[cfe-dev] C++11 and enhacned devirtualization

[Richard Smith]

On Thu, Jul 16, 2015 at 2:03 PM, John McCall <rjmccall at apple.com> wrote:

> On Jul 16, 2015, at 11:46 AM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Thu, Jul 16, 2015 at 11:29 AM, John McCall <rjmccall at apple.com> wrote:
>
>> > On Jul 15, 2015, at 10:11 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>> >
>> > Hi everyone,
>> >
>> > C++11 added features that allow for certain parts of the class
>> hierarchy to be closed, specifically the 'final' keyword and the semantics
>> of anonymous namespaces, and I think we take advantage of these to enhance
>> our ability to perform devirtualization. For example, given this situation:
>> >
>> > struct Base {
>> >  virtual void foo() = 0;
>> > };
>> >
>> > void external();
>> > struct Final final : Base {
>> >  void foo() {
>> >    external();
>> >  }
>> > };
>> >
>> > void dispatch(Base *B) {
>> >  B->foo();
>> > }
>> >
>> > void opportunity(Final *F) {
>> >  dispatch(F);
>> > }
>> >
>> > When we optimize this code, we do the expected thing and inline
>> 'dispatch' into 'opportunity' but we don't devirtualize the call to foo().
>> The fact that we know what the vtable of F is at that callsite is not
>> exploited. To a lesser extent, we can do similar things for final virtual
>> methods, and derived classes in anonymous namespaces (because Clang could
>> determine whether or not a class (or method) there is effectively final).
>> >
>> > One possibility might be to @llvm.assume to say something about what
>> the vtable ptr of F might be/contain should it be needed later when we emit
>> the initial IR for 'opportunity' (and then teach the optimizer to use that
>> information), but I'm not at all sure that's the best solution. Thoughts?
>>
>> The problem with any sort of @llvm.assume-encoded information about
>> memory contents is that C++ does actually allow you to replace objects in
>> memory, up to and including stuff like:
>>
>> {
>>   MyClass c;
>>
>>   // Reuse the storage temporarily.  UB to access the object through ‘c’
>> now.
>>   c.~MyClass();
>>   auto c2 = new (&c) MyOtherClass();
>>
>>   // The storage has to contain a ‘MyClass’ when it goes out of scope.
>>   c2->~MyOtherClass();
>>   new (&c) MyClass();
>> }
>>
>> The standard frontend devirtualization optimizations are permitted under
>> a couple of different language rules, specifically that:
>> 1. If you access an object through an l-value of a type, it has to
>> dynamically be an object of that type (potentially a subobject).
>> 2. Object replacement as above only “forwards” existing formal references
>> under specific conditions, e.g. the dynamic type has to be the same,
>> ‘const’ members have to have the same value, etc.  Using an unforwarded
>> reference (like the name of the local variable ‘c’ above) doesn’t formally
>> refer to a valid object and thus has undefined behavior.
>>
>> You can apply those rules much more broadly than the frontend does, of
>> course; but those are the language tools you get.
>
>
> Right. Our current plan for modelling this is:
>
> 1) Change the meaning of the existing !invariant.load metadata (or add
> another parallel metadata kind) so that it allows load-load forwarding
> (even if the memory is not known to be unmodified between the loads) if:
>
>
> invariant.load currently allows the load to be reordered pretty
> aggressively, so I think you need a new metadata.
>

Our thoughts were:
1) The existing !invariant.load is redundant because it's exactly
equivalent to a call to @llvm.invariant.start and a load.
2) The new semantics are a more strict form of the old semantics, so no
special action is required to upgrade old IR.
... so changing the meaning of the existing metadata seemed preferable to
adding a new, similar-but-not-quite-identical, form of the metadata. But
either way seems fine.

>   a) both loads have !invariant.load metadata with the same operand, and
>   b) the pointer operands are the same SSA value (being must-alias is not
> sufficient)
> 2) Add a new intrinsic "i8* @llvm.invariant.barrier(i8*)" that produces a
> new pointer that is different for the purpose of !invariant.load. (Some
> other optimizations are permitted to look through the barrier.)
>
>
> In particular, "new (&c) MyOtherClass()" would be emitted as something
> like this:
>
>   %1 = call @operator new(size, %c)
>   %2 = call @llvm.invariant.barrier(%1)
>   call @MyOtherClass::MyOtherClass(%2)
>   %vptr = load %2
>   %known.vptr = icmp eq %vptr, @MyOtherClass::vptr, !invariant.load
> !MyBaseClass.vptr
>   call @llvm.assume(%known.vptr)
>
>
> Hmm.  And all v-table loads have this invariant metadata?
>

That's the idea (but it's not essential that they do, we just lose
optimization power if not).


> I am concerned about mixing files with and without barriers.
>

I think we'd need to always generate the barrier (even at -O0, to support
LTO between non-optimized and optimized code). I don't think we can support
LTO between IR using the metadata and old IR that didn't contain the
relevant barriers. How important is that use case? We were probably going
to put this behind a -fstrict-something flag, at least to start off with,
so we can create a transition period where we generate the barrier by
default but don't generate the metadata if necessary.
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