[llvm-dev] RFC: Emitting empty invariant group for vtable loads

[Piotr Padlewski via llvm-dev]

2017-01-30 18:38 GMT+01:00 Richard Smith <richard at metafoo.co.uk>:

> On 20 Jan 2017 8:07 am, "Piotr Padlewski" <piotr.padlewski at gmail.com>
> wrote:
>
> Hi all,
>
> I would like to propose a new way clang would decorate vtable loads in
> order to handle devirtualization better.
>
> I've added *llvm-dev* also, because this can start a discussion about
> changing invariant.group to just invariant.
>
> PDF version of this RFC can be found here:
>
> https://drive.google.com/file/d/0B72TmzNsY6Z8ZmpOUnB5dDZfSFU
> /view?usp=sharing
>
>
> Background:
>
> Initial old design:
>
> http://lists.llvm.org/pipermail/cfe-dev/2015-July/044227.html
>
> My talk from LLVM Dev Meeting
>
> http://llvm.org/devmtg/2016-11/#talk6
>
> The problem
>
> Clang with -fstrict-vtable-pointers decorates vtable loads with metadata
> corresponding to mangled pointer type name like:
>
> void g(A& a){
>    a.foo();
> }
>
>
> define void @_Z1gR1A(%struct.A* dereferenceable(8) %a) local_unnamed_addr
> #0 {
> entry:
>  %0 = bitcast %struct.A* %a to void (%struct.A*)***
>  %vtable = load void (%struct.A*)**, void (%struct.A*)*** %0,
> !invariant.group !7
>  %1 = load void (%struct.A*)*, void (%struct.A*)** %vtable
>  tail call void %1(%struct.A* nonnull %a)
>  ret void
> }
>
> !7 = !{!"_ZTS1A"}
>
>
>
> This works well if the pointer type doesn’t change, but when it does,
> devirtualization might not happen like here:
>
> struct A {
>    A();
>    virtual void foo();
> };
> struct B : A{
>    B();
>    virtual void foo();
> };
> void g(A& a){
>    a.foo();
>    a.foo();
> }
> void clobber(A&);
> void f() {
>      B b;
>      clobber(b);
>      g(b);
> }
>
> The other problem is that when we combine 2 instructions with different
> invariant.group metadata, then we pick one of them, because for now we can
> have only single !invariant.group metadata.
>
> The solution
>
> I had some initial ideas how it can be solved, like
>
>    1.
>
>    introducing multi invariant groups
>    2.
>
>    having sub invariant groups - like inheritance, so we could figure out
>    that one group is subgroup of another
>    3.
>
>    decorating all loads with base pointer MD (doesn’t work with multiple
>    inheritance)
>
>
> The original idea was that the metadata would indicate the class that
> introduced the vptr; that seems like it should work ok with multiple
> inheritance. Is there a problem with that approach? (We may still be able
> to remove it entirely, which could still be preferable.)
>
>
You are referring to the last point, right? So it is possible to decorate
with the base class type (the introducing vptr class, which will be only
one for every vfunction). But there is problem with it. Consider example:

struct A {
  virtual void foo();
};
struct B {
  virtual void bar();
};
struct C : A, B {
  virtual void foo();
  virtual void bar();
};
void test(C &c) {
  c.foo();

  c.bar();
}


Right now call to foo and bar will get the same invariant.group, so we will
end up with only one load (the *!invariant.loads* - doesn't play any role
here):

define void @_Z4testR1C(%struct.C* dereferenceable(16) %c) {entry:
  %0 = bitcast %struct.C* %c to void (%struct.C*)***
  %vtable = load void (%struct.C*)**, void (%struct.C*)*** %0,
*!invariant.group* !7
  %1 = load void (%struct.C*)*, void (%struct.C*)** %vtable, !invariant.load !8
  tail call void %1(%struct.C* nonnull %c)

  %vfn2 = getelementptr inbounds void (%struct.C*)*, void
(%struct.C*)** %vtable, i64 1
  %2 = load void (%struct.C*)*, void (%struct.C*)** %vfn2, !invariant.load !8
  tail call void %2(%struct.C* nonnull %c)
  ret void
}

!7 = !{!"_ZTS1C"}
!8 = !{}



If we would decorate vtable load from c.foo() with !invariant.group
!"typeA", and from c.bar() with !"typeB", then

the second one won't be removed:



define void @_Z4testR1C(%struct.C* dereferenceable(16) %c) {entry:
  %0 = bitcast %struct.C* %c to void (%struct.C*)***
  %vtable = load void (%struct.C*)**, void (%struct.C*)*** %0,
*!invariant.group* !0
  %1 = load void (%struct.C*)*, void (%struct.C*)** %vtable, !invariant.load !2
  tail call void %1(%struct.C* nonnull %c)

  %vtable1 = load void (%struct.C*)**, void (%struct.C*)*** %0,
*!invariant.group* !1
  %vfn2 = getelementptr inbounds void (%struct.C*)*, void
(%struct.C*)** %vtable1, i64 1
  %2 = load void (%struct.C*)*, void (%struct.C*)** %vfn2, !invariant.load !2
  tail call void %2(%struct.C* nonnull %c)
  ret void
}

!0 = !{!"_ZTS1A"}
!1 = !{!"_ZTS1B"}
!2 = {}




Piotr




> I consulted my ideas with Krzysztof Pszeniczny, and he proposed something
> much simpler: we can decorate every invariant.group md with empty metadata.
>
> This should work because the lifetime of the object is strictly defined by
> invariant.group.barrier.
>
>
> If this holds, we can start discussion about if it makes sense to keep
> invariant groups, and instead have just “invariant”, that would be
> equivalent to having invariant.group with the same metadata.
>
> Do you have some thoughts about this approach? I don’t have a mathematical
> proof, but I am confident that it should be valid.
>
>
>
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