_private &= ~OPpCONST_STRICT;
          }

If you’re not used to reading patches, the first three lines tell you where we are: the seven lines from 6243 of op.c, which become thirteen lines from 6309 when the patch is applied. The lines starting with a plus are to be added.

Line 6243 is in the middle of the function Perl_ck_subr, which is the check routine called when an entersub op is filed. Check routines are fed an op and produce a `cleaned up’ version, performing any optimization or elimination of redundancies. Perl replaces each op in the op tree built during the compile phase with these optimized versions.

As we learnt above, we’re going to be in one of three situations at this point: ordinary subroutine, which will have a GV op attached to our entersub op; named method, which has method_named attached; or unnamed method, which has method attached. We care about cases when cvop is a method or method_named op:

 else if (   cvop->op_type == OP_METHOD 
          || cvop->op_type == OP_METHOD_NAMED) {

We’re fixing class methods, which means we try the conversion when o2 is a constant. We also want to fix up my Dog $sam; $sam->bark;, in which case o2, the object, is a lexical variable: (In internal-speak, a pad sv.)

 if (o2->op_type == OP_CONST || o2->op_type == OP_PADSV) {

In these cases, we try to convert the method to a subroutine, and return the new method if we manage it:

     OP *nop;
     if ((nop = method_to_entersub(aTHX_ o2, cvop))) {
         return nop;
     }

The final touch is a portion of the original source: in Foo->bar, Foo is obviously a class name, and shouldn’t fire off a warning if strict is checking against barewords. Hence, we remove the strict test from constants which are class names.

 if (o2->op_type == OP_CONST) {
     o2->op_private &= ~OPpCONST_STRICT;
 }

That’s ‘when’. Now let’s see ‘how’.

How to convert methods

Inside the check routine, the variable svop holds the method or method_named node that specifies how Perl should locate the subroutine that is actually called; o is the node that is executed just before this, that fetches the GV for the class name or the value of the variable that contains the object on whose behalf the method will be called. For example:

    Foo->bar():

    SVOP (0xa043b00) const  PV (0xa0411f0) "Foo"          <- o
    SVOP (0xa043b40) method_named  PVIV (0xa04ecf8) "bar" <- svop
    UNOP (0xa0d0500) entersub [1]
    ...

    $foo->bar():

    SVOP (0xa043b00) gvsv  GV (0xa04ecf8) *foo            <- o
    SVOP (0xa0d5720) method_named  PVIV (0xa0411f0) "bar" <- svop
    UNOP (0xa0d04d0) entersub [1]

There is also a method variable that holds the SV that is contained in o. Our function opens by getting the method name out of there:

 if (svop->op_type == OP_METHOD_NAMED) {
     methname = SvPV(method, methlen);
 }
 else {
     return Nullop;
 }

If it isn’t a named method, (that is, if we’ve got something like $foo->$bar()) we can’t do anything with it. We return the false value Nullop, which is a null pointer cast into an op, signalling to the calling routine that we couldn’t modify the program.

 if (cvop->op_type == OP_METHOD_NAMED &&
     o2->op_type == OP_CONST || o2->op_type == OP_PADSV) {

Now, we’re fixing up two things: class method calls where o is constant, and my Dog $sam situations where o is a pad SV.

Inside this function, we have three tasks:

Find the stash

A stash is a package symbol table, such as %Foo:: in Perl space. We need to find the class to which our method belongs, and bring out the symbol table from it.

Extract the GV entry

We’re trying to create an entersub op which calls, for example &Foo::bar. We’ve got %Foo:: from the above: the next task is to find *Foo::bar. At this point, we want to take care of inheritance.

Rewrite entersub

Finally, we fiddle with the op tree to convert the method form of entersub to a subroutine call which merely calls the GV we’ve just found.

Along the way, we also need to take care of the fact that Perl caches method lookups.

Finding the stash

We’re trying to find the GV *Class::Method so we can feed it to entersub. The first thing we need to do, then, is to find the package symbol table, or `stash’, for the class. There are two possible situations we’re in now, and the situation determines how we find the stash: either we’ve got a constant class, or the object is a lexical.

If we’ve got a constant class, finding the stash is simple: the PV of the SV hidden in the SvOP o tells us the name of the package, and gv_stashpvn returns the stash given a PV.

 if (o->op_type == OP_CONST) {
     STRLEN len;
     char *package = SvPV(((SVOP*)o)->op_sv, len);
     stash = gv_stashpvn(package, len, FALSE);
 }

Don’t be frightened of the scary casting in line 3: we’re just getting the string value from the const op.

If we’ve got a lexical (pad sv) object, my Dog $sam, life is slightly more interesting.

 else if (o->op_type == OP_PADSV) {

We fetch the actually SV itself from the current pad - the pad is where Perl keeps the lexical variables belonging to the current block, and it’s just an ordinary array:

     SV *sv = *av_fetch(PL_comppad_name, o->op_targ, FALSE);

We hope this SV exists, and is an object. If it is an object, SvSTASH will give us the stash, since Perl stored a pointer to the stash when we used bless to create the object.

     if (sv && SvOBJECT(sv)) {
         stash = SvSTASH(sv);
     }

Otherwise, we can’t do much.

     else {
         return Nullop;
     }
 }

Those are the only two cases - lexicals and constants - that can possibly happen, but computer programming sometimes makes the impossible possible. Doug stands firm against the demons of illogic:

 else {
     return Nullop;
 }

Extract the GV entry

If we’ve got a stash, we can now try and find the GV. Thankfully, there’s a function in gv.c which does this, and even takes care of inheritance for us as well.

 if (!(stash && (gv = gv_fetchmeth(stash, methname, methlen, 0)) 
             &&  isGV(gv))) {
     return Nullop;
 }

If gv_fetchmeth fails, or if what it returns is not a gv for some reason, then we couldn’t find the package that actually contains the method, so we give up and return Nullop. Otherwise, gv points to the GV that contains the subroutine we want entersub to call. Or it will do the first time the method is looked up. If, however, our method is inherited and provided by another stash, gv_fetchmeth provides an alias to the real GV in the object’s stash.

In order to get at the code for the real subroutine, we have to get the original GV from this alias; we use the GvCV macro to do this. If the GV we get from GvCV is in a different class to our method, then we have an alias. We need entersub to point to the original GV, and not this alias:

 if (GvSTASH(CvGV(GvCV(gv))) != stash) {
     gv = CvGV(GvCV(gv)); /* point to the real gv */
 }

In his comments, which I’ve snipped, Doug notes that this cached lookup will only be valid if inheritance doesn’t change at run time. There’s currently no way to freeze @ISA at run time yet, (hey, there’s a job for someone!) so Doug suggests as an alternative a simple return Nullop to abort if we have a cached lookup.

Rewrite entersub

Finally, we have enough information to rewrite the entersub op: We have a GV, which contains a CV, which we can now call directly. Let’s arrange things to do that.

First, remove the strict test from the method name as before.

 o->op_private &= ~(OPpCONST_BARE|OPpCONST_STRICT);

Now find the op_method_named op, and change it into the GV to create a subroutine call.

 for (mop = o; mop->op_sibling->op_sibling; mop = mop->op_sibling) ;
 op_free(mop->op_sibling); /* loose OP_METHOD{,_NAMED} */
 mop->op_sibling = scalar(newUNOP(OP_RV2CV, 0,
                             newGVOP(OP_GV, 0, gv)));

Triumphantly, we return the converted op:

 nop = convert(OP_ENTERSUB, OPf_STACKED, o);
 return nop;

Conclusion

We got a lot of mileage out of that patch; we’ve seen how Perl finds and calls methods and subroutines, and shown how Doug’s patch can turn methods into subroutines. This should almost totally remove the overhead in using object-oriented Perl, but for the following two points.

Exercises

Doug’s patch needs a way of validating the inheritance tree at compile time; the obvious way to do this is to freeze @ISA. Write a pragma which does this.

For bonus points and god-of-the-week status, find a way of validating the method cache at compile time while keeping @ISA variable.
This only works for named methods; find a way to make it work for methods that are fetched from variables or via other means. That is, make Class->$thing() convertible to a sub.

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